MGB: more dizzy in Hudson Valley

Hello and help! OK, point dwell perfect, timing on the mark, carbs balanced, mixture correct.... Race engine to 3500, 4000 rpm and after 1 to 2 minutes engine sputters, misses, gets rough, allow to return to idle, idles rough but smooths out within a few seconds. Could this be valves in need of adjustment ? Hope not broken springs. Will pull valve cover tomrrow and inspect.. Any suggestions.

What dizzy and coil are you using ?

Check your gas filter. Take it off and see if you can blow through it easily, if you can't it's clogged.

Great minds think alike, and (as my daughter says) fools seldom differ!! :o) Fuel filter!!

I'm with Big Al on this one. Swap the coil if you have a spare.

Wouldn't the coil continue to stay degraded though, and not recover? I think that is where the fuel filter diagnosis is coming from.

The one that has gotten me time and again has been a bad condensor. Runs, quits, let it cool and it runs again. It happens so rarely, that I forget it until I have done everything else then go "Doh!".

I'm w/JDW...
Fuel filter or pump not keeping up for some other reason.

Could a partially blocked gas tank vent cause this problem?

Float level way too low? Sounds like there's enough fuel available for idle, but the engine sucks more fuel than is available at higher RPMs. When back at idle, the problem goes away one the fuel supply is replenished. Clogged fuel line or faulty fuel pump might also be the culprit. Anyway, I'll bet that if you shut the engine down while it was running it's worse, you'd find a lack of fuel in the bowls.
I doubt that it's the coil, since the problem appears to be strictly RPM related. If it was quitting when hot, you'd probably have to shut the engine down to let it cool before it would return to normal.

Not if the tank was only half full, and you would really hear it when you opened the cap!!

Stiill dizzy in the Hudson Valley, Swapped fuel pumps, fuel filter, coils, distributors, condensors, ign wires and carbs, opened floats and full, level correct, needles clean, idles fine, rces to 2000, still ok, go to 2500..3000, sputters, backfires and spits out of rear carb, enginne rourh, valve train gets clanking, rattling, returns to idle and smooths out!!!! Pulled plur wires, ne at a time and all are working and each cause engine to get rough at idle, as it should. Compressione 170 t0 180. no smoke. Going crazt, valve springs look good and clearances correct. I fear a burnt valve ?

PS , Thanks for all the previous suggestions to all. All help is appreciated. Desperately need to hit the roads in the B!!

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