2003 yz 250 "detonation"

[SCHLAVA]

Hey guys i have a 2003 yz 250. It has stock jetting and the pro circuit gnarly pipe on it. Under load coming out of a turn the dam thing sounds like a cummings engine...Is this a sign of being to lean. I've always read about yz's detonating. My jetting is all stock..I was thinking of going up 1 on the main and pilot. I also run 94 octane pump gas at 40:1. The bike flat rips but Im scared to get on her since she sounds like a diesel under load and then it sounds like it's going to catch a ring on top. Sorry but that is the only way to describe it. Oh yeah. It was about 40-45 degrees sea level east coast woods riding. Any help would be appreciated.

Mark

 

[Spine_Thrust]

You sure it's not your chain? Make sure it's in spec...When was the last time you changed it?

 

[pace]

What throttle position(s) does this occur at? I'd recommend you make one change at a time, rather than changing both the pilot and the main. It only takes a few minutes to swap jets, and if you tinker with several things at once you won't know what's helping and what's hurting.

Read the jetting guide on Eric Gorr's site.

-pace

 

[SpectraSVT]

Your running lean. Mine does this with the PC platinum and FMF Fatty. Also, some YZ's require race gas when it starts detonating. For some reason aftermarket pipes cause the YZ to ping alot. But if the jetting is close it shouldn't ping hardly at all. Try richer main to start. Mine usually acts up around midrange than stops when I'm WOT. I need to go up one on the main for starters. Haven't ridden in a couple weeks so haven't had a chance to fix it.

Also, my YZ will instantly tell you if the pilot is correct or not. Too lean and it revs high from the get go. Too fat and it won't idle unless the air screw is all the way in. So after you get the detonating under control with the main/needle, go after the pilot if required.

 

[steve125]

Your running lean. Mine does this with the PC platinum and FMF Fatty. Also, some YZ's require race gas when it starts detonating. For some reason aftermarket pipes cause the YZ to ping alot. But if the jetting is close it shouldn't ping hardly at all. Try richer main to start. Mine usually acts up around midrange than stops when I'm WOT. I need to go up one on the main for starters. Haven't ridden in a couple weeks so haven't had a chance to fix it.

Also, my YZ will instantly tell you if the pilot is correct or not. Too lean and it revs high from the get go. Too fat and it won't idle unless the air screw is all the way in. So after you get the detonating under control with the main/needle, go after the pilot if required.

All good advise here, my first thought was the pipe. The FMF gnarly and PC platinum are off road pipes to enhance bottom power. In order to do this they tend to over-charge the lower end of the rev range more than the stock pipe, this can cause detonation in the rpm of the power increase. Richer jetting in the range of the ping or higher octane fuel will be the fix. Well??, U could put the stock pipe back on too, if not already damaged??

 

[SCHLAVA]

Yes it usually it gets rid of the detonation when she get up on the pipe but then i get this sound like a shearing noise...I put her away cause i didn't want to burn it up. I'm wash it tomorrow night and will start with the main. And as the description for the pilot it idles great so maybe I will leave it alone. Until i work out the detonation. Thanks all i'll be on it wed with the new main and bring a pilots with me as well

 

[Spine_Thrust]

Please, check chain specification. 10 links in 6 inches.

 

[SpectraSVT]

Be sure to check you radiator fluid. I ran my bike lean for about 5 rides(oops) and it went down alot. No more running lean for me.

 

[Lorin]

As to running lean, maybe it is just where you are at. My 03 is at sea level, and runs a 40 pilot. It has an fmf pipe \ silencer and v-force reeds. I went down (one step leaner) on the main jet, and run 93 octane at 40:1. No pinging problems here. My riding partner runs an 03 with the pc pipe (and same jetting) with good results as well.

 

[Spine_Thrust]

Have you checked the chain yet?

 

[SCHLAVA]

No i haven't, i will readjust when i get my new sprockets today. Bike runs great after changing the jetting but it was only riding around in a field. I'll see this weekend on the mx track and the following in the woods where it was most underload in sandy jersey! And I will adjust my chain!

Mark

 

[Spine_Thrust]

You may need to get a new chain if you have never replaced it. I am betting a dollar to a dime that this is your problem.

 

[SCHLAVA]

sorry, oring chain and new sprockets about 3 months ago with not much riding time due to broken leg/ankle recovery. But I will check tension.

Mark