Kiss my Keyster goodbye…. the Frankenbike 350 story continues

I have commented on the quality of the Keyster brand of motorcycle carburetor kits for years now, and am joined by many others who have suffered from the lack of accuracy of their components. I generally spend my repair time with 250-305 twins, but occasionally I have to work on bikes from a later vintage, often times 350 twins and an array of four-cylinder models. The challenge is that Keyster is one of the few remaining companies that make an attempt at offering repair kits for vintage motorcycles of all brands. 

You would think that a company that goes to the trouble of manufacturing carb components would have the expertise in duplicating the original parts accurately, but that is often not the case. We have found that the slide needles for the CB77 kits are too rich for Super Hawks, but work okay for Scramblers, which use a different needle taper. Likewise, all of their bowl gaskets are too wide where they fit into the forward slot in the carb base and don’t take into account that there are two little bumps that need clearance.

In recent years, the kits for the 305 Dreams started coming with #130 main jets which are way too rich for a stock bike that came with #120 main jets from the factory. Every time someone contacts me about difficulties with plug fouling on a Dream, the first question from me is: Does it have a Keyster kit installed with a #130 main jet? Often, lately, that is the answer to the problem. Early Honda models have JIS thread pitch jets, so you can’t plug in the later versions which are ISO pitch now. One of my suppliers www.4into1.com does supply JIS main jets, separately from whole kits.

Usually, the plan is to reuse any OEM hardware pieces like jets and needles. I have discovered that the dimensions of the Keyster float valves are a little off, as well, causing the need to bend the float tang excessively to get the float height to be correct.

If you read my recent blog post http://www.mrhonda.guru/2022/11/frankenbike-honda-350-twin.html

which was supposed to have been published back in September, you can see what kinds of problems can occur when you are using aftermarket components, like Keyster carb kits. Well, the story didn’t end there after all.

After a few days of happy driving, the bike became difficult to start and would stall out after a few seconds. I shared as much guidance as I could over text messages with the owner, but despite his best efforts the bike refused to come alive again. Part of the difficulty is that the bike is 30 miles away in Leucadia near my sister’s home. I only go up there about once a month for my nearby chiropractor’s appointment so I had hoped to be able to resolve the problems via text messaging. This was not the case, but my next appointment came due so I packed up tools and equipment to do a house call after my appointment.

Having tools on hand proved fortunate as my chiropractor needed some carb cleaning work on his Vespa Scooter, so I was able to trade services and save $125 for the visit. After that work, I drove down the road to the awaiting CB350 Frankenbike and its frustrated owner. Let the fun begin again….

Checking for battery voltage first, I measured 12.4v so that was a good starting point. The bike would try to start, then die as soon as the throttle was turned at all. There was backfiring going on during the startup attempt too, so that is generally an air leak or incorrect ignition timing. It was apparent that the right side cylinder was misfiring and not really catching on, so I focused on that side first. 

One of the other problems facing the owner was that the throttle cables wouldn’t synch up properly, which was eventually resolved after the carb cable extension to the left side had slipped out of the junction, leaving uneven lengths. The aftermarket cables have big fat junctions, unlike the originals, which can get trapped beneath the fuel tanks when the tank is set back on its mounts. The second problem was that either the threads in the carburetor cable holders were stripped or the cable end dimensions were undersized so that you can’t tighten the cable adjusters onto the carburetors. After a lot of fiddling, I got them as close as possible to matching each other and left it at that.

With the point cover off, the left side points didn’t seem to be opening very far which resulted in retarded ignition timing. The point adjustment was all the way open and the point backing plate was rotated as far as it could go but still, the timing was 20 degrees retarded, which explained the backfiring on that side. Eventually, I had to bend the point base contact outwards to get some point gap established and corrected the ignition timing. The bike started up and kept running but the right side wasn’t taking the throttle off idle, but it was idling okay. I thought perhaps the carb diaphragm was damaged, so removed the carburetor and pulled the top off to inspect the diaphragm. It looked fine but THE SLIDE NEEDLE WAS SEPARATED AGAIN! I don’t think that Honda’s Keihin carburetor slide needles are made in two pieces, but these certainly were. I tried to crimp the end of the needle slightly to make an interference fit with the little top hat end and tapped it back together again. It seems to be secure for the moment, but I don’t trust it at all now.

