172 Main Jet?

[shaggy01]

I know there are 6 million thread on how to jet your 400. My question is, putting on a two brothers slip-on and they recommend a 172 main jet. Seems kinda big considering HMF and other companies usually go with a 150-165 range. Any opinons? Got it pulled apart and would rather have a better starting jet size to work off of.

Exhaust is the only mod right now. Considered venting/removing airbox lid to compensate for the big main.

[CJM]

Yea thats a bit high. I ran a 172 with ALOT of mods.

Your gonna be closer to 160. Buy jets from 150-165 in increments of two so your covered and put a 42 pilot in there.

[shaggy01]

yeah, got a 42 pilot. But that was my thought, that the 172 was pretty big for a stock motor. Any downside to running the 172 for one ride? Dont know if ill have time to pick a few other sizes before this weekend and tune it.

[dxcody]

No down side, you just prolly wont be able to go above half throttle or it will cut out.

[shaggy01]

Well good to know i wont mess anything up but no full throttle is no fun. I didn't think it would. It'd just run really rich. I guess ill have to make time this week to get a few smaller sizes.

I guess this answers my question. Thanks guys.

[rooster300ex]

Originally posted by shaggy01
Well good to know i wont mess anything up but no full throttle is no fun. I guess ill have to make time to get a few smaller sizes.

I guess this answers my question. Thanks guys.

U could atleast try it. It might not bog out at WOT. Some people just lower the needle e-clip. That will allow more fuel at WOT. Most people not knowing what they are doing just put an aftermarket pipe on and not jet at all, and it runs fine and have no engine issues what so ever. I've seen it done first hand. But I wouldn't do it I always rejet if needed. I'm just sayin if u wanna ride u can do that.

[shaggy01]

Yeah, ive seen alot of people slap on an exhaust and ride it as well. no adjustments. But I figured I should do it right so i can get the most out of the exhaust and the motor.

I guess i may try it and see what happens. Worst comes to worst, i gotta pull it back apart again and try another size.

[rooster300ex]

Originally posted by shaggy01
Yeah, ive seen alot of people slap on an exhaust and ride it as well. no adjustments. But I figured I should do it right so i can get the most out of the exhaust and the motor.

I guess i may try it and see what happens. Worst comes to worst, i gotta pull it back apart again and try another size.

I think the same way. Might as well get as much power as u can out er.

[shaggy01]

Yep. Raced a couple GNCC races this season and finished decent but there were times were i wish i had some more power so i was hoping a slip on and airbox lid removal would give it a tad more low end grunt for the tight woods sections. Hopefully, if i can get the jetting close it'll help.

[shaggy01]

Put it back together and ran it around the neighborhood last night and it seems to run great. no bogging or anything. Sounds like a slight bit of backfiring on decel after a hard run but nothing too bad. Other than that it seems to be good. Noticable power increase!

And now i know why everyone swears by the 42 pilot jet....amazing! could never get mine to idle quite right and cold starts weren't great but now, totally different. Starts right up with no choke (quicker start too) and idles great.

I guess ill leave the lid off and rock it this weekend and see how it does.

[russellsroost31]

i put a 170 in mine and i couldnt even get it to start

[shaggy01]

Yeah, i was wondering if it was going to cause a problem but seems to be fine. Im going to run it some more after work today to make sure. Haven't done a spark plug test yet to see whats going on inside but atleast i know its not running lean.

[ben300]

Originally posted by shaggy01
Put it back together and ran it around the neighborhood last night and it seems to run great. no bogging or anything. Sounds like a slight bit of backfiring on decel after a hard run but nothing too bad. Other than that it seems to be good. Noticable power increase!

And now i know why everyone swears by the 42 pilot jet....amazing! could never get mine to idle quite right and cold starts weren't great but now, totally different. Starts right up with no choke (quicker start too) and idles great.

I guess ill leave the lid off and rock it this weekend and see how it does.

i had a 42 in mine, and it ran great, but at really low rpms, when i would punch the throttle hard, it wwould just bog for a split second. so i put a 40 in, and this totally fixed it

[300exBorg]

I just put a new pipe on my 400ex today and when I rejetted I put a 42 pilot jet in my card too. OMG I can't believe how much better it starts up and idles. Like you said earlier I could never get mine to idle right either. I fired my bike up from a cold start and all I can say is WOW. I am def. amazed at how much better it is than the stock 38 jet.

[shaggy01]

Well the ride this weekend went pretty well. Was about 1500 feet higher than where i live and still ran fine. Only issue was it didn't have the pop in the lower rpms like if i went to pick the front end up over a creek crossing or something. It wanted to fall flat on its face. but other than that, it would run all the way to WOT with no issues. I'm thinking that i should drop down do a 165 or so. That should fix the bogging correct?