71 350 has rough idle and very poor acceleration with some backfiring through the carb if I really get on it.

Asked by John Jun 03, 2014 at 09:44 PM about the 1971 Pontiac Le Mans

Question type: Maintenance & Repair

It is a 350 -2bbl originally, and 4bbl conversion many years ago by the looks of it.
Starts well, but just has no acceleration.  If I couldn't feel plenty of exhaust coming out
of the  pipes, I'd swear they were plugged up, that's how restricted it feels when I
accelerate.  Last point to make is that 6 of the 8 sparkplugs all appear to burn
differently.  Do you think this is carburetion or a timing chain problem?  I've heard these
motors had plastic coated chains that sometimes skipped a tooth or two when the
coating cracked.

25,455

chassy answered 10 years ago

you got everything happening at once . restricted exhaust,blow back thru carbs,4 barrel stuck on by who knows who,2 plugs firing well & 6 not or 6 firing well and 2 not {your question doesn't say}.... Fire in the carbs is timing or valve...... Plugs different is maybe wires,lousy rotor,points,condensor....... Lousy exhaust pressure ( using your hand as a gage isn't exactly according to Hoile) but if you have enough experience you can get a feel for whether it's off. might be because the engine is running like s__t.......So where to start??? I'd start with making sure the timing is at specs [never going to run right if timing is off , then work forward from there.

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25,455

chassy answered 10 years ago

run straight pipes until you get the engine running well because if the back pressure is indeed too high you aren't going to get it running well to start with

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