XT600 4PTA idle Idle speed remains high
 

I have an idle problem with my Xt600 model 4PTA. Model year 2000. :unsure:

The idle speed often stays high, around 2100 rpm. The symptom is worse when the engine is really warm.
This does not happen every time when revs drop. Sometimes the revs drop to normal and when they stay high, the revs can sometimes drop to normal after a while. If I load the engine by starting off at idle or against the front brake, the revs often drop to normal. if the throttle is not touched when starting off.

The intake side rubbers have been replaced with new ones and the carburetor has been cleaned and checked.
Neither the linkage nor the cable seems to be holding up.

When spraying with brake cleaner, there are no leaks at all, unlike when the rubber grommets break.

I have tried adjusting the pilot screw (idle mixture screw), but it does not seem to have any effect on this problem.
The default setting was 2.5 turns open when I put the carburetor back on.

What could be causing this problem?

hi.
with my somehow limited knowledge, thinking about three more or less possible reasons -

carbs might be out of sync.
diaphragm movement/condition issues.
slide/body mating areas might be leaking some air (due to wear), taking some fuel as well.

also, not sure what you mean by "control screw" - idle screw (on the cable attachment) or idle mixture screw (in front of the fuel bowl, harder to access)?

Do you mean the air-cut valve diaphragm or the other throat's big diaphragm?
Could the air-cut valve cause this if it doesn't work or leaks?

There was a typo. I meant the pilot screw. The one that is awkwardly downwards.

Here I was wondering a little about the effect of the 2nd throat and diaphragm effect to idle.
Was it the case that throat 2 is not connected to throat 1 other than through the hose going to the air-cut valve (number 2 in the picture)?

Did the 2nd throat diaphragm get its vacuum through that opening in the picture (number 1 in the picture)?

If this is the case, then in principle the damage to the diaphragm of throat 2 could not affect idle, because the throttle valve is mechanically controlled and closed in throat 2.

But at idle, could excess air enter the idle circuit through the air-cut valve, if there was a defect in the air-cut valve diaphragm or something else in the air-cut valve?
It is connected to throat 2 by that hose (2), so it freely sucks air also at idle, because it is connected to the cylinder side of the throttle valve?

I did not renew that when servicing the carburetor.

I hope you can clarify something about my wondering about this:rolleyes3:

yep, sorry for unclarity, i meant the smaller diaphragm (luckily accessible without removal of cabrs), which actually works as an enricher. guess it could be a reason.

passages numbered 1 and 2 are not directly connected. "1" is something like a "pressure intake" for cv diaphragm, 2 is linked to enricher diaphragm.

leaking the excess air could increase idling speed, by the means of collecting some fuel as well.

pilot mixture screw must be adjusted when the engine is hot (extra challenge).

mainly because of irregularity/randomness of the problem, for now my general idea is that it's less likely to jetting/adjustment issue, more about leakage/mechanical malfunction of one or more parts.

recommending complete inspection/overhaul of the carbs.

some history fo problem is always helpful. problem since you bought it or started all of the sudden? following something you did to the bike?


"cleaned" means bowl off and air blown or fulll dissasembley and ultrasonic cleaning? quite some difference :)


in "inspection", have you checked for play of throttle slide in bore?


not sure what you mean by "rubber seals"


not easy to diagnose with our dual carburetors so try something a bit radical. block the secondary carb air entry with something rigid like a plastic cap that cant be sucked into the engine. the bike can run on just the primary one so ride and see if problem persists. if yes, then you can focus on the primary carb, if not, then its the secondary giving trouble


@n67 hos is your bike running with the new carbs?

hi )
throttle shaft issue is almost sorted.
and recent winter's inspection displayed no wear-related problems so far.
and bike is running pleasant enough to evading me to think about other options.

good to hear, doing the carb transplant with the little tech support you have there is not easy

When I couldn't find anything wrong with the air cut valve and otherwise, I decided to try loosening the intake rubbers from the carburetors and carefully wiggled the carburetor in place and tightened the rubbers back up and the problem went away! :D

So the engine was getting air between the intake rubber and the carburetor even though I thought I had tightened them carefully and I checked them a couple of times.

The leak also didn't respond to brake cleaner, like it did when the intake rubbers were nroken. Maybe the leak was so small.

well, good to hear that your carbs are in good condition and the problem is solved.
try to keep carbs clean/healthy, it's pretty good system and hard to find an used kit in good condition.