XT600E carburator issue at low RPM and bad start
 

Hi, to best explain my issue I have created a youtube video. Please have a look:

https://www.youtube.com/watch?v=0XKQ5Lroe2E

The issue in short: When warm, it does idle (but never for long). When driving low speed, it is not smooth, it stutters and hesitates. It seems to do this only at low RPM.

What I have done to this motor so far:
- Top end overhaul
- Cylinder, rings and piston inspected and good
- new Yamaha OE gasgets and rubbers where possible
- Valves lapped
- Valve clearance ajusted
- Cam shaft bearings ''refreshed'' (see other post). I ended up sanding down mating surface to as best as possible get bearing clearances. Seems to work ok for now but planning to do better in future.

To the carburetor:
- inspected membrane (2), look ok
- Cleaned all ports
- set screw (to be beleaved air screw to 2.5 turns out)
- replaced rubbers where necessary
- Inspected boots to engine and checked for vacuum leaks with brake cleaner

All input welcome with regards what I can try. Thanks!

hi
first of all, i'll repeat common question asked here about such situations -
can you tell when, how or after which step of service started that problem?

otherwise, there could be couple general suggestions.
2.5 turns of idle mixture screw (on this bike you regulate fuel amount with it, not air) is mainly a general reference; your airbox/filter etc. setup might be requiring different settings. normally you adjust it while engine is hot and idling.

other less possible thing is to make sure that engine is spinning normally, i.e. freely. maybe worth trying with spark plug out. you could use wrench from alternator side or rear wheel (in 5th gear).

as for timing chain condition (according to video), if it's getting overly stretched, most likely it will affect all rpm range. anyways, you could check it by the tensioner - how many clicks left there.

thanks for your input.

Good point to check where it started. In this case it is all-ready from when I bought it unfortunately. It just did not occur to me then some how. Since I have it, I have been doing a top end re build to it as it had a blown head gasget (was smoking and low compression). This all seems ok now but did not check the compression again yet, as all seems good. But seems like a good sanity check so will do this next weekend. Also the cam bearings are now ok and turns over smoothly. Cam chain just seemed a bit long (tensioner not fully extended but quite a bit). Why I think it may be to long is because I had difficulty setting the timing where I could not really find a really ''good'' point lining up with the mark on flywheel. As per service manual I pressed at the chain guide when checking timing/ position of cam chain sprocket. Air intake side all stock and looks ok. No broken boots or air leaks I could spot.

I'm looking for things to check or swap out to get more grip on this issue. Perhaps buy a second hand caburetor from Ebay just to see if that is any different? Or should I look into measuring the ignition system parts could something like this occur due to bad/ low spark?

If it will run fine at higher RPM (Although throttle position is the way to diagnose), then I would at this point stop worrying about the motor.

It is most likely to be the carb. Start again !

Start with stock size jets and parts. And not Chinese ebay rebuild kits. Genuine Yamaha or OEM quality only.

Then set the float height correctly.

And then set the freeplay between the two carbs.

Agree with the assumption that is must almost be the carburetor. And as I have checked all I can with the one that is on the bike and cleaned it very well. I will go for swapping it out (even if just to test).

I have ordered a 2nd hand one here in the netherlands and will intstall it coming says. Owner states that it comes from a bike that was running well.

Will update about findings soon.

Succes!!

I got a second hand replacement to test the hypothesis wether the carb is this issue and yes, it is. I have swapped it now with a carburetor of a TT600 2000 model (my bike is a xt600 1990). It did not perfectly fit on the air intake boot side (left side on carburator was larger then boot diameter). But I was able to press/ force it in. For de rest the fit was quite ok although some differences.

It now starts great and it idles perfect perfect. Both it did not do before. But.. There is only one issue with this new carburetor. At high RPM. It seems to not get enough fuel. Something I need to look further into. I did not look into this carb much so I fist want to adjust the float hight and butterfly valve adjustment between the primary and secondary carb. I expect this to potentially fix this high RPM Issue.

If any other suggestions to check for the high RPM seems to get not enough fuel condition, please le me know!

The jetting and needles on a TT600R carb are very different ie larger than an XT600e carb. The TT600R has a way bigger more freeflowing airbox. Would be surprised if it worked well as is and suspect the xt600 would run very rich with it

Well, Good point. Because I saw that the main jet is a 150 on the TT600 carb instead of the 130 I have on my XT600E one. But, the XT600E runs better the ever before on the TT carb so I am happy for now. Would the TT600 then pull more power out of the same block or something. And, what you guys think I should do? It is running quite well on the TT carb only not at very high rev.. leave it like it is?

Then, I have a additional question that I cannot get answered by the service manual, It is about the tube/ valve displayed below. I thought this is the ''roll over valve'' according to service manual but also not sure about that. So my question, what is this tube/ valve for? and how can it effect carb performance? And, does it need to be in a certain position for the carb to perform at its best, or.. what position to put it in to make sure it functions poperly?

https://i.postimg.cc/WpFrjKqJ/202505...0660-i-OS.avif