The bike started back up and was running on both sides, but idle mixture settings were off specs and there was still hard starting and some backfiring going on. I hit the intake manifolds with a blast of brake cleaner which I had brought along and the RPMs jumped instantly. A closer look at the manifolds revealed that the paper gaskets that were supposed to seal the manifolds to the head were blown out causing a huge air leak. The nearby Auto-Zone was a source for a tiny $6 tube of high temp RTV gasket sealer and the manifolds were glued back onto the cylinder head, solving that problem.

I had been on the job for more than 2 hours, improvising repairs and troubleshooting but finally had a functioning motorcycle, at least at the moment. I feel like starting a GoFundMe page for my suffering friend who still is sleeping in his car, next to the railroad tracks, living day to day from his meager income from photography work in the local area. He goes out and shoots local surfers in action, plus has other photography gigs in the county, but it isn’t enough to really get him settled into an apartment where the mean rent prices are in the $2k range. Surprisingly, he’s adapted to his situation the best he can for now, with a hopeful attitude for future work and a warm spot to call his own. See his work at https://www.abstractseagull.com/ 

I’ve done what I can do for the Frankenbike 350 for now. It needs a whole host of OEM parts to get its reliability improved, but it is what it is at the moment.

Bill Silver aka MrHonda

11-2022

Frankenbike Honda 350 twin…

I received a desperate call from a man who needed some electrical work done on his “CB350” as his Prius had just died after spending $2500 on new batteries. Apparently, a cooling hose leaked out all the coolant on the freeway and fried the gas engine. I am not sure if he had already owned the bike or bought it as a temporary solution to his transportation needs. Unfortunately, the bike began to blow the main fuse, then with help from semi-knowledgeable friends wound up with a new, aftermarket wiring harness installed but not fully completed.

Day ONE--- Delivery

I didn’t ask all the right questions like, “Is it a stock CB350?” etc. so I agreed to have a look at it. After finally rounding up a friend and his pickup truck to haul it 35 miles down the coast, it arrived late on a Thursday… Arrgh! The bike was someone’s version of a “cafe bike” transformation, which still had the stock tank and frame, but the rest was non-stock, to say the least.

The fork tubes were shoved up in the triple clamps about 2” compressing the fork boots down. There were no fenders, front or rear. The stock seat pan was reupholstered and beneath it a tiny 4 amp battery was strapped down to the battery box. I was puzzled as to where the starter cable was hiding but then noticed that there was no electric starter on the front of the engine! It was an SL350 engine swapped into the CB chassis! The carbs were still the stock CB350 CV carb set, however.

The later SL350 engines have a special cylinder head with small ports and a pair of 24mm carburetors to enhance the low-speed torque and power for off-roading, but probably a complete mismatch for the 32mm CB carbs and their calibration. Oh, of course, there was set of noisy-looking aftermarket mufflers attached to the header pipes, which affects the carburetor calibrations, as well.

The “cafe” handlebars were the drop-down kind but all of the original length cables, normally on a set of much higher handlebars, were still in place looped around and around the front forks and handlebars. The clutch cable jutted out on the left side and was zip tied to a too-long spark plug wire, both hanging out in space. The owner brought a box of spares including another set of stock cables, which were useless on this application. There was a tiny aftermarket speedometer hooked up and a large H4 headlight mounted up front. The turn signals were tiny aftermarket units, as well. Pretty much anything that came from the factory was no longer present.

The fuel lines were that colored plastic stuff, that was then zip-tied to the fuel fittings. I had to drain the fuel tank through one of the fuel lines from the Chinese petcock which has reversed markings for the On-OFF-Reserve from normal. I barely got one of the lines off, but the back one pulled the fuel fitting out of the petcock body. I hate this stuff!

Looking over the electrical connections, it appeared that the condensers and coil wiring was incorrect, but all of the wiring colors are different on the aftermarket harnesses, so a 12 v test light will get a workout as I sort through the connections. An aftermarket rectifier-regulator was installed but the wiring colors don’t seem to match up.

