Big Bore 285cc Help

[mikemay]

Hi there,

I have recently rebuild this bike and I seem to be having exhaust popping on deceleration and acceleration. The bike also will not rev down probably when the throttle is popped the throttle will hang for a bit then it goes down. I pulled the plug and it's as white as it could be other then some dark around the ring. I then pulled the carb and noticed the hot start cable threads were stripped the plunger was still in the carb body(Would this cause a extreme lean condition)?. This bike has new valves, crank, cylinder and piston etc etc etc 0 hours on it Valves and timing good. Below are my specs any help would be greatly appreciated.

2007 YZ250F

285cc

Stroker Crankshaft

Heavy Duty (+3mm stroke)

Big Bore Cylinder Kit

(+3mm Big Bore Kit)

Stock Pipe & Silencer

458ft above sea level

Jet Needle OBEPQ Clip #4

Main Jet #178

Pilot Jet #42

Needle Jet #32

Starter Jet #72

Pilot Air Jet #105

Leak Jet #80

Edited April 18, 2013 by mikemay

[mikemay]

1. I am have no idea why my plug would look so lean with the big bore? I thought you actually go with leaner jets with the big bore? Would a strpped hot start cable make it this lean?
   

[indy rider]

I'd replace the hot start. With a #178 main jet you should be rich. I'm at approx. 800' elevation and I run a #162....but mines also ported.

[mikemay]

I am going to replace it for sure. thanks. It appears to be lean on idle to 3/4 trottle also. I wouldn't think the hot start cable thread being stripped would cause this if the plunger was still inserted into the carb..

Edited April 18, 2013 by mikemay

[mikemay]

I should add I had white deposits on the electorde ground that I was able to rub off with my finger. I ordered a richer pilot and leak jet today but all that I have read said do leaner. I am stumped why this thing is so lean if I am reading the plug correctly

Edited April 18, 2013 by mikemay

[mikemay]

Checked the float height and it seems to be around 2.5mm lower then it should be if I am measuring it correctly which I believe I am most likely. Using the manual.

Edited April 18, 2013 by mikemay

[sawworcs]

I had the same problem when I went to my big bore. Everyone said I needed to go leaner but I ended up going richer on the pilot and main. 35 hrs on the Athena 290 kit and the bike rips

[peaceofwork]

i,m running the 290 kit with needle obelp 5th clip,(4th sucked) winter 45 pilot 185 main. recently i was trying out 178 182 but 185

worked best.

webb cams and fmf pipe open air box

pacfic north west

[mikemay]

Thanks guys I order a 45 pilot and a 70 leak and new hot start cable nut. I should have got a richer main also but I will when I go to pick up parts. I am going to drop the clip on the needle as well. I will post back my resualts hopefully next week. Love this forum thanks again