Big Bore 285cc Help

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

DSCF1259.jpg

Edited by 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?

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.

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 by 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 by 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 by mikemay

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

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

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

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now