Chevy 350 runs and idles fine. But has no passing power. Also sometimes diesels (continues to run a bit) when shut off. What could be the issue?

Vince Moore

Asked by Dec 07, 2015 at 06:38 PM about the 1975 Chevrolet Impala

Question type: Maintenance & Repair

Engine did have a bad cam so I installed a
new one. I can cruise around town but if I try
to pass someone or speed up it goes no
where. Will sometimes backfire while
crusing. Also causes the trans to shift funny
when on the gas. Any help would be greatly
appreciated. I made sure firing order and
timing were correct. All vaccum lines are
hooked up. Even the one to the trans. Wich
would normally cause the shifting problem.
I'm thinking it could be the carb getting to
much fuel seeing as the carb was off
another 350 that wasn't running right. So it
was adjusted to run decently till I got this
engine going. I do have a mechanic as a
father but he hasn't had the time to be able
to look at it. So I'm stuck trying to figure it
out myself and am kinda stuck.

11 Answers


What do the spark plugs look like? lean, rich or OK?

Vince Moore

They were ok when i put them in but Haven't checked them since it started. What would it look like if it was lean or rich?


Black if rich and burnt if too lean.

Dylan Detlor

If the timing is right u maybe getting to much fuel but it maybe backfiring because the timing is off a degree have you tried to tune it by turning the distributor and like full of regrets asked what do the plugs look like if it is running rich then u will have a high build of carbon on the plugs, something that happened to me years ago is I did a tune up on my truck plugs wires cap and rotor it ran fine until I got on the high way then it would spit sputter and die out when I would step on the gas when I opened the hood the brand new plug wires I had just installed had a defective coil wire and it was causing the rest of the wires to arc out in other words the engine compartment was lighting up like a Christmas tree I put the old coil wire back on and problem solved


Another chart that is easier to read --- m

Dylan Detlor

Is the coil on the engine or is it on top of the dist cap?

Vince Moore

The second link didn't work? But the first one is useful. Still have to check the plugs. And coil is in dis. Cap

Dylan Detlor

Okay it diesles u said that means the timing is off just a bit loosen the dist and turn it until it runs smooth at an idle and runs smooth under rpms it will take a few trys to do it but it sounds like this may help. Be sure to do this with the engine running and warm. TURN IT A LITTLE AT A TIME

Vince Moore

Found out vaccum advance was bad. And some choke items bad. Fixed them. It now runs good. But still has no power when trying to pass. Any suggestions?

Dylan Detlor

Try advancing the timing just hair and see if that makes a difference

Horacio Perez

I had the same problem on my 75 impala,it would run good but as soon as i would try to go pass 40 mph it would slow down backfire and lose speed...The problem with my car was a dirty fuel tank/fuel tank sending unit..after getting this replaced and tank cleaned my car runs good now..(i bet thats your problem)

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