I went online and quickly ordered a set of short cables from 4into1.com along with 5 feet of 5.5mm Honda fuel line, with payment for 2-day delivery which cost $37!

Day TWO- Investigation.

I had a leftover Li-Ion battery that was larger than the one in the bike and might work out better for this bike. Even though it showed fully charged after a few hours, the Li-Ion battery failed the self-test, so the little 4 amp battery was charged overnight and reinstalled. Then the fun began…

Turning the ignition switch to ON blew the 15amp fuse. Looking over the wiring to the points and condenser, it appeared to be miswired, so after reconnecting the condenser, coils and points, the switch was turned ON again… and another blown fuse. I did an ohm test on each coil and the left side which was a made-in-China Tec coil had shorted out to .6 ohms. The opposite coil was about 3.8 ohms. In the box, of spare bits was another coil, which tested out okay and when installed the fuse stayed intact. One small step towards success. Unfortunately, the replacement wiring harness had wire colors unknown to Honda’s engineers, so it took time and getting a copy of the revised wiring harness from 4into1.com to help guide me through the process, one wire at a time.

Using a 12v test light, I was able to probe various wires for power or ground and eventually got everything that was supposed to be connected… connected! Power ON showed headlight (both beams), tail and brake light functions, ignition power to the points and power to the rectifier/regulator. I tried to kickstart the bike, but it didn’t give any signs of wanting to run. Compression checks revealed about 150 psi on each side, but the engine sounded noisy when kicking it over.

Putting a wrench on the rotor and turning the engine forward/reverse demonstrated some slack in the camchain, which was reduced with a camchain tensioner adjustment. The tappet covers were almost unmoveable except for my adjustable Sears 6 point 12” long wrench which finally broke them loose without breaking anything. As suspected the tappet clearances were WAY off. I noticed that the little index marks for the ends of the rocker arm adjusters were not all in the usual 4 and 8 o’clock positions, so that was all reset.

Next step was to tear the 722A carbs off and see how they were doing inside. The first thing was that there was a lot of unmarked parts indicating some off-brake carb kit installation. The float levels were in the 19-21mm settings, which is fine for early carbs but doesn’t match the 26mm suggested settings from Honda. After the floats were reset, the carb tops were removed to check the condition of the slide diaphragms. The left side had the usual little locating tab, but the right side did not. When the slide was inspected the non-adjustable needle had separated and the top was left inside the slide body, while the rest of the needle was sitting in the needle jet inside the carb throat. I didn’t realize that these needles were made in 2 pieces! I tapped the top back on the needle portion and hoped for the best.

When the right side carb was inspected, it had an incorrect float level setting as noted above, with a non-indexed diaphragm. The needle was also dislodged, but not separated. Whoever put the needle back into the slide didn’t secure the wire clip that holds everything down in place, so that needle was jumping around too. The owner said that the bike had been running previously, but not well. Duh! I am surprised that it ran at all. With carbs done, I awaited the new short cable set from 4into1.com to come so I could install the new throttle cable, along with the clutch and front brake cables of a more appropriate length.

Day THREE… updates

The Priority Express mailing of the cables failed to arrive in the 2-day period. Ordered on Friday, they finally landed on Monday, which normally the regular Priority Mail shipping ($15) would have sufficed. 4Into1.com contacted USPS and actually got a refund for failed 2-day delivery. The old cables were already disconnected at the handlebars, but not without more difficulties. The cable adjusters were somewhat corroded into the lever mounts and the ends of the cables were corroded as well. Prying them apart finally released the cable ends and the adjusters went into some Metal Rescue for de-rusting. With non-standard handlebars, the best cable routing has to be determined by trying various routes to allow the cables to move properly and not be pinched by the fuel tank. The clutch cable goes into the left side cover where it connects to the clutch lifter hardware. Finding the right path took a few tries, but eventually raising the handlebar angles helped make it all fit.

