Wednesday, November 16, 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 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 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 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. 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.


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

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