NOTE: I wrote this story after doing all the repair work in 2021 but never published it! Now in 2024, I purchased the bike back from the owner, as he can no longer ride the bike. The title showed that he bought the bike in Aug, 1968! So, enjoy the re-cap of the extensive repairs done 3 years ago...
The CA license plate tag shows 1980 as the last time the bike was registered for the street. The bike has been hibernating in the mountains to the east of Los Angeles for 4 decades after being built for competition or as a sleeper for the street.
When it came off the truck, the first challenge was to prop it up as there was no centerstand or sidestand equipped on the bike. The owner was in his 20s then and just leaned it up against whatever was available between rides. The resident stand was an automotive jack stand which nestled up against the solid rear-set footpeg on the left.
The dramatic paint scheme was an eye-catcher and was done to a high standard. Underneath the tank, there were dual push-pull petcocks, with fittings brazed into the bottom of the tank. The reason for the dual petcocks was that they were each feeding Mikuni carburetors which were attached to the back of the cylinder head via machined 2 piece adapters. A lot of specialized handwork was done on the bike including welding the adapter plates to the intake ports and the ports were blended into the cylinder head which was sporting 34mm intakes and 28mm exhaust valves as it turned out. The big reveal only happened after an initial compression check which showed 175 psi on the right side and 90 psi on the left. Most likely, the left side valves were left open for a long time and corrosion built up on the valve seats and valve heads. When I removed the tappet covers I was greeted with shiny alloy valve spring retainers which indicated the installation of high-performance racing valve springs.
After more than 40 years, the owner really couldn’t remember all of the bike’s engine build history. He had discussed big bore pistons and a camshaft with his riding buddy who apparently did most of the engine mods, but there were no clear memories of just what went into the engine build so many years ago. Obviously, if it was going to be a runner, the engine would have to come out for at least lapping the valves to increase the compression readings. So, we pushed it up onto my HF bike lift and strapped it down to hold it steady during the motor drop process. The owner headed back on his 150 mile return trip to await the results of the investigation and I went to work.
The first shock was that when the right hand muffler was removed and stood on end, whole corn kernels rained down onto the ground! Turning it 180 degrees yielded more corn kernels and then it ran dry of the remaining vegetables. The bike had been built with lots of self-locking nuts just about everywhere I looked. This slows the engine removal process, as each nut needs to be backed off with a ratchet to the very end. It took about a good ½ hour to remove all the engine bolts and hardware in order to allow the engine to come down out of the frame. The frame had been gusseted in the back section behind the battery to help chassis stiffness apparently.
Carefully, the engine was lowered down on a floor jack and wrestled out of the frame and off the bike stand to the ground. I put the floor jack back underneath it near the workbench and raised it as high as I could towards the edge of the bench. These engines weigh about 100lbs and are somewhat unwieldy to manhandle on and off the work surface. Once it was propped up securely the top end teardown commenced and new discoveries revealed themselves with every disassembly step.
Once the top cylinder head cover was removed, it was apparent that the baffle plate was installed backwards with the drain holes at the top of the engine instead of the bottom where the oil can return back to the crankcase. Ordinarily, this creates a LOT of oil being pumped out the breather tube fitting and out the breather hose. In this case, there was no hose attached to the fitting and no signs of oil spraying over the top of the engine. The lack of oil spray at that point was due to a crankcase fitting that was installed into the top engine case. The transmission cover had been modified to allow the fitting to poke through the sheetmetal and a breather hose was attached at that point. This crankcase relief point apparently prevented the usual oiling problem from the cylinder head cover when the breather plate is installed backwards.
With the top cover removed it was evident that there were big lumpy reground camshafts that were opening the valves against the S&W racing springs. Everything inside the engine still looked new and shiny. Once the camchain was disconnected and the two small nuts removed from beneath the spark plugs, the cylinder head came up with just a bit of prying. Ah! More surprises! I recognized the piston crowns as belonging to the Honda CB350 twins, which have a 63mm bore. Flipping the cylinder head over revealed LARGE valves installed and a big bevel cut into the combustion chambers to clear the crowns of the big bore pistons. Someone had done some very serious work on this engine so many years ago.