The bike was fired up, finally, with way too loud mufflers blasting my quiet neighborhood. The bike ran unevenly but finally went down the road under its own power. Kickstarting was kind of random when the key was turned on, eventually, that problem was traced to an erratic aftermarket ignition switch function. I finally had to “test” the switch with the horn button to see if it actually had turned ON or not. I dismantled the switch and smoothed out the contact plate, but it still seemed to have some dysfunctional moments.

Day FOUR… too much drama

I told the owner that I thought the bike was ready to ride after about 6 hours of labor and adjustments of the engine and fuel system. I also had to seal up the leading top edge of the fuel tank with some tank sealer as there was a fuel leak right at the seam. The owner had to come down to Spring Valley on the Coaster train and then on a bus to my neighborhood. I had just returned from appointments, so the timing was about right. He appreciated all the work that I had done and wanted to add value to the transaction by shooting some photos of me and my NT650 in action and in some scenic spots nearby.

We rode out to Mt. Helix which is a panoramic viewpoint in the La Mesa area. The bike seemed to run okay as he trailed me on the Hawk GT. We took some photos around the top, then when we went to leave the bike wouldn’t start back up. It appeared like the left coil wasn’t firing again and it just wouldn’t light up on the right side. I rode back home, got my Tacoma, and returned to load up the 350 and bring it back home for diagnosis.

It appeared like the left side coil still wasn’t firing properly. I replaced it with another spare from the extras in his parts box. The bike started up and I drove it over to the 7-11 to fill up the tank for the journey home. The bike started dropping one cylinder again, but this time it was the right side! I would not fire up again, so the owner was summoned to bring my truck over, and pick me up so I could return home and get the ramp and tie-downs again. We were only about ¼ mile from home, but we were losing light and the owner had been planning to make a coastal journey back home which was looking less and less likely.

Troubleshooting the bike, I finally tested the battery voltage which had dropped to 8 volts with key OFF and 5 with key ON. Okay, a combination of a failed charging system and perhaps an undersized battery were the current causes of failure to keep the bike alive. I wound up taking my owner friend back to the train station in downtown San Diego, which is a 14-mile drive, one-way, and promised to get to the bottom of this problem the next day. I had to take responsibility for the fact that I never checked the charging system output during all of the running and electrical repair work.

Day FIVE… Battery hunting

Calling the local Honda dealership, it seems that the correct battery for an electric start CB350 was NLA at any of the other motorcycle dealers in the SD region! I called Interstate battery and they said the battery was back-ordered. I finally found a battery store out in Santee (12 miles away), that had two in stock! I made the trek out to the store and secured the new battery, turning in the old 4 amp battery and the leftover Li-Ion spare as well.

Dropping the already-charged battery into the bike, I noticed that the static voltage was about 12.5. With the key ON, it dropped to 12.25, which is pretty normal. The bike sputtered back to life again now with full voltage to the coils and cleared off some of the unburnt fuel in the cylinders and on the plug tips. Revving the engine up, the voltage failed to increase, so there was more hunting to do.

I had to remove the left carburetor, which is easy with no air filter box to have to remove. This gave me access to the electrical plug that connects the stator windings to the main wiring harness. Pulling the plug apart, revealed that two of the female connector pins had pushed out of the connector, thus no AC power going to the reg/rectifier. Finally, the last smoking gun had been discovered. Reassembled again, the voltage increased to nearly 13 volts with no lights on and held steady around 12.5 with the lights back ON again. Another run down my test circuit was successful with the engine pulling towards redline and running well on both sides. Repairs finally completed… or not!

The engine was drooling oil down from the dyno cover side gasket. From the appearances, I would guess that someone made a gasket to fit, as most CB350 gasket kits would have supplied a dyno cover gasket that incorporated the electric starter function which this bike did not have. Digging through a pile of misc gasket sets, I discovered what appeared to be the correct gasket. That will be a job for the next day.

Day SIX

To prevent oil loss, the bike was leaned up against a wall so the dyno cover gasket could be changed out. The rear engine cover needs to be removed first, but that only happens after the shift lever is removed, the footpeg bolts loosened up, and then the cover was

removed. The gasket replacement was successful and the bike was sent back home for local transportation needs. I can only hope that it continues to function given the difficulties that were presented with this Frankenbike 350.

