Thursday, September 22, 2016

Vintage Honda Tips, Tricks, Tests and Troubleshooting

An wide array of vintage Hondas have shuttled through Casa De Honda since the first of the year and with each one there is the gift of knowledge to be learned and shared. Understanding the basics of how mechanics and electrics all work on the older machines is a sound foundation to work from, especially when the bikes try to pull a fast one on you. Despite owning hundreds of bikes over 50 plus years, there is always something unexpected to discover.

I started out the year with a couple of CB92s, one a race-kitted 1960 model and a 1961 bike with an odd provenance. The 1960 bike had a California assigned engine sticker on the motor apparently due to an engine change, perhaps because of a blow-up moment. While the top case was correct for the year, the bottom case was actually from a CA95, which has different muffler mounts cast into the part. These were not compatible with stock CB92 exhaust parts, so they were removed to aid in proper fitment of the megaphones. The 1961 bike was a mystery, in that the original MSO paperwork from American Honda showed the bike as a CB92R model, which comes with all the factory race-kit parts. The MSO was sent to a dealer in MA, where the dealer sold the machine to a young woman. The bill of sale showed a note about the inclusion of stock mufflers in the deal, but the only problem seemed to be that the bike was NOT a CB92R. Apart from an add-on tachometer mount, the bike was a stone-stock CB92 street bike. There was no sign of CB92R parts anywhere else on the bike. I sold it to a local friend and wound up rebuilding the engine and it was all stock inside.

On the other hand, the 1960 bike did have YB92 pistons, which require side-cap spark plugs because the domes are so tall that they interfere with the normal spark plug gaps. The valve spring retainers were alloy parts on both engines, which apparently were stock for the first 2 years. The intake port had been hogged out to fit a larger carburetor, but the original 18mm mixer was replaced with just a 20mm CA95 unit which was later done at the factory as an upgraded part. I expected to see a YB mark on the camshaft, but it appeared to be a stock unit. The engine was freshened up and sounded like a mighty mite when it was fired off in the back yard. That bike was sold to a man in Indonesia.

Simultaneously, a nice, original-looking CL72 popped up on eBay, portrayed as a 1964 model; however the tail light was a “short-lens” unit which made me think that it might be an earlier model. It turned out to be a late 1962 machine, confirmed once I was given the serial numbers. Few people were aware of the fact and I was able to buy it at a reasonable price. Both the CL72 and CB92 purchases were done in the dead of winter and the bikes were located in New Hampshire and Montana, so cross-country transport seemed out of the question for immediate delivery. However, the U-ship system managed to contact some willing drivers and the bikes did arrive within a few weeks of purchase.

The Scrambler had most all of the original patina and only needed few air filters/tubes and a general going-over to be serviceable. On a couple of occasions, the bike was hard to start and the float bowl on the right side was found to be empty despite a freshened up petcock and a half tank of fuel. Flipping the float up and down didn’t seem to affect it, so I blew into the fuel tank opening and suddenly the fuel was flowing once again. The bike ran well after that, so whatever little blockage seemed to have worked itself out.

I had a “time-out” in February for knee surgery, but was back in the saddle within a month. But in the interim before surgery a derelict CB77 popped up on the local Craigslist. The bike had low miles, but had been poorly stored for years and at some time, the bike had been lying on its side causing the protective oil film to be lost from much of the engine’s internals. The engine was “stuck” and the transmission wouldn’t shift. After a laborious disassembly most of the engine’s internals were severely rusted with the shift forks and drum basically a single, unmoving unit. Lots of time was used to clean parts and replace the whole transmission and shift drum with good used parts. I had a spare top end that was mostly NOS parts and had been used sparingly on another engine build, so the whole original top end was replaced with later model parts.

I don’t advertise doing repair work, but people do find me by word of mouth, so various interesting repair jobs show up. One was a hard-starting XL350, which indicated that power was coming out of the stator coils, but there was no spark at the plug, even after replacing the coil and condenser. In the end the stator’s primary coil was somehow defective and a good used one fixed the no-spark problem.

Something that shows up often is hard-starting, poor performance due to poor battery function or maintenance. At least four bikes have arrived lately with lead-acid batteries that were basically dry inside. Recently a cherry, Scarlet Red Honda S90 with 2500 original miles was brought by for poor throttle response, but would start and idle okay. Everything was set to specs by another shop, but apparently they failed to check the battery voltage and condition. Even with about a cup of distilled water added to the battery, it only showed 4 volts available. After a few hours on the charger it came up to 6 volts and once installed the bike ran great. There was a little hesitation off-idle that was eventually traced to a low float level coupled with the needle clip being in the wrong slot. With a fresh battery and carb adjustments it ran perfectly, but ALL of the light bulbs were blown out due to the engine running with a dead battery.