The piston crowns and combustion chambers had a lot of burned oil deposits on the surfaces, probably due to a couple of factors. First of all, these engines do not have any valve stem seals, depending upon the little air passages that lead to cross-drilled valve guides which help break up the oil film on the intake valve stems so that less is drawn into the engine on the intake stroke. This bike was a 1967 vintage and Honda eliminated the air passages in the cylinder head for reasons unknown. The other probable cause for the oil consumption/burning is the fact that the cylinder hone cross-hatch was quite coarse instead of a lot of smooth swirls. It may be that the rings have never seated or were delayed seating due to the rough surface of the cylinder walls. The piston rings had the old-style one-piece oil rings, which are not as efficient as the modern three-piece oil control rings
The whole rest of the cylinder head was a jumble of non-stock pieces. The reground cam lobes have big lift/duration but are not marked by the grinder. The cam bearings all were notched at one point where little set screws lock them into the cylinder head. I have never seen this type of modification before. Once the cams and rockers were removed, the camshafts came out fairly easily, but the long through bolt for the tach-drive end was apparently short and the end nut which draws the shaft into the end of the camshaft to lock it into the camsprocket was staked around the end to keep it in place. There was no lock washer underneath it and I am not sure if the shaft was shortened up or there is some other unexplained reason for this setup. I had to use an air gun to spin the nut loose on the end of the shaft enough to allow it to relax it’s hold inside the left camshaft. Even removing the valves took extra time and care as the high lift camshaft caused the valve stem ends to be mushroomed a little bit, so the edges needed to be smoothed over so the guides didn’t get broken as the stems worked their way down the guide surfaces.
I removed the rings and wire-brushed the pistons to clear off the carbon deposits. Checking the end gap of the rings will be in order along with piston clearance checks, but there are no unusual wear marks on the piston skirts or the cylinder walls, so the fit must be pretty good.
During engine removal, somehow the wire that comes from the point plate to the coils was trapped behind the right side carburetor adapter. I removed the 4 set screws that hold the outer plate to the inner one, but that didn’t release the wire harness. I couldn’t really see how else the adapter was attached to the cylinder head, so tapped around the edges with a small hammer and suddenly it broke free and fell to the floor. Looking at the way it was attached to the head revealed that the adapter had been welded into the intake port and then smoothed up for good air flow. The adapter had two countersunk holes drilled which actually lined up with the old cylinder head stud holes, but the adapter installation wasn’t using them. With the piece broken off of the head, I smoothed out the mating surfaces and lined up the holes, installing the adapter with two countersunk screws instead of having them re-welded. When the final fit for the adapter is ready, I will use JB-Weld to help seal the two mating surfaces and to fill in the gap left behind when the adapter was rotated for the holes to align with the cylinder head stud holes. I am leaving the left side carb adapter as it is, as the mounting seems to be sturdy for the moment.
The biggest challenge to overcome will be replacing the special head gasket. The main supplier for these kinds of gaskets is Cometic Gaskets, but apparently they are back-logged for 6-7 weeks in order to make a composite gasket with a 64.5mm fire ring. The backup plan is to use a solid copper gasket, which can be had much more readily, but my experience with copper gaskets is somewhat spotty as they have a tendency to leak oil when hot.
There is a lot of work to do anyway. I happened to have a used centerstand available to install on the bike. It needs cleaning/painting or a powdercoat treatment, plus I need an extra centerstand pivot bolt, which are apparently only available as aftermarket parts from Thailand. A quick check of eBay turned up a used centerstand, with one correct bolt and a spring for $31 including free shipping! I hit the BIN button and waited for delivery.
I fished out a used OEM dimmer switch that needs a new sheath to clean up the appearance, which will help solve some of the lighting issues with the bike. Unless the owner specifies that an electric starter upgrade be performed, the lack of starter button on the right side is moot for now.
The monster Bridgestone 3.50x18 KR96 racing rubber on the rear, made in the last century, will need replacing, of course. The front tire is a Yokohama Racing tire in 3.00x18” size, both on stock steel DID rims.
I can see a couple of different paths for this bike, one being taking it back to near-stock specs with a stock head and cylinder that I have in stock; just rebuilding what is there now and hope that it is streetable for future riders or de-tune the racing engine with stock cams and valve springs to lessen the hit when it hits the power curve. The choices await a conversation with the owner.
Decisions…
I started the day by hauling the cylinder block and valves to my friendly machinist who took a few minutes to hone the cylinders to a fine finish and reface the valve stem tips. Back at the shop, I spent a couple of hours cleaning off RTV and old gasket material, degreasing the parts and carefully assembling the valve train. I noticed that when the camshafts were installed and the set screws were tightened the camshaft began to turn roughly as if there was a damaged bearing. Backing all the set screws out again freed up the camshaft to turn freely on the bearings. Once the cams were set properly, I removed all the set screws completely as I really can’t see a purpose for them unless there was a concern that the bearings were turning in the head.
The cylinders needed cleaning and Heli-coil inserts into three of the holes that mounts the camchain tensioner to the block. The rollers all looked very serviceable and it seems like the engine wasn’t really run all that much since it was built. I checked the piston ring end gaps and they were mostly out around .020” which is the far end of the “new” specifications. Going online, I located a set of new steel rings that have a 3-piece oil ring that will probably be more efficient at controlling the oil on the cylinder walls, especially after being honed smoothly now.