Factory AHMC photo of what the bike looked like originally

Bill Silver aka MrHonda

Sept 2022

MrHonda takes in another stray…. Honda CB750 Cafe Racer #2 of 2 (part two)

 Deja Voodoo…. The evil twin takes the place of the Orange CB750

I dropped off the Orange CB750 Cafe bike to the owner, after more than a week’s worth of dismantling, ordering parts, cleaning, adjusting and doing a deep dive into what I had to work with. The owner was thrilled to have it back, but then pleaded his case about having me “take a look” at his black CB750 which is a mismatch of K0 engine parts, an unknown frame, custom 4-into-4 exhaust pipes, Clubman bars, and a set of pod-filtered K2-ish carburetors.

The quick rundown was that he was putting a battery in it every few months and the bike kept dying due to charging system issues. Beyond that, the custom exhaust “mufflers” (there are some baffles inside but they don’t do much) were hitting the end of the swing arm on the #2 pipe, and the side stand was wedged in between the #1 and #2 pipes so that the shortened stand arm was getting stuck between the mufflers and didn’t fully retract which was causing grounding issues on left-hand turns.

Also mentioned was that the engine was an 836 big-bore motor with “low compression” in a few cylinders. There was a misfire in #3 cylinder and rattling noises coming from the engine cylinders as well as a very noisy clutch.

We loaded the bike up and I drove it back to Casa de Honda for a going over. With the tank removed and the side cover removed, I had access to a Medusa-like nest of wires that were zip-tied and taped up that were going every which way. The first tests are on the stator and field coil with the harness lead disconnected and the VOM first checking continuity with the buzzer function. All three yellow wires were connected and not grounded. The field coil checked out okay, with a 7.9 ohm resistance value. The bike had already had a solid-state regulator-rectifier installed which was a Hail Mary attempt at solving the charging system issues. Working backward from the connector to the Reg/Rec unit into the harness I discovered that one of the three yellow wires wasn’t making a connection down to the control unit. Unwrapping old, oily electrical tape and cutting a few zip-ties got me to a section of the harness that had been zip-tied to the carburetor throttle cable bracket. Pulling it away and unwrapping the wiring revealed a separate yellow wire that was smashed up against the carb bracket and the friction and vibration had worked the wire into two ends. I cut back the ends and crimped a butt connector into the harness wire. Checking with the VOM again, I had continuity all the way through the three stator wires.

I fired up the bike with the gas leftover in the carburetors and watched as the voltage increased as the revs built up. So far, so good.

The compression test came back with 140 psi in #4, 160 psi in #3, 190 psi in #2 and nearly 200 psi in #1. The high compression numbers are consistent with the 836 big bore piston kit, but the two lower numbers, side by side, often indicates a blown head gasket between the two cylinders. The #3 spark plug came out with a much different color than the other three, which indicated either a lean condition or has something to do with the blown gasket.

This bike had the same removable frame section kit installed like the Orange bike, so with that frame piece out of the way, the tappet cover could be removed for a quick look at the overall condition of the engine internals. Everything was pretty clean and well lubricated but when the engine was turned to 1-4 TDC for a valve clearance check, most of the “loose” ones were too tight, including the intake valve of #3 cylinder.

With the difference in the cylinder pressures so great, it was time to pull it apart and find out what was wrong with it. Wrestling the exhaust system off was the first challenge as the original header pipes had somewhat baffled muffler bodies welded on, but only the outboard mufflers had brackets to the frame. The inboard mufflers were attached to the frame with radiator clamps. The #2 muffler came off and had a big hole worn through it from where it was rubbing against the sidestand/bracket area. That required a trip down to my friend, Rob North, for some creative clearance work building a little box into the muffler body.

The carburetors were a chore to remove from the manifold stubs, as the heat and age had hardened them into solid blocks of plastic. I had to use a Dremel cutoff wheel to remove the manifolds from the cylinder head. A check of the carb bank showed float levels a bit low, but #140 main jets installed. That is pretty big for most applications, but the spark plugs seemed to be reading okay.

With the camshaft disconnected and removed, the zillion head nuts and various bolts for the rocker arm stands were all extracted, but I could feel that there were stripped 6mm bolt holes in the head. Two of the bolts were not installed at all, owing to the fact that the rocker box gasket holes were out of place, so they decided to just leave them out.

