Not VMX but hey I'm sure someone can help

[Paul552]

I just bought a 2010 KTM 85 for my son.
It is supposed to be just rebuilt top and bottom including new crank. (so I paid too much)
Any way it runs great at idle and full throttle but if you hold it at 1/4 throttle it pops and carries on. Miss fires bad.
If you roll the throttle on it is fine.

Any way I played around with jetting and it made no change( I thought the pilot was lean)

Tried cleaning carb new plug new fuel

What could it be?
Sounds electrical? Bad CDI?

Could air leak cause it weird only at 1/4 throttle

[adam.m]

Blown base gasket possibly

[Paul552]

Blown base gasket possibly

Do you recon that that would only effect 1/4 throttle??

If you roll it on it is fine?

It pops so bad it sounds like it will stall.

Throttle it more and it screams?


Do you think spraying it with Wd 40 to check for a leak?

[ozktm]

Check the reeds??

[Paul552]

Check the reeds??

Reeds were perfect

[Mike52]

Blocked needle jet ?

[OverTheHill]

Probably not relevant but food for thought. Years ago i had a small carb with a blocked airbleed to the needle jet [emulsion tube], was a small hole & had a bit of bum fluff in it stopping air flow then ran super rich by memory at that throttle opening, started altering slide cutaway & dropping needle etc before finding the cause. Just another [lesson learned] cock up i'll never forget from 45 years ago. Was a Yamy CS2E 180 Twin [one side problem only].

Just to add another curved ball!!, i did a crank job on an older KTM125 a few years back & it ran terrible getting through midrange, couldn't get it to go through clean [was real bad] & "was" the reeds [replaced with good aftermarket set]. Lesson learn't again.

Back in our bucket racing days of "BIG" porting i had the same blubbery [un-rideable bad] between idle & midrange--just where the power came on in big bounds & some smarty suggested doubling up the reeds [steel original reeds they were], so did that & sorted it 100%. Was a short lived motor with a big reed block welded into a mudbug100 cylinder but anyway--food for thought [reeds]. Another lesson learned--just so much to learn!!. Cheers.

[OverTheHill]

Just to add one more [reverse idea] maybe they have a "screw in air bleed" to the emulsion tube in carb mouth that's has come out [gone through motor i hope not], which would make it very lean in that area, or someone's drilled out the air bleed to try & overcome a problem caused by something else. Sorry, not helping much i know--just more ideas.

[OverTheHill]

one more thing before i pull my head in. maybe a bit off track or unlikely but i don't suppose someone has shortened the chamber in the mid section or header pipe!!.

[Paul552]

Nope on all over the hill.

I going out now to have another look