Tuning/repairing old vintage Hondas (and other makes/models) often requires going beyond whatever it says in the factory repair manuals. When bikes are brought back to life after many years, sometimes the process is straightforward and direct; clean the fuel system, replace the battery, adjust the valves, set the ignition timing and off you go! Other times, they defy all the standard settings and you are on your own to discover the blocks to a good running machine.
My recent experience with swapping a rebuild S90 engine into a CL90 chassis lead to many problems, just from a design and structural viewpoint. The “early” S90 has a different set of engine cases, intake manifold system, exhaust port angles differ, different clutch cover, dyno cover and other smaller features which created parts problems and finally required re-rebuilding the engine using the correct matching CL90 engine cases. Even though these 90cc OHC engines were part of a long-running design, substantial changes occurred in the first year or two and that created more problems to be considered.
Once the engine components were swapped back into the stock cases, the carburetor settings were different from one series to the other, calling for careful tuning steps to be followed in order to get the bike running properly again. At first, the engine was running rich (starts with no choke) and fouling the plug, while wafting a bit of oil smoke out the exhaust pipe. The engine wouldn’t take even half-throttle with no load on it. Backtracking on my adjustments, it was discovered that the point gap was out around .018” instead of the normal .012-.016” settings. This can cause a loss of coil saturation and subsequent misfiring that can mimic carburetor problems, if you aren’t careful and observant. In trouble-shooting and/fine tuning efforts, you have to make ONE change at a time, re-evaluate and then try something else if necessary. In this case, the point gap wasn’t a significant issue, so I moved onto the carburetor settings.
Current “wisdom” is that with today’s fuels, you need to richen up the fuel curve about 10% in order to compensate for diluted gasoline energy potentials. In several cases, I have found that the carburetors needed to be richened up about one main jet step to help smooth out the fuel delivery to the engine. I had raised the needle up one notch and was using the stock S90 main jet which was #90. With continued richness on the plug and poor performance, I lowered the needle and re-tried the engine performance once again. There was little or no change, so the main jet was reduced to the stock CL90 size, which is #88. Each time there was just a little cleaner running, but not anything that was really acceptable. Dropping the main jet to #85 seemed to be the magic setting, as the engine started to run cleaner and without the mid-range hesitation.
This may or may not be the final answer in that the air filter is a simple conical unit, standing in for the stock air cleaner/filter setup which was not available with the bike. Once the original S90 engine configuration was altered, the air filter for the S90 was no longer compatible with that of the CL90. More run-time is needed to confirm that the engine is set properly and won’t seize due to an incorrect main jet choice. Sometimes, the changes to the carburetion are masking an ignition system problem. In several cases, I have discovered that a bike will start and run at idle with a failing ignition coil, but when the throttle is cracked open a big misfire occurs and the bike fails to move forward with authority.
One of the other steps was to check the spark plug cap for high-resistance. This specific engine still carried its “Hm” logo’d spark plug cap, which are generally viewed as “non-resistor” types. When this one was checked, the resistance value measured was 33k ohms; whereas the “resistor” style plug caps have only 5k ohm ratings. Swapping out to a new resistor cap helped the performance to some degree, as one of the trouble-shooting steps. It is beginning to appear that many of these spark plug caps might well be faulty as they age and go through many heat cycles.
The ignition coils on the OHC 90 engines are mounted directly to the top of the engine cases, along with the condenser. You can check the primary winding ohms values by disconnecting the engine wiring connector plug and probing into the black wire terminal. Disconnecting the points wire connector allows for access to the other end of the primary windings set. So a probe into the black lead and one in the disconnected points wire connection will give you a reading of the primary wiring ohms resistance. If you need to service the condenser or replace the coil, the motor will have to come out again; or at least be tipped down off of the lower rear mounting bolt.
For the moment, the bike is running smoothly and may have a new owner waiting in the wings who can help take it to the next level. Given the amount of money spent on new parts to make the bike fully functional and the time invested in multiple engine work efforts, it will definitely not go down as a money-maker for MrHonda. It did provide a good platform to continue to hone my tuning skills, though, and perhaps some of these tips will resonate with my readers out there who are wrestling with similar issues. If they won’t run on the OEM spec settings… give them what they want!