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

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.

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.

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.

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

 

thanks to everyone for all the suggestions

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

Thanx!