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Followup To
Question -
I have a 70 Trail 90 that starts easy and runs great until the throttle is opened all the way under load. It then cuts out badly (varies from bad to half bad) and I have to back off on the throttle. It has new points, timing has been set, advance mechanism cleaned, carb disassembled and cleaned twice, gas lines cleaned and filters installed and air cleaner cleaned. The spark plug looks new. I am puzzled where to go next. I'm concerned about using the bike for an upcoming hunting trip. It appears that it'll do everything I need it to do since I will never be opening up the throttle. All travel will be at relatively slow speeds. My only concern is there some deteriorating condition (something like a coil going out) that could result in me being stranded 5 miles up a logging road in these Idaho mountains. Any thoughts/suggestions will be appreciated. Claude
Answer -
Claude, if your bike is a K2 model (1970) CT90-3XXXXXX then the float level should be 20.0mm, with a#65 main jet and #35 idle jet. Needle set at 3rd groove.
A K1 has 23.5mm float, #78 main jet, just for reference.
Check the camshaft in the head to be sure that the cam bearings are not worn out, causing the camshaft to flop around and the advancer/point cam to throw the spark timing all over the place.
If you used a KEYSTER carb kit, the jetting may not be correct. I have found many of these kits to be poorly calibrated on a lot of models.
Put a little choke ON when it gets to full throttle and see if that makes it worse or better. Worse, then the main jet is too big or fuel level is too high. Better means that it is running lean at full throttle. Change main jets one size in the direction indicated and retest.
If you have changed ANYTHING on this bike, in regards to exhaust pipe or air cleaner parts, the carb settings will probably no longer work as designed.
A failed valve spring could cause early valve float, but that doesn't happen very often.
These are BATTERY ignition bikes, so be sure that the battery is fully charged and that the charging system is fully functioning up to about 7.5v output. A low battery will cause loss of spark energy at high speeds.
Check the spark plug cap resistance.. probably either ZERO ohms or around 5K ohms. Look for tiny cracks in the cap structure.
Compression should read about 150-175 psi. Check valve clearances at .002" (cold) both intake and exhaust.
Bill Silver
Bill: Sorry you're dealing with a dummy. What needle are you talking about and what does a 3 groove setting mean?
AnswerClaude, if you remove the top of the carburetor and pull out the slide assembly there is a tapered needle hanging down from inside the slide. By compressing the return spring you can remove the slide from the throttle cable.
The needle is held in place with a spring clip that sits on top of the e-clip, which is clipped into one of 5 grooves. Raising the position of the clip, lowers the tapered needle in the needle jet, leaning out the mixture. Lowering the clip position raises it and richens the low-speed/mid-range mixtures.
If you are only having problems with WIDE OPEN throttle conditions, then leave the needle/clip position alone.
The parts are tiny and need to be handled carefully, so something doesn't go flying and get lost.
That info was for reference only. You mentioned previous carb cleaning, so I thought you were aware of the internal parts and adjustments.
Bill Silver
