I have the exact same problem as the original Poster of this thread.
If I ride on trails, it is just fine. If I keep the speed under 50, it runs fine.
If I take it over 50, and try to maintain a steady speed, it starts sputtering. If I give it a little gas, the sputtering stops, and I accelerate. It only sputters when trying to cruise at any speed over 50 mph.

What I've done.
Rebuilt the carberator, and taken apart several times to clean.
Cleaned the air filter

The needle has no adjustment on it. So maybe a new needle might do the trick, as I think that should only cost about $10.

One more thing that I've really noticed, is the gas mileage is horrible. I think I only get about 35 mpg. I can fill up the tank completely, and I know when I get close to going 100 miles, I'm just about out of gas. It has the stock tank, and this is while driving in the city to my work. I'm thinking for a one cylinder, the gas mileage should be a lot better than my Honda Accord.

If anyone else has any solutions, or suggestions, please post.
Thanks,
Craig

sounds a bit like what i was having happen on my 03 xt cleaned carb new filter even had it on the dyno.still no improvement so did a compreshion test and it was well down .the top piston ring was worn .

I had a XT600 would run fine untill you tried to cruise at 60mph after about 10 minutes engine would stutter and surge like it was running out of fuel, sometimes it would accelarte past this but then start to surge again. If I stop and let engine cool it would run fine until you tried cruising at 60mph for more then 10 minutes.
It turned out to be the pick up coil breaking down when engine got hot.
The easy way to check is get a complete generator and pick up from a good friends bike just swap the complete side case.
I could not get a pattern pick up for my 3AJ same as 1VJ so had to use second hand one, Yamaha only sell the pickup with the generator about £400

hello on my bike i had great difficulty getting it to run at slow speeds so i cleaned out the carb and put the petrol pipe straight to the carb from the lh petrol tap to eliminate the pump and i noticed i had very fine fumes leaking from the rubber seal which seals the float bowl on carb i think it had gone hard after 20 years so i got a new one from yamaha fitted this and it run ok at low revs and through the range because before this if opened the throttle it would suck air in and cut out unless full throttle and it would rev ok once i sorted carb out i put petrol pipes back to how they should be and it now runs ok .

Last time that happened to my 1990 XT600E it was nothing more than and obstruction above the needle. That caused the bowl to run out of fuel in moments of high fuel consumption (above 60MPH).

When I took it apart, that little white screen above the needle valve was packed with crap up to the fuel inlet. So I took it out and threw it in the trashcan.

Now I always use an inline filter and no problems yet.

I'm going to check my compression then. I'll post if this fixes the issue.

It does not sound like the coil, as the sputter happens right away when I hit 50mph. It is a California model, and I live in Colorado. The previous owner did have it jetted, but maybe not correctly.
I've cleaned and rebuilt the carberator, with no change.

This is very fustrating. I like everything about the bike except for this sputtering. Just about ready to sell it if no fixy.

Hi All,

I'm new to this site, so bare with me!

I have a 1984 Yamaha XT 600 that I got free. Bike was sitting in a back yard in weeds for about 3 years. I massaged it back to running order. Pulled the carbs and rebuilt them, fluid changes, replaced plug, etc.... I'm very mechanical. The bike misses and sputters at 3/4 to full throttle accelleration. I can even pull in the clutch and it will continue to miss/sputter as if it has a rev limiter. I can't figure if it is starving for fuel or it is an electrical issue. The bike has a Super Trapp and I went up 2 main jet sizes on both carbs. Checked the vacuum diapham on the secondary - good shape. No fuel restrictions of any type..... fuel system is clean! The plug and Super Trapp are somewhat black, showing plenty of fuel.... maybe a little on the rich side. I've had this problem prior to enriching the fuel carberation. The kick stand switch has been disconnected and bypassed. Any Ideas are greatly appreciated.

I purchased a 1986 xt600 with 15,000 on it. Although I haven't yet really cleaned the air filter, I did have my carbs professionally cleaned with no noticeable effect. Later, I ordered a second set of carbs from fleabay. I put them on and...vroom! Smooth sailing at high speeds! The second set was from an '86 TT, and aside from the choke setup, the carbs look like they are the same.

