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philipstjohn

Typical Jetting for a uncorked XR650R

5 posts in this topic

I live in Florida, at sea level. Most charts tell me to run a 168-170 main, along with the 68S pilot and B53E comp needle. But many posts tell you to run 175 main. What should I do? :thumbsup:

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Run 175 main, 68s B53E needle middle clip. richer is always better. If it is to rich (at see level it shouldn't be) then step down. Better to be rich then toolean and burn a hole in the piston.

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...richer is always better. If it is to rich (at see level it shouldn't be) then step down. Better to be rich then toolean and burn a hole in the piston.

While I wouldn't quite disagree with that advice, I will say that I think most people error on the rich side further than needed. And the XRR seems pretty tolerant of rich mixtures so it's not a biggy...BUT my advice would be to start at the rich end of what you think is correct. Test from there by stepping down (or up--if it turned out that way) and find the best performance.

Maybe the easiest main jet evaluation technique is the roll-off method. Basically, you do a fourth gear WOT acceleration. Accelerate for a few seconds, allowing the engine to run at/around the meat of the powerband. Quickly roll-off the throttle to about 7/8. If you actually gain power as you roll-off than the main is too small--you're lean and need to jet richer. If the engine studders/stumbles/hesitates then the main is too large--you're rich and need to jet leaner.

The roll-off technique works because of the temporary rich/lean conditions that occur in carburetors when the vacuum signal within the carb's venturi changes quickly. The roll-off creates a temporary rich condition. It's the "opposite" phenomenon of the bog that occurs when you whack the throttle open quickly from idle.

If your main is already a little rich than creating the temporary rich condition will make it far worse--to the point where it is obvious. If you're already lean, then the temporary rich condition will actually improve the mixture creating a little extra HP that is felt as acceleration.

The slightly better but more complicated main evaluation technique is the roll-on method. But it's tougher to do and at that point you might be better off doing the tuning on a dyno. But my bottom line is: most people will be far better served by leaning good jetting techniques than searching for some indisputable turn-key jetting spec or trying to boil down various suggestions into one correct answer. Let the bike tell you what it wants...

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will give it a try... I am buying this bike from a buddy in Utah and he has a 172 main, 68S and comp needle third seat.

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I use a 172 main and like it better than the 175. I use the 175 in the winter months.

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