Long time lurker/first time poster.
Working on a CA160 (single carb) for a customer. Bike came in with many engine issues due to PO trying to rebuild the top end. Worked through all of that and now have an engine with strong compression, cam correctly timed, valves set to 0.002" intake and 0.003" exhaust. Points gapped at 0.014" and timing set both static and dynamic (timing light) dead on the F mark. Advancer free and working fine. New plugs, condenser and points. Strong spark (for a 160, anyway!).
Carb has been ultrasonically cleaned (three 30 minute cycles) and all jets and passageways checked, double checked and triple checked with compressed air and spray cleaner. Floats set at 20.5mm (spec 19.5 + 1.0mm for ethanol fuel). Floats (brass) are symmetrical and free of holes. Air screw initially set at 1.5 turns out. Reused stock brass (scrupulously clean and in good shape), even though my rebuild kit was Keyster and very high quality. Jet needle is in middle position. Main jet is #95 and pilot jet is #40 (also tried a #38). Carb vent is clear. Carb is correctly assembled with no missing parts per the fiche. O-ring on air screw is new and sealing well. Confirmed no vacuum leaks by spraying brake cleaner on all intake joints while engine was running.
Symptoms are like this: Engine starts immediately. Once warmed up, it revs just fine, but the idle is uneven and misses. Exhaust smells rich to me. Adding choke only makes the problem worse, indicating that mixture is too rich. Tried backing out the air screw to lean out the mixture, but no effect, even if air screw is completely removed. I have to keep blipping (or slightly lifting off idle) to keep engine running. Anything above idle the engine sounds great. No smoke, and immediate, strong revs.
Yes, I double/triple checked the pilot jet air passageways in the carb body (actually, both pilot and main jet passageways).
- Blew air through hole in intake horn to air screw hole, clear
- Blew air into air screw hole and got strong air flow through hole in intake horn, through where pilot jet screws in as well as through the small hole in the base of the carb throat.
I thought the float level might be too high, so I shut off the gas with the engine running and waited. Sure enough, once the fuel got low enough, the engine settled into a nice, smooth idle with no missing. Of course, once the fuel go too low, it'd quit, but I thought this gave me the answer. So, I raised the floats (dropped the fuel level) a few mm. No difference. Tried raising floats some more; still no difference. Finally got to the point that I had the floats so high that when I put the bowl back on, it pushed the floats up, closing the needle valve so no fuel would flow at all.
So I went back to square one and reset the floats to 20.5mm. I then placed a clear battery drain tube into the drain hole in the bottom of the bowl and used it as a sight gauge to see where the fuel level actually was. It came right up to the top of the bowl, just a hair below the body of the carb. That's what I'd expect for proper float height. I shut off the fuel and started the engine. Same symptoms as before. But I watched the fuel level in the bowl and it slowly came down as the engine consumed fuel. When the bowl got to about half-full, the engine suddenly began to idle perfectly. I could rev it up just fine and had good throttle response, at least until the fuel dropped so low the engine wouldn't run anymore. Turning fuel back on got the engine running again, and if I played with the fuel shutoff to keep the fuel level in the middle of the bowl (which seems WAY too low to me) the engine ran great.
So, why would I need an abnormally low fuel level to get the engine to run correctly? What say ye, gurus? I'm not often stumped by carb problems - I do a lot of rebuilds here on way more complex carbs than this and usually have great results. But this one is whipping my butt.