The head was lifted off and the condition of the head gasket seemed to be fine. It definitely was a big bore motor, so a Cometic special gasket was sourced from an eBay seller. The bore spacing on these engines are pretty wide, so the chance of a gasket blowing out between cylinders seems remote. That left the valves to be inspected. All of the valve heads were marked “Made in Japan” which indicates that they are NOT OEM parts. The intake valves wouldn’t come out of the guides until I filed down the ridges that had formed around the valve keeper areas. The valves and seats were making good clean contact, so that left the exhaust valves to be the source of low compression. Sure enough, the exhaust valve seats in the head looked pretty good, but all of the exhaust valve faces were pitted all the way around and all were exactly the same. Honda wants $70+ each for an exhaust valve, so I tapped into my account at www.cmsnl.com and ordered up a whole set of valves (same brand), plus a set of reproduction intake manifolds and a few spare bolts/nuts for about $250 delivered from Holland.

I had to reorder a set of Heli-coil repair inserts to repair all of the damaged 6mm holes. An eBay seller has a nice kit for less than $20, with 25 inserts, plus all the installation tools. After de-greasing the head and installing all the inserts, it was ready for new valves and stem seals. I bought a different brand of gasket set (NE) from an eBay seller and instead of ill-fitting gaskets, I was dismayed to discover that the “complete kit” was missing all 6 of the rubber seals, the two small o-rings for the rocker stand feeds and the 8 sleeve rubbers that are used on the later engines. The kit was labeled 1969-76, but was sorely lacking in content. I complained to the seller and was refunded $11. It’s another case of wondering why some company would go to the trouble to make a gasket set and get it all wrong….???

After waiting for all the incoming parts orders, the reassembly commenced. The camchain was a little worn, so you can see it moving a bit on the sprocket. Camchains on 750s are the same length as those on the 250-305, 350 engines, but they are endless instead of having a master link. To replace the camchain requires a full tear-down of the engine so you can loop the camchain over the crankshaft sprocket.

Initially, I missed the cam timing by one tooth and a quick check of the compression readings came up in the 120 psi range! Shifting it back to where it was supposed to be yielded 170 psi readings across the board. Slowly, the rest of the top end was reassembled and buttoned up. The new intake manifolds were almost as hard as the old ones. It took about a half hour of shifting them around and pushing the carbs on with a pry bar to get them seated and clamped down correctly.

Wrestling the exhaust pipes back onto the chassis was another twenty-minute task, as all the clamp bolts were rusted and needed to be replaced and the pipes positioned correctly. The inboard pipes were reattached with hose clamps again as there were no other options. Working on these “custom” bikes has worn me out with endless tasks to make the parts go back onto the bike working with someone else’s ideas of what is “cool” and custom.

I fired the engine up on a remote bottle feed to see how it would run. The mufflers appear to have baffles on the end, but they are basically straight-through, so quite loud. I’m sure that my neighbors will be pleased not to hear that roar for too long.

The fuel lines cracked when I pulled on them to hook up the petcock fittings, so those needed to be replaced with 5.5mm Honda hoses. The rear brake pedal hits the muffler unless you position it just right and adjust the brake rod to find a happy place between being too high and hitting the muffler below.

Eventually, it all came together and it went out for a test run around town. The bike has a bit of a flat spot at 4k then comes alive after that. Idle is good but there is a little backfiring happening in that #3 cylinder that must be carb related, as the manifolds were all changed and the jetting is the same as all the other cylinders. 

With some miles run in with the new owner, we'll see if it heals itself or needs a round of carb work which is quite time-consuming with the later carb sets. I dropped it back off to the owner and he's happy to have both bikes back and running with a lot fewer problems and more understanding of what they need and perhaps a deeper knowledge of what to look out for in future purchases.

Bill Silver aka MrHonda

10-2022

What this bike would have looked like from the factory.

MrHonda takes in another stray…. Honda CB750 Cafe Racer #1 of 2 (part one)

What it looked like new from the factory.

