YZ250 Weird Idle?

I just got back from riding at pismo beach. While I was there I noticed that my dad's 06 yz250 had a weird surging idle, then about 7-10 seconds later idles normal. It started about 7 minutes after riding hard, lots of wot and jamming from gear to gear. It only does this after the bike is warm and lots of wot and then stopping. I think it may be detonation as it sounds like knocking. I didn't notice any pinging while riding under load at low throttle opening, just surging at idle. I turned the air screw out half a turn to see if it would help and didn't notice a difference. It has been jetted for 40:1, pro cicuit pipe and silencer running 91 octane fuel. I may try some av gas but not sure if it is safe for the bike?

It is lean when it does this. Turn air screw in to richen pilot circuit. If not better, get a richer PJ.

It is lean when it does this. Turn air screw in to richen pilot circuit. If not better, get a richer PJ.

Oh shoot! Turn air screw in? I thought it was the opposite!! Haha, that's why it didn't seem to get any better!!

Oh shoot! Turn air screw in? I thought it was the opposite!! Haha, that's why it didn't seem to get any better!!

RBYM, your confusing the air screw with a fuel screw as found on 4 bangers.

2T's use an AIR screw - Turn IN to richen mixture (turning it in limits the incoming air, thus richening the mixture)

4T's use a FUEL screw - Turn IN to lean mixture (turning it in limits the incoming fuel, thus leaning the mixture)

Bergermeister

RBYM, your confusing the air screw with a fuel screw as found on 4 bangers.

2T's use an AIR screw - Turn IN to richen mixture (turning it in limits the incoming air, thus richening the mixture)

4T's use a FUEL screw - Turn IN to lean mixture (turning it in limits the incoming fuel, thus leaning the mixture)

Bergermeister

Thanks, I guess I was making it worse instead of better! I'm going to check the plug and see what color it is. Do you think the engine is still ok?

Thanks, I guess I was making it worse instead of better! I'm going to check the plug and see what color it is. Do you think the engine is still ok?

I wouldn't be getting too worried yet mate. Check the plug and make sure its not pitted. You could always wipp the head off and check its condition along with the piston crown.

You'll do more damage running lean at high rpm (lean main jet or needle circuit).

I reckon you'll be sweet. But I'd definately check that plug. It should look like a nice tan color, not white.

I think we've all buggered up our idle circuit jetting from time to time. I wouldn't be getting too stressed at this point.

Bergermeister

Just a thought as I had this problem last season.... check your cylinder head... if it's not tight you may be sucking air into the cylinder. It could also be the head gasket. Mine was doing this and I didn't pay much attention to it... then halfway through a super gnarly cross-country race my bike got so hot I blew the rad hose and did some damage to the cylinder.... I thought it was the pilot jet, air screw or reeds.... but when my mechanic pulled it apart... he discovered it was the head... a new cylinder, gasket and piston kit later... the bike runs super! No more surging at idol... I learned a hard lesson... but it's one mistake i'll never make again. Cha-ching :worthy:

Just a thought as I had this problem last season.... check your cylinder head... if it's not tight you may be sucking air into the cylinder. It could also be the head gasket. Mine was doing this and I didn't pay much attention to it... then halfway through a super gnarly cross-country race my bike got so hot I blew the rad hose and did some damage to the cylinder.... I thought it was the pilot jet, air screw or reeds.... but when my mechanic pulled it apart... he discovered it was the head... a new cylinder, gasket and piston kit later... the bike runs super! No more surging at idol... I learned a hard lesson... but it's one mistake i'll never make again. Cha-ching :worthy:

Man that sucks, I'll definetly check the cylinder head.

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