wreckless1 Posted August 7, 2012 Report Share Posted August 7, 2012 can someone tell me whats causing backfire soon as i go to let off throttle ? just put together a 4 mill and this is my only issue so far. thanks for any help i can get. Quote Link to comment Share on other sites More sharing options...
wreckless1 Posted August 7, 2012 Author Report Share Posted August 7, 2012 4 mill play/dune port by FAST, 7 degree timing 280 main jets and whatever stock pilots are toomey t5s and pay check after pay check of evreything else.thanks Quote Link to comment Share on other sites More sharing options...
VoOK Posted August 7, 2012 Report Share Posted August 7, 2012 Possibly the timing? It may to too advanced, unless F.A.S.T. to you to run that. I once had a backfire due to a bad reed. Quote Link to comment Share on other sites More sharing options...
wreckless1 Posted August 7, 2012 Author Report Share Posted August 7, 2012 Possibly the timing? It may to too advanced, unless F.A.S.T. to you to run that. I once had a backfire due to a bad reed. ok ill check them if there fine ill back the timing to 4 . the thing is that it doesnt happen every time . ill bring it off in 2nd and as soon as i let out after wot it lets out a quick pop . thanks for the help . Quote Link to comment Share on other sites More sharing options...
shakar Posted August 7, 2012 Report Share Posted August 7, 2012 maybe lean??whats your altitude?im guessing your up there ?did you ask what fast recomended for jets?? Quote Link to comment Share on other sites More sharing options...
wreckless1 Posted August 7, 2012 Author Report Share Posted August 7, 2012 maybe lean??whats your altitude?im guessing your up there ?did you ask what fast recomended for jets?? i think im around 450 in north kentucky , jeff suggested 320s then drop to 310s and go from there. It would smoke even after it warmed up, so im running 280s now only because thats the jets that my builder said would be right along with my friends that ride banshees . to lean im thinking . does the needle setting have anything to do with the pop or just the mains? i have the needle on the 2nd from top if that matters . but i have to say even with the pop every now and then , its nasty from mid to top . FAST really done these cylinders up right. now just getting it dialed in .. Quote Link to comment Share on other sites More sharing options...
sprinklerman Posted August 7, 2012 Report Share Posted August 7, 2012 Sounds lean to me. Try middle clip and bump your mains. Quote Link to comment Share on other sites More sharing options...
moledigger Posted August 8, 2012 Report Share Posted August 8, 2012 Im running a fast dune play stock stroke +4 timing and 310 main. 20 cc domes at sea level if that helps Quote Link to comment Share on other sites More sharing options...
wreckless1 Posted August 8, 2012 Author Report Share Posted August 8, 2012 Im running a fast dune play stock stroke +4 timing and 310 main. 20 cc domes at sea level if that helps cool . thats what im gonna try . thanks everyone for your info . greatly appreciated . Quote Link to comment Share on other sites More sharing options...
ticktock Posted August 14, 2012 Report Share Posted August 14, 2012 25 pilots? This may or may not be relevant. A 4 stroke will pop when letting off because the pilot circuit is too lean. Most Harley's are this way. With your mods you might want to be a 27.5. Turn in your air screw to 1/2-3/4 out and try it. Quote Link to comment Share on other sites More sharing options...
Rustynail Posted August 14, 2012 Report Share Posted August 14, 2012 way lean i ran 27.5 and 280's on some bikes that were mostly stock with pipes....4 mill with porting you will need bigger mains for sure. Quote Link to comment Share on other sites More sharing options...
cattledog Posted August 14, 2012 Report Share Posted August 14, 2012 it is most definatly because you are lean, i have stock stroke, v3 reeds, very aggressive trail port, t5s, plus 4 timing, stock milled head, 350 mains,30 pilots and have put hundreds of miles on without fouling a plug, and plug color is good not lean. if jeff from fast or any of the site engine building sponsers suggests a starting point for your jetting you should listen and bet they will get you close. do not continue to run it with less than 310 mains and bigger pilots or you may burn it up. if damage has not been done now. Quote Link to comment Share on other sites More sharing options...
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