I had the good fortune of an online forum member in Arkansas offer up a “big bore” cylinder head gasket for the cost of shipping. My private source of the little clutch retainer wires has offered to send another batch of used ones that he removes from his engine builds. These wires are NLA on the open market, but as a backup I discovered that there is some .020” stainless safety wire that can substitute for the real thing. All you have to do is to drill one extra small hole next to the original ones and then twist the ends together inside the clutch hub. That will help with clutch disengagement issues while running. I have some lighter clutch springs that will lessen the clutch pull effort. Also the clutch adjuster threads are wobbly for the clutch lifter arm, so that will be replaced as well.
A quick peek at the carb float bowls reveals little in the way of old gas deposits leftover from the last 40 years, so they must have been drained not long after the bike was put to rest.
Reassembly...
I forgot to consider that the 350 pistons leave NO taper in the bottom for rings to slide into, so I had to come up with a different assembly method. I put the pistons/rings into the bottoms of the cylinders with the pins sticking out, then turned it all upside down and fished for the rod ends to get pins into without dislodging the piston rings in the process. Eventually, it all worked, but it wasn't the easiest assembly I have done with one of these engines.
I was starting to gather up parts to install the electric starter but hit another roadblock...
Apparently, the crankshaft in the engine is from a CL77, as there is no oiling hole for the starter clutch, as there is for the CB/CA crankshafts. I have to consult with a friend in the UK who has been rebuilding crankshafts lately to see if he knows if the end of the crank can be drilled to tap into the oiling source or if the whole end of the crankshaft needs to be replaced. I can't put a starter clutch hub on the end of the crankshaft without an oiling system, otherwise it will just seize up in a few minutes.
A message from my machinist friend in the UK states that the crankshaft snout is hardened steel and won’t drill easily, so the whole electric starter setup idea is scrapped now.
Changing the rear tire revealed that the brake drum lining was rusted over, not surprisingly. The brake cams and pivots were all sticky with 40-year old grease that had congealed and hardened, so the whole brake plate required disassembly for cleaning and lubrication.
After rounding up a serviceable centerstand and having it powdercoated, I discovered that the hook that is welded to the frame for the centerstand spring had been sawed off! I created a workaround by going to the local Home Depot to find an eye-bolt that could be installed once the frame backbone tube was drilled through for the installation. Finally, it does have a centerstand, but the next challenge will be how to make a stopper for it, as the original design used the bottom of the left muffler for a centerstand stop.
As the bike was reassembled it seemed like the centerstand stops up against the inside of the left muffler ever so gently, but good enough to keep the stand from coming up too far and getting into the drive chain with the right side stand foot.
With the ignition timing rechecked and the carbs installed on the engine, it was about time to fire it up and see what happens. With a good battery, I kicked it over a couple of times without any response. I had put the chokes ON at first, but then turned them to the OFF position on the enrichment levers. A couple more kicks and it roared to life, sounding deep and throaty, but definitely running crisply on both cylinders.
I had to borrow the seat from my 1963 CB77 just so I could take it out for a test run. The bike seemed to settle in quickly with a little bit of smoke out the back which was probably a combination of old oil burning off inside the mufflers and new rings which had to bed into the cylinder walls once again. At any rate, the engine sounded good and I rolled it down the street being mindful of the upside down shift pattern due to the rear facing CL72 shift pedal.
It purred along for about 15 minutes as I ran the bike up to the gas station for a drink of premium fuel and then down my ‘test track’ roads to let the bike speak for itself. Despite the over-sized tires and a few other mods, the bike handled very well and pulled through the gears with some real muscle. The 32t rear sprocket gives it some extra punch, so running it up through the gears brought a smile to my face. After probably over 30 hours in the teardown, rebuild and install/tune process. It was rewarding to hear it run and feel good in local trips around town. Being that the registration tags are 41 years old and expired, I thought I would keep the test rides to a minimum.
The bike was picked back up on 5/29/21 and the owner had his first ride on it after 40 years. Initially, it didn’t want to start, no matter what we tried with choke/no-choke. Pulling the plugs revealed spark plug side electrodes bent closed due to some interference with the over-sized valves. I remembered that the old spark plugs had little spacers installed so I recovered them and installed the spacers after re-gapping the plugs. It still didn’t want to start, so I pulled the points cover to check for power to the points and plugs. With the point cover off the bike fired up suddenly and a closer look revealed that one of the points wires was getting pinched between the cover and the cylinder head cover. With the point cover reinstalled, it was time for a long-awaited test run by the owner.
I could hear it echoing around the neighborhood as the revs rose and fell between gear changes. A smiling owner returned to the driveway with a pleased grin. You could imagine that many memories were being triggered by reconnecting with the long-stored bike once again.
It was both an educational adventure and a challenge to bring the bike back to life and to make it more functional for future use. I’m happy that it had a successful conclusion.
Bill “MrHonda” Silver
5/29/2021/ updated 0707/2024
www.vintagehonda.com
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