However: the butterfly on the 2nd carburetor on the replacement set was adjusted to open much later than what I had before. Also, there was a plug that had been drilled out, exposing an adjustment screw. Interesting note: When I had my 1st carbs cleaned, the mechanic was surprised that the plug had not been drilled out to expose the screw.

Today, I adjusted my original set's 2nd carb's butterfly to open like the replacement set, and went through the effort of switching them out again. It did not solve the problem. I don't know everything that has been done to the replacement carbs, but I am sure the answer lies in the carburetors.

Something extremely tedious that could be helpful to clue into the solution, is to run the bike at the cursing speed and let it sputter. Then, hold in the clutch and flip the kill button; don't let the bike back into gear until it comes to a complete stop. Then, remove the spark plug and inspect it. If it is white-white, that means it's too lean. If it is blackish, then it is running too rich.

If I learn the secret key to this problem, I will post. If anyone knows more about this, please, please post.

Check the resistence readings on your ignition source coils as well as the pick ups.
i've seen several running issues disappear since getting mine rewound.

I have read a couple threads regarding this issue. I would just like to make my case known and see if there are any similarities with my motorcycles problems and if there are any solutions.

I have a 1994 Yamaha xt600, I bought it just recently, I have no idea what any previous owners have done to it. However, upon buying it I proceeded on a whole slew of TLC for the bike. I cleaned the air filter, cleaned the carb (mostly, I couldn't get the float pin out), changed the spark plug, and changed the oil.

Ok, so now the issue. I take my bike up to 60+ mph either taking it easy getting it up there, or going full throttle. Both ways, once I am cruising in 5th gear it seems to do just fine for the first little bit. Then it will do one of two things, start to sputter and slowly lose speed as I am getting no horsepower. Or, it will go a little further but when I accelerate past 1/4 throttle it will start the sputtering thing. When either situation occurs it sputters and loses speed and sounds on the verge of dying all the way till either, it dies, or I have been able to downshift enough without it dying that it picks back up again. Both times I pull to the side of the freeway embarassed going slower and slower. If it dies I can always start it right back up right away. It feels almost like it is running rich, but I when I took out my spark plug it was mostly whiteish like it was running lean. I do have blackness on my exhaust (super trapp) which leads one to think it is running rich.

I was thinking maybe the side stand switch would be the next best bet, but this sure doesn't appear to be anything electrical related. Especially since my bike will stutter all the way down to 25 or 30 mph before it comes back. So after that my next option is a rebuild of the carb (with the diaphram included?) So I just need to find a good kit for this or get hosed by the Yamaha repair shops.

Any input on this is greatly appreciated!

Now this is a long running thread.
It would be great if those that have resolved their issues could post their resolutions!
Just to add my 2 cents worth.
I've had a few XT's over the years and a few other various motorcycles.
But XT's.
I recall once on a road trip on the XT550 would run fine but once I tried to sustain my revs over about 4000rpm for longer than 3-5 minutes it would begin to surge and lose power. Being on the road I didn't have too many tools at hand but I did have a phillips screw driver, I did the obvious things like drain the carby bowl etc.. and I disassembled the fuel cock. In it I found a nice piece of compacted fluff. It was probably the best fuel filter I could have had as it was full of those nasty bits you don't want in your carby. Anyway removed the fluff and all was good.
Another XT 600 I owned would occasionally missfire when HOT. Only when hot, really hot. This issue went from an occasional missfire to an occasional cut out to eventually I would have to push start it, at which time it would fun very poorly. It was an issue with the pickup. I never did solve this as a fellow bought it before I fitted the part.

Always wondered if he managed to get home on it. He was a German fellow who was going to ride it around Australia. (If your there I hope it went well)
Anyway I'll stop my ravings now and finish my ..

Resolutions? I have a 1984 Xt600 with the same issues... Hope it wasn't formerly owned by one of the previous posters!

