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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 19 Mar 2003
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carburettor adjustment

I have been rebuilding my bike over the last months (a 3DS '88) and did following modifications:
1. Changed the carburettor (the Swiss 3DS with original jets and needles) for a 1VJ '87 carburettor with original jets and needles.
2. After 500 kms of running checked the plug and found it to be bright white, so I changed the jet of the primary carb from 145 to 150.
3. Installed stainless steel headers, with about 5 mm more in diameter.

The problem now is that the idle speed mixture screw screwn in totally gives the highest engine speed.
Float level is according to specs (25-27 mm). I have the standard air filter.

Idle running is not very neat; I have the feeling that the engine vibrating more than it did before I changed the carburettor.
Does anybody have an idea what should be done to get a) the idle adjustment as it should be and b) what could be done about the vibrations. I was thinking along the lines of changing the setting on the needles.

Any help much appreciated!

Auke

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  #2  
Old 19 Mar 2003
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Hi Auke,

Hmmm. The Swiss jetting/carburator is probably mainly based on higher altitude = less oxigen in the air so basicly smaller jets. Is that right?

If I remember right: the main jet only controles the mixture from half to full throttle. the needle controles it from 0 to 3/4 throttle, the idle jet when throttle is closed. Off course these controles overlap.

As far as I know the right procedure to 'read the plug' ('Coffee with milk' should be what you want) is the following:
To check if the main jet is correct: ride full throttle, if possibly up-slope or with head-wind, for a while and than cut the engine with the righthand switch (and use the clutch off course). Then check the plug (after you stopped :-)
Do the same with half throttle to check the needle position.
When you check the plug after the engine has idled you probably only check the idle adjustment. Which in your case could explain that indeed you cant adjust it right. The screw fully in and still not right probably means it gets still to much air ---> white plug.

So I wouldn't change the main jet until you are sure it to lean at full throttle.
Make sure the needle position is original 1VJ (needle up = clip down... can be confusing in the manual)
If it is try to lower the needle one position en check if you can get it idle properly. If not you may have to change the idle jet for a smaller one.

I'm not sure what you mean bij 'headers' but probaly the original (1VJ) size gives the least chances on problems.

Peter
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  #3  
Old 19 Mar 2003
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Sounds like your engine gets too much fuel @ idle somewhere. Maybe something wrong with the carb, especially the idle circuits.
I assume that when riding-in the fresh engine only the needle made your mixture.
Are you 100% sure the cam timing is correct?
What exhaust muffler are you using?

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