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Stumbles and backfires through intake when I press the gas

Tiny
RICHARD48
  • MEMBER
  • 1985 CHEVROLET CORVETTE
  • V8
  • 100,000 MILES
It has new wires, distributor cap, and spark plugs non of which has helped. Engine only has about 35,000 miles on it. Checked the compression on several of the cylinders and they all measured about 150. This problem started off as solid miss at low speed and progressed to a miss with the backfire.I have put #1 TDC and confirmed the rotor button is pointed to the #1 terminal on the distributor cap.

Any help would be appreciated.

Thanks,
Richard
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Saturday, April 25th, 2020 AT 1:30 PM

3 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
If timing is correct then we need to suspect a worn camshaft. This will not show up in a compression test because the valve is able seal but it is not able to lift the valve enough to allow proper air flow which causes the air fuel mixture to back up in the intake and then ignites.

Here is a guide that talks more about this:

https://www.2carpros.com/articles/engine-backfires-while-running

Have you taken a running compression test? Basically the same thing as a compression check but you let the engine start and run for a few seconds.

The real way to confirm a cam lobe being worn is to measure the lift. I attached the spec. You should have just short of a half inch of lift on this engine so I would suggest we measure it if we find low running compression.

Let me know if you have questions. Thanks
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Saturday, April 25th, 2020 AT 4:02 PM
Tiny
RICHARD48
  • MEMBER
  • 6 POSTS
Thanks for the response. I started with the easy approach, cleaning the MAF bit it didn't correct the problem. I think the next step is to remove the valve covers so I can see what is going on there.

Thanks again, I will let you know what I find.

Richard
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Monday, April 27th, 2020 AT 4:41 AM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Sounds great. Keep us updated on what you find and we can assist if needed or you can let us know what fixed it because I am curious about this one. Thanks
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Monday, April 27th, 2020 AT 6:05 PM

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