1984 TT600 to be specific. I can get it to run OK, but the tuning is wrong. As it's jetted now, it takes a few kicks to catch, and then barely stumbles at 800rpm with choke on, and dies with any throttle. After 30sec to 1 min it will idle at around 2000RPM. After some stoplight waits in 70F weather it will begin to idle slower, but straightens out if I run the pilot mix screw leaner. The average ride will leave me with a somewhat yellowed fender from carbon soot.

So essentially, I can't get the right range for it to start happily in the morning, and run right during the day. Does this sound like a float bowl issue? The TT600 carb runs a comparatively low float level, with richer jets, than the XT600 carb. Is it too lean, which is why I am overcompensating with the pilot screw?

Well, you do not say running under load is off, so let's concentrate on the idle running. I experience the cold-start not accepting throttle thing also, but at much lower temperatures.
I found out that there's a setting for the idle screw for winter and for summer. Remember; screw in = leaner (clockwise), screw out = richer (anti-clockwise).
What I do to set the pilot screw, with reasonable results is the following:
- warm up engine
- start with the factory setting for the pilot screw, something like 2,5 turns out
- Screw out (make richer), engine probably increases speed
- Keep screwing out till engine speed drops
- Turn back (leaner) about 3/4 turn

Remarks:
- In this way the idle setting is pretty rich which makes for good starting
- Be careful not to unscrew the pilotscrew entirely, it'll poooing out and you might loose it and is a b*tch to get in again
- Older engines seem to need a bit richer idle setting
- When starting, do not use / open the throttle, just grap the bar and engage front brake, if starting is bad just increase the idle speed a bit with the other screw
- Higher setting of the idle speed screw makes for better starting, just accept that it does not idle nicely and slowly

With respect to your complaint that the engine speed reduces at traffic lights, check the throttle cables / mechanism. It might be that they are not smooth enough to return to idle position easily and need some time and vibration to

Auke

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Auke
http://aukemarieke.blogspot.com

auke, thanks fo the reply. I did roughly what you described to tune the pilot screw. Setting aside my suspicions about running rich when warm at idle (I actually got off at a red light and leaned it out, which fixed the idle), is there any explanation for its cold start issues? That is what is really keeping me from leaning it out more.

I can understand the sensitivity to throttle, but why does the choke not bring it right up to 2000RPM, but instead barely lingers around 800RPM until it warms up. This does not seem normal. My KLR is pretty good about going to 1500-2000RPM when choke is on, and rising to 3000rpm as it warms.

The jet for the choke may be partally blocked, It is located in the bottem of the float bowl where the dirt likes to settle.

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Winston
85 xt600Z
02 YZ250