previously posted about intake manifold backfire in my 1996 Volvo Penta 5.8L ford 351 Winsor. Having a complete engine rebuilt. Heads rebuilt twice.

carb rebuilt, eliminated all electronics( went straight to battery.) coil is good. timing is set correctly according to the mechanic that rebuilt it. new spark plugs twice and new wires to plugs in order. pvc valve assembly is good. did a smoke test and found a leak in intake manifold gasket and I fixed that and retested and its good.
I'm at a loss. If I spray a steady stream of WD40 into carb while running in about 2 to 4 seconds it stops backfiring. When I stop spraying in about 2 to 4 seconds it comes back.
If I pull the #8 spark plug wire off while running , the backfire stops and engine idle goes up about 100 rpm and seems to run smoothly. What is going on???? Anybody ever have something like that happen? I've had all kinds of suggestions and pretty much tried them all, its been a mystery to a lot of people.

Sounds like a lean backfire; did you go thru the carb? What do all 8 spark plugs look like?

The way you are describing if may be a wiped cam lobe. You need to pull the valve covers and turn the engine over watching if any of the rocker arms are not moving as much as the others.

Richard,
Were any of the recommendations suggested in your last thread on this topic (link) carried out?

ESGWheel said:

Richard,
Were any of the recommendations suggested in your last thread on this topic (link) carried out?

Click to expand...

most if not all.

My suggestion is to perform a leak down test. Start with No. 8 but work it thru all the cylinders. Post results.

Thanks for your reply, My mechanical skills are extremely limited. I'm not sure how to do that?

It requires a tool (about $100, link) and an air compressor. If you do not have a compressor would be best to take it somewhere to do this. But if u do, even a smallish pancake compressor used for nail guns would work. Do u have one? Also, the results can be a little tricky to interpret. The reason I am suggesting it is that all indications are leaning towards no. 8 not sealing up properly and this should confirm it. IMO, despite all the work done and based on the symptoms described, there is a fundamental mechanical problem with the motor and a leak down test can help figure it out.