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weird idle and sluggish on top end. New phat head with pump gas dome


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Hey guys. So I recently rebuilt my 2000yz250. New wiseco crank, wiseco piston, brand new vforce3 reeds, phat head with pump gas dome, procircuit platinum pipe with silencer. Clean carb new airfilter.

When I bring it to a stop the idle hangs up a tad for a couple seconds then returns to normal idle. Any suggestions?

2nd - I know I'm suppose to do my plug chop to see where my jetting should be. I just want your thoughts on it. Ok pilot is 50 which I'm not worried about and the main is 180. I'm at 300ft elevation. This was the jetting before with stock head, boysen reeds and the procircuit pipes. Now that I have the phat head with a pump gas dome, it seems to be a little sluggish up top. Should I go with a 182 main or a 185. Thank you and I appreciate your guys thoughts on this if you could toss some knowledge at me

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Well what main sounds better to go with. A 182 or a 185main. The dome that is in it is a 21.1cc dome. It's obvious that it creates more compression then the stock head but still good for 93pump. I guess 185 would be good and if anything it will be a little richer for protection

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Get your idle hanging figured out before jacking with main jets. If it is an air leak then the main jet is not the problem. I would do a leak down test, a quick search will show the parts and process to do it. It is cheap and relatively easy. If you have no leak then check the air fuel screw setting, start at 1-1.5 out then adjust idle and see if it still hangs. If you have to go out more than 2 turns the pilot is too rich, if you have to go below 1 turn out (i.e. 0.75 turns) the pilot is too lean. The air fuel screw adds air to the pilot circuit. With the screw closed you get no air, with the screw further opened you get more air. This is controlling the first 1/8th throttle mainly but slightly affects the entire range. Once this is clean do a wide open plug chop (quick search) to determine main, I dont usually do this but can rely more on feel. Once that is good you can play with the needle setting to clean up the low-mid and mid rpm.

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Ok. I just brought the bike to my shop and I'm going to let it warm up till the idle comes down then going to spray some starting fluid around and see what happens. I appreciate your guys iinput cause I know this has been talked about 100+ times on here. It's just nice to get input directly sometimes then just reading threads. Thank you. And I'll post up later around 5 ish to let you guys know if anything pops up. I hope it does and it's something stupid

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If quarter,half, etc. throttle is lean and you're running there then your idle will hang briefly. If it's only after a wot run then up the main.

Otherwise try a clip richer on the needle

Edited by seer
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Ok I'm going to throw a 185main in. And I'll raise the needle and see what happens because I have no air leaks. Checked yesterday. Thanks guys. I will report back. Waiting on some gaskets from yamahapartswarehouse for the top cap and Bottom Boulevard with a new air screw because my plastic ones worn to shit

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