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Yamaha XT 600 - Max speed only 120-130 km/h Yamaha XT 600 - Max speed only 120-130 km/h
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Yamaha Tech Originally the Yamaha XT600 Tech Forum, due to demand it now includes all Yamaha's technical / mechanical / repair / preparation questions.
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  #1  
Old 14 Mar 2020
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Question Yamaha XT 600 - Max speed only 120-130 km/h
Hello everybody.
I have a question for you.
A problem came up with my XT 600 bike (3TB 1992)
For some reason the bike has no real top speed.
In the fifth gear with half throttle, it goes with 120-130 km/h, and it does not matter if i turn the gas handle to max, the motor not accelerate further.
I think i bought the bike with this problem exist.
But i don't have any clue whats the real reason of this.
It has a 15-47 chain ratio on it.
I use "34L" cylinderhead, and stock "3TB" (Y26PV) carburetor.
When the engine is hot, it starts perfectly.
A cylinder and cylinderhead are fully repaired, valve clearance are good, it works as it should.
And it also has stock exhaust muffler too.
It think that the main jets are too small, but i disassembled the carburator and everything is set up as it is in the Service Manual (In 3TB manual).
Is the problem that the engine head is from a different XT series bike (34L), and bigger jets needed?
Thanks for your help in advance.
And sorry for my bad english.
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  #2  
Old 14 Mar 2020
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If the bike is modified, intake, exhaust, engine parts etc - then "by the book" tells you nothing.

So BEFORE you get into changing jets, you need to do a "plug check". This involves running the bike - in this case flat out then pull in the clutch and hit the kill switch, coast to a stop and pull the spark plug. What colour is it? White, brown or black? Report back and ideally post a photo. You can also search the internet for "spark plug colour" and you'll get loads of photos of what's good.

But first check the air cleaner - if it's dirty that will kill the top speed.

Other questions: is it misfiring? Does it go just as fast at half or three quarters throttle as full? Can you hook up a tachometer - is it revving to maximum because it's geared low? (I have no idea what the stock gearing is, so compare the ratio you have to stock).
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Old 15 Mar 2020
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Quote:
Originally Posted by Grant Johnson View Post
If the bike is modified, intake, exhaust, engine parts etc - then "by the book" tells you nothing.

So BEFORE you get into changing jets, you need to do a "plug check". This involves running the bike - in this case flat out then pull in the clutch and hit the kill switch, coast to a stop and pull the spark plug. What colour is it? White, brown or black? Report back and ideally post a photo. You can also search the internet for "spark plug colour" and you'll get loads of photos of what's good.

But first check the air cleaner - if it's dirty that will kill the top speed.

Other questions: is it misfiring? Does it go just as fast at half or three quarters throttle as full? Can you hook up a tachometer - is it revving to maximum because it's geared low? (I have no idea what the stock gearing is, so compare the ratio you have to stock).

- In 5th gear, there is no difference between half and full throttle.
- New air filter
- When i use the engine brake, there is a very little misfire in the exhaust.

spark plug:
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Old 15 Mar 2020
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There probably is something not quite right with your bike but my experience of that era 600 singles is that they do 160kph on a test track with a journalist flat on the tank and 130kph out in the real world.
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Old 15 Mar 2020
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first of all, 15/47 is quite a bit sorter than stock , so that could be one explanation, try a 45 rear and see if it helps.



agree with putting a new plug and doing a "plug chop", i.e. read the mixture by looking at the plug color, plenty of howto's on the web.


also agree that if you are sitting upright on the bike, if you are a tall and big guy and have some luggage, you wont see much more than 130kmh.... to see more you will have to laydown on the tank....


you are talking about "your bike", but not clear to me if you just bought it and then repaired it or if the repair was done before you bought it.



Anyway,after you change gearing, do the plug chop, take a good pic of the plug and get back to us
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  #6  
Old 24 Apr 2020
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I have 14/45 chain and sprocket setup, and it'll only do 120 or so. Its not a fast bike, nor was it intended to be. The punch down low is where its at on these bikes.
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