A nice-looking Z50A came to the shop due to “not shifting” problems. The bike had been stored for years out along the coast, but inland a mile or so. The whole history was unknown, but the owner got it up and running, then discovered that it wouldn’t shift gears no matter what he did.  He did mention draining the oil which looked terrible and then refilled with ATF to try to flush out the old gunk inside. He drained it again and brought it over for a diagnosis. Once the clutch cover was removed, the “high water” line where the old oil had been sitting spelled more than just another oil change in order.

Most of the internals were pitted with rust and there was an eighth-inch of sludge at the bottom of the cases.  Rust attacked the shift drum, just like the CB77 had suffered, locking the shift forks in place. The camshaft lobes were pitted, as were several of the transmission gears. Even the crankshaft bearings had rust on the ball bearing races and retainers. The cheapest way to address most of these problems was to just replace whole units. There were killer deals on eBay for a complete cylinder head ($61 delivered), a four-speed transmission for $155 and a couple of crankshaft main bearings for $22. The camchain was replaced, as well. The cylinder was still on STD bore size and the piston was still a good fit. Fresh rings brought the end gap down to specs, so a good honing was all the machine work needed. A local shop just opened offering wet blasting of the engine cases, so the main motor castings went to them for a good scrubbing and the whole motor looked pretty nice at the end. In the end, this engine repair ran to over $900. 

When replacing clutch covers on any of these engines using gaskets, it is always wise to start all of the screws in a few turns before cranking them down tightly. Gaskets have a tendency to shift around enough to where one or two of the screw holes are not matched up with the cases without a little massaging of the gasket while the outer case is still loose. Get all the screws started, then you can go to town in tightening up the fasteners.

With today’s alcohol injected fuels, it is imperative to run the bike regularly or drain the fuel out of the float bowls, using stabilizer to maintain what is still in the tank. The alcohol attacks the old-school rubber carburetor parts and the float bowl gaskets invariably will swell up once they are removed from the carburetor body. Best defense against this problem is to have duplicate float bowl gaskets on hand. Once the alcohol dries out of the old gaskets, they will shrink back to normal size after a day or two and you can reuse them once again.

For owners of the 250-305 engine series machines, there are a lot of tips to help keep the bikes running well. Fresh fuel is a must, and be sure that the gas cap vent holes are open and clear. Check your ignition timing with the engine running, using a dynamic automotive timing light. Static timing doesn’t take into account for weak advancer weight return springs and side-play in the point cam, where it turns inside the right side camshaft. Over-advancing the spark timing will cause engine seizures under full-throttle. The CB77s seem to like running #140 main jets on the new fuels vs. #135 stock sizes. Check your wet-cell battery levels at least once a month.

If you are assembling the top end of the 250-305s, a trick to help keep the cams steady is to loosen up the valve adjuster screws while the camshaft is being timed to the crankshaft. The right side cam lobes are on OVERLAP (not compression stroke) when the camshaft is being timed, so both valves are slightly open on that stroke. Turning the adjuster screws in slowly by hand will allow the spring-loaded rocker arms to put pressure on the lobes. The adjuster screws can be manipulated back and forth on the intake/exhaust sides to a point where the camshaft sprocket is dead level with the cylinder head. The left cylinder exhaust rocker arm must be backed off as well, but the intake valve on that side is closed so no affect on the cam timing. You will find that the camshaft is rock steady once you have it centered with the RH side adjuster screws. Attach the camchain when the right side piston is at TDC (T mark on the rotor) and your engine cam timing is perfectly correct.

Honda’s rectifiers can be replaced with little Radio Shack bridge rectifiers or some companies now offer solid state regulator/rectifiers which allow the charging system to run full blast all the time, but still regulated to keep from overcharging the battery. Honda’s system only adds in the last leg of the stator when the lights are turned ON. The new solid-state units rout all three AC legs into the control unit all the time and then it decides how much voltage to allow into the system to keep the battery fully-charged.

Remember to use Honda GN4 engine oil which is specially formulated for motorcycle engines. We “old-timers” always remember using Castrol GTX car oil in our bikes back in the 1970s-80s, but it really doesn’t do the job for long, due to the shearing forces in the transmission gears. Use a motorcycle-rated oil in your motorcycle for long life and best results! Always take a moment to check the oil level, tire pressures, chain adjustment and fuel levels before you ride out on your vintage machine.

Bill “MrHonda” Silver


  1. On the Honda GN4 motor oil which weight do you recommenced?

  2. I use 10-30 for the small engines (50-200cc) and 10-40 for the 250cc-larger models.