Having just cleared the backlog of bike repairs, a customer referral caused me to bring home another bike for a “minor” electrical repair. The 2-3 cylinders were dropping out intermittently after the bike had been at a shop for 8 months for a top-end overhaul. Even with an upgrade to Dyna Ignition and Coils, the bike continued to quit on the middle two cylinders.

It seemed like a repair that I probably could have done in the man’s garage, but time was tight and it looked like there was more to do than to hunt down the electrical gremlins. So, it was loaded into the Tacoma for the 30-mile drive home and unloaded the next morning for a closer view. The bike had had a rattle can silver paint job on the frame, some Clubman handlebars, a very fat 120/90-18 rear tire, a drive chain way too tight, oil leaking from around the left side of the cylinder head, an oil pressure warning light that turned on when the high beam was selected and a few other issues. The carbs were fitted with little cheap cone-shaped pod filters and the rubber sealing boots for the top of the slides were all MIA.

Fortunately, the dead ignition symptom was still present, so after the fuel tank was removed, access to the ignition system connectors was easily performed and what seemed to be a decent installation turned out to have a pair of undersized bullet connectors that were floating around inside the insulated ends. I replaced the bullet ends with larger sized versions and tested the system again. After clearing out some leftover un-burned fuel in the dead cylinders, they came back to life and the engine started sounding a whole lot more like a normal CB750. Mission accomplished… or not!

The owner was impressed with my findings and wanted me to work on his OTHER CB750 after I was done with this orange one, however there was a pressing need to fix the oil leaks that appeared to be from the newly installed head gasket. The saving grace for this particular bike is that the frame sections above the head were cut out and then then special tubing splices were installed to reunite the frame section after the gasket repair. I probably wouldn’t have even taken on the job if I had to pull the heavy lump of a motor out of the frame. With the owner’s permission I ordered a set of top end gaskets from my friends at 4into1.com and tracked down a $90 set of the OEM carb boots for the carburetor tops.

There was a lot of fiddling to get the frame section loose. The fasteners were all SAE nuts and bolts and all the splices had to be slid aside to get the frame piece out of the chassis. Eventually, it all came apart and I was able to begin removing the top end components. The top cylinder head cover was all held down with stainless steel Allen bolts, some of which came out with some difficulty. This is usually an indication that the threaded holes are damaged, which isn’t unusual for a lot of the Honda Fours from the 1970s. There are a lot of little nuts, washers, bolts and screws that hold the top end together and they all have to be dismantled in a certain sequence. I removed the one accessible cam-sprocket bolt which holds the cam-sprocket to the camshaft, then kept working on the other parts. Suddenly, I noticed that the cam-sprocket had fallen off the camshaft when I had brushed up against it! There was NO second bolt and the one I removed was not very tight. Looking around the base of the valves, I spied the missing cam-sprocket bolt all mangled and unusable. There was a random piece of aluminum that had broken away from somewhere but I didn’t see the source right away.

Eventually, the cylinder head was ready for removal and cleaning. The tops of the pistons were barely coated with some burned oil carbon deposits, so it hadn’t been run for too many miles. In checking all of the 6mm bolt holes in the head, the first thing I did was to flush the holes out with brake cleaner and I was surprised to see a lot of old oil deposits that had been lurking deep down in the holes. The contamination inside the holes can cause the ends of the bolts to bottom out inside the hole and that in turn, damages the threads. Eventually, I wound up repairing seven 6mm holes with Heli-Coil thread repairs.

After prepping the head for re installation, I loosened up the drive chain so it had some slack to it and then spent another 20 minutes dismantling the back of the speedometer to access the various light bulb sockets and then determine which ones needed to go where. I checked the wiring inside the headlight shell and found a lot of either disconnected wires or ones that didn’t match up to normal OEM Honda wire colors. Eventually, I sorted them all out so that the neutral light came on in the green lens and the oil pressure light came on behind the red lens.

The gasket set which was ordered on a Friday morning, went out to the post office that day and landed in the mailbox on Saturday afternoon’s mail. I was all ready to open the kit and start installing new gaskets and o-rings from the D&K branded gasket kit. I had a bad experience with a CB650 gasket kit from these makers a few years back and hoped that those problems had been long-solved.

