carbed EJ18 misfiring and stumbling

[LPGsuperchargedBrumby]

My carb an distributor ej18 has developed a misfire/stumble at under about 2500rpm and low vacuum/open throttle.

 

it was running great until i drove over some very soft sand and had to hold nearly wide open throttle in 1st gear for about 1 & a half minutes to keep moving....once back on a hard surface and less rpms this misfire appeared

 

it starts and idles perfectly and revs all the way to redline while stationary without a problem, the misfire is most pronounced in 3rd,4th&5th gear below 2500rpm under fairly heavy throttle

 

at over 2500rpm(on the road) it runs like a champ from only having the throttle open enough to hold 2500rpm right up to WOT.

 

i thought it was the coil breaking down so swapped that for a new one that i had....that made no difference. I also cleaned the contacts on disty cap and rotor (i can't get new one's at short notice),and checked the plug gap.

 

after doing that i took it for a test lap....and its now backfiring as welli'm

 

starting to run out of ideas of what the culprit is....anyone have any suspects?

 

Thanks

[hatchsub]

Does your distributor have a vacuum advance? If so check to see if it still works. Without that it could cause a stumble. I drove around without vacuum advance for awhile so thats how i kno.

[TomRhere]

Two things that come to mind is;

 

Dirt in the carb, Vacuum leak. Either one would cause a mis-fire or backfire due to the fuel mixture being lean.

[idosubaru]

bad valve stem seal could cause this too. i'm a carb idiot so hopefully those other suggestions get you fixed up. if everything keeps falling short a compression/leak down test might show something.

[LPGsuperchargedBrumby]

found the problem....plug leads were failing under heavy load....new plug leads and the problem disapeared.

 

thanks to everyone for all the suggestions

[Loyale 2.7 Turbo]

Could you Post Pics of your Carbed EJ18? ... I have never Seen any Carbed EJ Series Engines.

Thanx!