If anyone has had a chance to lock down the issue, please do share. I've replaced the air filter with a brand new UNI filter and upped to a 130 main jet which helped at idle but the sucker still doesn't like to run without a choke on then pops on decel when cruising at 50MPH and then dies. Easy to restart as stated above but embarrassing as all get-out.

I'd like to say I could read the plug but I have a slight oil leak at a valve seal and that makes the plug always read "shiny, wet and black" (yeah I know Mom, I'll get around to fixing it soon.)

Any new developments? it would seem everyone is out riding and forgot about the forum or sold their scoots and don't care anymore...

Things wrong with my bike:
-I know previous owner tore out some of the stock muffler baffling
-I have a Uni filter installed. (stock one was AWOL)
-Kickstand switch bypassed.
-Slight oil leak from a valve seal

Things I have tried:
-Added a inline fuel filter and cleaned tank and carbs
-replaced ignition coil with universal unit thinking a cracked coil was acting up when warm
-replaced jets, air screw, carb o-rings & gasket, needle valve and the seat (with filter still on) all with new OEM parts.
-Replaced all fuel lines with new hose and clamps. (except long breather that goes up to the tank.)

Things I might try soon:
-Replace enrichening diaphragm
-Replace secondary carb diaphragm
-drink until the problem stops bothering me

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Perpetually fixing an XT600 as old as I am...

Ok, so I have been cheating and looking at other forums for other makes of thumpers and they mentioned the vent hose that attaches to the tank can get clogged and it could cause similar sounding symptoms. Also I know my muffler is more open than it should be so I will attempt 2 experiments to see if I can remedy the issue.

1: I will replace the vent hose with known good hose and inspect it for blockage (did I mention when I got the xt600 it had a rust problem in the tank?) I figure that could cause it to starve itself of gas theoretically when the throttle is released and the enricher diaphragm kicks in and leans the mixture, and the vacuum line being clogged doesnt allow the main jet to suck enough gas to keep her running... (just a guess since I'm new to the carb game, please correct me if I'm wrong.) I will post results by Monday.

2: I will also see if restricting the exhaust using my welder and tack welding some washers to the tailpipe makes a difference. I'll only do this if the first experiment proves unfruitful because I know my Xt600 needs some back pressure or a bigger jet due to fuel/air screw tests.

I know I'll actually post on this again because I'm a desk jockey and will be in front of the computer again Monday... and either be excited or pissed given the results of tomorrow's tinkering

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Perpetually fixing an XT600 as old as I am...

Mainjet has little effect on the idle!

bike dieing on idle a popping sure sounds like either the PILOT jet is clogged or it isn't big enough!

good luck!


Vando

UPDATE!

First I want to clarify, When I said "main jet" I meant the one in the bowl, I was unaware the other jet on the secondary carb is also called a "main jet" on sites like bike bandit, I'm sitting at 130 in the bowl right now.

So the popping was due to a lean condition I went through the bike to look at every possible angle for leaks... and found one right at the snorkel that attaches to the top of the airbox. Wow, what an idiot I can be. That fixed I took it for a spin: no more lean issues. So I learned to be careful to seat that fully when I clean the airbox next time.


That took care of the lean condition and I was able to turn the air-fuel screw in to factory specs again (2-3 turns). But the bike still dies at highway cruise speeds. I replaced all of the fuel lines previously with clear lines. I noticed a crap-ton of bubbles in the fuel line after it died on the highway. If I unplugged the vent line that runs up to the tank, bubbles disappear and fuel flows as usual.

I don't know what its called specifically and its not shown on the parts diagram but the black screw in "tank vent valve?" on the bottom of the tank looked suspect. I replaced the line running up to it and the problem persisted. The valve deal was dirty looking and I noticed it has a check valve or something inside of it and rattles if shaken. I tried to blow it out with no luck. (valve apparently works) but when I tapped it on a paper towel there were black flecks that came out. I couldn't try it out again today as the weather turned shitty again.

Has anyone else had issues with tank venting etc or know of any fixes?

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Perpetually fixing an XT600 as old as I am...