Motorcycle Repair: CT90K1 Carb issue, honda ct90, k1 usa

Question
Rebuilding a CT90K1, 1969 for a fellow. It starts hard and after it gets going it revs up.  I adjust the carb to get it to idle and it runs fine for a minute or so and revs back up.

I cannot find a rebuild kit for it.  It is an early model with a 4 screw bowl and nobody has a kit for that type carb. I have cleaned it twice, adjusted everything including valves and timing.

I am a pretty darn good Honda mech but this thing has kicked my rearend. Is there another model or year of carb for this thing?

Could it be low compression as well?

Answer
Johnny....

Try looking up either the individual carb parts or the kit number from this reference:

http://www.cmsnl.com/honda-ct90-trail-90-k1-usa_model620/partslist/E++13.html

16010-077-305 Up to Serial CT90-234851
16010-102-305 or after that number, depending on the carb type.

A 16100-077-004 carb came up on Ebay

http://www.ebay.com/itm/NOS-Honda-1969-CT90-Trail-90-Carburetor-16100-077-004-/2

but missing some parts.

I found a kit listed for early bikes. Apparently you may have a K0 and not K1 if it has the 4 screw bowl. That is an odd one that isn't even shown on my Honda Carb catalog listings.

http://dratv.com/carin.html

Looks like they have other carb options available if that is the actual cause.

Assuming that you have good compression, then the things that make an engine idle speed hang up high or rise up like you describe are:
Air leaks at carb flange or intake manifold flange.
Sticking throttle cable/slide
Spark advancer springs are weak and/or the point cam is sticking on the advancer base.

Carbs are stamped on the base or manifold with a code. CT90K0 carbs are T9A, TD90DS, 572A with a #72 main jet.

CT90K1 carbs are stamped K9A and a #78 main jet.  Honda shows either 21.5mm or 23.5mm float level, depending upon the carb type. If you have a K0 carb and set the float level at 23.5mm, instead of 21.5mm, then that can also a contributing factor for engine idle speed changes.

Honda specs compression at 171 psi.  Low compression shouldn't be much of a factor in the engine speed condition, unless it is very low, which would require more idle speed screw setting than is specified/normal. When you crank the idle speed screw up too high, then the idle mixture screw effect lessens. And the higher the idle speed is set, to keep it running, causes the spark advancer to start kicking in, which results in a spiraling idle speed.

Hope this gives you some new direction on items to check out and verify.

Bill Silver