Sadly, the CB750 head gasket had numerous mismatches where the 6mm bolt holes were supposed to line up It was just off here and there, but clearly could not be used as-is. I used a rotary file tip to enlarge the holes in the correct direction so that bolts could be installed once the cylinder head was dropped down in place. This probably took an additional 30 minutes of test fitting and whittling on the gasket, which can be easily damaged, but finally I felt like it was going to work.

Bolt by bolt, I installed the four external 6mm bolts which are exposed to the outside, then the front and rear bolts in the center of the head, followed by the all the washers and nuts on the cylinder studs. The sealing plugs were a nice tight fit, but I glued them in place with Gasgacinch gasket adhesive. The old sealing plugs were about 1mm thinner and not a tight fit around the edges of the holes where they are fitted. It appeared that the last mechanic had chosen to reuse most of the rubber parts and all of them had flattened out over the years.

With the head bolted down and the cam towers installed, the rest of the process is getting the cam timing correct and getting the rocker arms and rocker arm shafts all installed, followed by a valve adjustment. I had to order new cam-sprocket bolts, which are 7mm, from an eBay seller. Those came down in 3 days from the Delaware, and were installed with a dab of blue Lock-tite to ensure that they don’t try to back out once more.

It takes a bit of time to dismantle the carb tops and linkage bits to refit the rubber sealing boots. They are not really critical for operation, but keeps corrosion and dirt from damaging the adjustment nuts. They are fussy to install into the grooves of the adjuster sleeves and you have to pull them away when you are syncing the carburetors.

Slowly, all the hardware and brackets were all reinstalled and the bike readied for a start-up with a newly sealed cylinder head. There was a little hiccup with installing the new camshaft sprocket bolts. Despite looking at the threads of the side where the bolt flew out, the new bolt didn’t want to start into the hole. This was one of the last steps before putting the top cover on, so everything was in place like rocker arms, etc. Fortunately, my aged MAC tap and die set had a 7x1.0mm tap in the set, which was never used in the last 30 years. I rolled the engine over so that there was a little bit of room to insert the tap and I turned it over with a 7mm open-end wrench and was able to clean the threads up with everything still in place.

I had set the top cylinder head cover aside when it was dismantled, and when I grabbed it for some cleaning up, I finally noticed a quarter-sized hole that was punched through when the bolt made its exit. Fortunately, a friend in Lakeside (16 miles away) was the CB750 whisperer for many years and had a spare solid cover available. It was a bit scruffy, but I saw that he had a bead blaster in his shop. He offered to let me use it and that made the price more palatable. Once home, a good wash job and some aluminum paint made it look pretty good.

The reassembly process continued with the carburetors and wrestling the 4:1 exhaust system back onto the spigots and all clamped down. The fuel tank was installed and fuel lines were attached (OEM 5.5mm hose not that plastic crap). The bike fired up almost immediately and sounded like it was hitting on all four cylinders evenly. After a warm-up, I took it out for my local test run for about 5 minutes, then returned to check for leaks and problems. Some of the leftover oil and grease had heated up and was drooling down the engine surfaces, but there were no active leaks on the top end now. Of course, after the second good run on the freeway, oil leaks surfaced from elsewhere.

The engine dyno and shifter cover gaskets appeared to be weeping and then there was some oil that was coming from around the countershaft sprocket. With the cover removed, there was a shiny oily patch in front of the countershaft sprocket/chain. Closer inspection revealed some chain marks on the engine case, where the drive chain had been run too loose and it was lifting off of the sprocket and rubbing against the engine case, leaving a couple of wear mark tracks. That required removal of the chain and countershaft sprocket, cleaning the engine case carefully, and then mixing up a batch of JB-Weld to patch up the damaged area.

The gasket and seal kits came on the same day, so with some careful cleaning and a little bit of Gasgacinch sealer to help the sealing process, the gaskets were installed successfully. I had to add some extra sealer around the rubber grommet for the stator winding wires, but it looks like it is ready to return back to the owner.

I suppose the next task will be to look over his other CB750…..

Bill Silver

aka MrHonda

10-2022