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tuning a brute

blue beast2010-08-11 09:45:05 +0000 #1
my buddy had his 07 750 brute rejetted ,it helped alot, but at around 10 or 15 mph it spudders. get past that and it runs great. can u dial this problem out with the screws or something . it has better throttle response,better top end and lots more power now .oh i forgot to mention it has 2" snorkes and stock muffler. he is gonna try to tune it this weekend do yall have any suggestions . trying not to have to rejet if it can be adjusted somewhat,need a list of what to check and how to adjust it before we tear it apart agian. thanks in advance guys
nmkawierider2010-08-11 09:48:44 +0000 #2
Sounds like the slow or pilot circut is in need of adjustment. If too lean, then you will have to go to #40 pilots. If too rich, well, that 2" snorkle is too small. The actual airbox intake port works out to just under 2.25" so even a straight piece of 2" is already too small, then add the 90s and 45s and its restricted enough to have to rejet the mains...maybe enough for the pilots depending how you did it. Should go to a 3" anyway.
blue beast2010-08-11 09:59:44 +0000 #3
Sounds like the slow or pilot circut is in need of adjustment. what are theses and how to
nmkawierider2010-08-11 10:33:15 +0000 #4
Quote:

Originally Posted by blue beast

Sounds like the slow or pilot circut is in need of adjustment. what are theses and how to

Pilot or slow circut supplies fuel from idle to about 1600 rpm where the mains take over the rest of the way. That fuel system gets its through the pilot jets. Its next to the main jet in the carb. That path takes two directions, one to the A/F screw which adjusts idle mixture and the other to an intermeadiate port close to butterfly. The stock pilot jet for the BF is #38. A few mods like CDIs, pipes and a few other things can cause the need to increase that jet size to #40s. A sign of this is leaning mixture to the point of popping and power loss just before the mains kick in. After the mains start, it seams to be OK. Adjusting the a/f screw out a little can help, but the result is rich idle mixture. The #38 just can't supply enough for say..the 1300 - 1600 range no matter what.

Leaning on the mains can be fixed with shims on the needles, lower the clips on DJ needles which raises the needle or increasing main jet size depending how much is needed.

Carbs 101. Hope that helps.
blue beast2010-08-11 11:34:31 +0000 #5
thanks man maybe we can get started on it......I havent ever done a whole lot with anything that is carbed, so i really appreciate the help
Bootlegger2010-08-11 10:31:26 +0000 #6
what throttle position is it doing it at? If its between 1/4 & 3/4 you can get it out with needle adjustment...but if its lower than 1/4 you pilot jets will need to be a #40....but that is unusual with a stock pipe. Did they use a Dynojet Kit? Also....when you say sputter...is it losing power and kinda falling on its face or is it a pop and crack type of noise?
blue beast2010-08-11 12:32:06 +0000 #7
ill have to check the throttle position, it was a dynojet kit(is that bad?),pop and crack type noise. as far as i saw it dont just lose all power just kinda hesitates.but u probably would like to know the throttle position before you can tell me what to do .ill check on it , we both had to work this weekend so we havent got to do anything to it.
Bootlegger2010-08-11 12:39:56 +0000 #8
You can just guess and be close to what throttle posistion...but I would lower the clip on the needels down one nothc from wherever they are and try that.

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