My CT won't run unless the choke is on... why?
CTs have a variety of possible problems with the carb. The original Keihins were fiddly carbs and while usually reliable, occasionally you might see a problem where the CT doesn't seem to want to run unless you choke its air supply off. True, a CT requires some choke to start on cool days or if it's sat a while, but in this syndrome, you warm the bike up, take the choke off, and it dies. In extreme cases, you have to even partially block off the intake to get it to run, and even then it seems to run lean at high speed.
There are a couple of possible causes.
The first thing to look for is a clogged or partially-clogged main jet. The idle jet can supply a little fuel if the main jet is clogged, but it can't spit out enough fuel to run the bike much above jogging speed. The main jet can be removed and cleaned with a fine piece of wire. My personal favorite is a piece of wire from the inside of a twist-tie (like on a bread bag). Do not at any time try to ream the orifice -- don't use a drill or anything like it to clean out the jet. If you can't get the crud out of it, so that you can see daylight through the jet, soak the thing in Berkebile
Chem-Tool
or Techron or Gunk or similar. If you still can't clean it after soaking overnight, replace the jet.
If that doesn't work, check the setting of the main needle. If it has been misadjusted, or has come loose, it may not be pulling up sufficiently out of the main jet orifice. Check it against the service manual for your year (the settings sometimes change).
If that's not the case, and you've also verified that the ignition timing and spark plug gap are right (CTs have ignition sickness much more often than fuel sickness), there's one more rare but possible cause. If the valve chain has not been adjusted or tensioned, there's a chance the chain has jumped a tooth, messing up the valve timing. Basically, the thing can't breathe right and the only way to compensate is to... choke the heck out of it. Check the valve timing and make sure the valve clearance is set correctly.
Don't try to resolve this problem without a decent service manual.
Last updated:
03/10/2002