Friday, May 5, 2023

Mini-Trail with maxi-problems…

A local referral brought a nicely restored CT70H (4-speed with clutch) bike to me to “make it run.” The owner couldn’t get it started after installing an aftermarket CDI ignition system. I don’t work on these little bikes very often, but how hard could it be? Right?

Well, initially, having pulled the spark plug out and grounded it against the head, I could see what is typically the little wispy CDI spark jumping across the plug gap. CDI ignitions have a fast rise time and the sparks are not the big fat arc that you see with a magneto or battery ignition system. So, it is easy to miss the spark arc, especially on a little 10mm spark plug.

Knowing that I had a spark, but not knowing that it was timed right because the CDI module controls the spark timing, I went to the next step.. fuel. The bike had older gas that was infused with “stablizer” to keep it from going off. Age of the gas was not revealed, but it seemed like it was probably good enough to fire off a 70cc single. So, off comes the carburetor to check the jets.

As expected, the pilot/idle jet was blocked off with fuel solids. I have some tiny tapered jet reamers that will poke through almost anything in the #35 jet, so once the orifice was clear, the jet was reinstalled. The carburetor was a Chinese copy of the OEM CT70 carburetor with #35/60 jetting which was stock for a stock CT70. The float level looked off, so I reset it using my Honda float level tool, but it wasn’t clear if the setting was from the gasket surface, the base below the gasket or the raised flange around the outside of the bowl. I opted for the gasket surface but found out later that it is measured from the carb body itself.

Putting the carb back on, I noticed that the stack of the gasket and insulator were reversed when removed. Eventually, I had to remove the manifold and put it on a sander to flatten out the gasket surfaces. The mounting bolt holes in the manifold were “enlarged” apparently to compensate for the replacement cylinder head mounting bolt holes being a little bit “off” from what the manifold was made for.

Then, petcock started drooling at the lever so that needed attention. The dual fuel hoses were pinched off and the lever was removed. The back side of the lever was uneven, so that was smoothed out and the parts were reassembled with no further leaks. The float bowl gasket was typically swelling up so it was washed in soapy water and left in the sun for a while.

Once everything was reinstalled, the bike fired up on the second kick with a full choke. So far, so good… or not. The bike ran okay at idle, but the first test ride yielded some part throttle misfiring through the mid-range and towards wide-open throttle. Using the choke, it picked up power with about half the choke closed, indicating a lean condition. I tried a number of needle clip positions, but it seemed to like the richest position for mid-range and top end, but you could hear it “8-stroking” just off idle and the plug was sooting up. After quite a while of messing with settings and texting to the owner, he reminded me that the fuel cap had a variable vent feature which was in the OFF position. The cap seemed like it might have been an aftermarket copy as the vent passages were somewhat restricted, so I spent another 10 minutes massaging it, so that the vent function was fully operational.

As the text conversations continued it was revealed that the engine was not 70cc but 88cc and more variables came into my head as to what was happening with the carburetor jetting. The 88cc kits on eBay included a performance cam, which will upset stock carb calibrations to a certain degree. All signs were that it was running lean, so I reamed out the main jet to about #70 size and the bike began to pull strong through the mid-range and WOT but was still too rich off-idle. Moving the clip up to lower the needle made the bottom-end leaner but then the mid-range went lean. Looking carefully at the needle jet, it appeared that the top edge was not beveled like most of the ones I have seen. I have run across this problem with Chinese-made carb kits and it appears to have a big effect on how the fuel is distributed coming out of the jet edges.

As an experiment, I had some aftermarket needles for CB400F carbs, which are a little longer but thinner. I clipped it as lean as possible and tried it out. The power improved in the mid-range and WOT but it was still too rich off-idle. I contacted the owner who told me that he had three more carbs, including an OEM that was going to be rebuilt, so I asked him to bring it all to me for evaluation.

In the midst of all this testing, an oil leak developed at what seemed to be the shift shaft, but with the left side covers removed, it was coming from the big o-ring that surrounds the ignition/charging system mount plate. I had to pop off the rotor, remove the two mount screws, and borrowed the o-ring from the OEM plate to seal it up. While apart, I noticed that one of the CDI coils appeared to be rubbing up against the inside of the rotor, as there were witness marks and a bit of metal debris floating around inside. I tried to reposition the coil slightly and replaced a failed flat washer that held it in place. Like the movie, these bike projects can become “ The never-ending story” when the combination of OEM and aftermarket parts collide in a vintage Honda.

Certainly, when you install a big bore piston kit on a stock cylinder head and manifold/carb setup, the airflow needs are going to be increased quite a bit. The compression readings were about 160 psi, not unusually high considering the engine setup. The intake manifold mounting bolt holes were enlarged around the bottom portion as if there is a mismatch between the head and manifold dimensions for some reason. A lot of this information would have been helpful when the bike was received, rather than learning mid-repair cycle after hours of fiddling with what should have been an easy repair job. Oh, in the last visit, the owner felt that the gasoline might be a couple of years old, but had some stabilizer added when parked!

As a final attempt, I put a #65 main jet in one of the new replica carburetors, reset the float from 22 to 20mm, put the stock needle in the middle notch, and bolted it all back up again. I did, as an experiment, use a large drill bit to flare out the edges of the needle jet in hopes of improving the fuel delivery flow.

The bike started quickly (with a new spark plug installed) and pulled well at medium driving speeds, then with WOT it seemed to pull cleanly to whatever redline was at a thrilling 30 mph. Hopefully, the owner will approve of the current setup and it can go back home. With some fresh gas, it should be good to go again.

Bill Silver

aka MrHonda



  1. Mr. Honda, you have an amazing amount of patience. I worked on a CL77 years ago. It kept fowling plugs. The culprit was that the owner installed an after market carb kit. The kit had the wrong size main jets and the owner didn't notice. It was one of those kit brands that you have warned us readers about.

  2. More amazing detective work. I love these reads. Thanks again, Thailand Paul