1978 454 missing/backfire
 

I have a 1978 454 that seems to have a bit of a hesitation and a little backfire out of my exhaust? I've been fighting this for a few weeks now. I have replaced wires, cap, rotor, plugs, vacuum advance... I can't seem to find the problem? It makes a slight squeel that sounds like it's coming from #1 just as it sends out a little backfire through the exhaust? Does anybody know what this might be?
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Re: 1978 454 missing/backfire
 

First thing I would do is a compression check. Have you pulled the valve covers to look to see if all the rocker arms are moving like they should? Motors need three things, fuel fire and compression. Looks like you have just worked on fire.
Jimmy

Re: 1978 454 missing/backfire
 

I will get on it ASAP and let you know! Thanks Jimmy!!! What compression should I expect? It's had a little head work, headers and a small cam. All done prior to me getting it.
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Re: 1978 454 missing/backfire
 

A slight pop out the exhaust could be ignition, valves to tight and/ or not seating, weak springs, flat cam lobe, and a/f ratio wrong. Before the compression test I'd adjust the valves. Any that are to tight will cause a miss because the valve is hanging open. Far as the numbers to look for I don't know for a bbc.

Thanks TJ! I appreciate your feedback and will be checking on your thoughts.
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Okay, I checked my plugs and the gaps were at about 60. Is that normal? I re gapped them to 47. Also, my timing seems to be jumpy? What causes that? My vacuum advance is new and working fine. I can't bind any other vacuum leaks? This thing starts right um and after about 2 minutes sets at a steady idle of 550-600 but as soon as I try to give it any fuel it has hesitation and continues to hesitate even at a steady speed? If I'm rolling down the road at 25mph holding it there it's slightly jumpy and when I give it gas it's even a bit worse with the hesitation? Anything helps...
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Re: 1978 454 missing/backfire
 

Maybe just a lack of initial timing!

What's your timing curve like?

Re: 1978 454 missing/backfire
 

Like Geezer mentioned, possible lack of initial. Do you have the vacuum advance on Ported or Manifold?