emttrevor

I got a 302 with a 512 lift cam gt40x heads, 670 holley street avenger carb, hei distributor hooked to an msd 6a box. All was fine then I put a parker intake on and still all was fine bar some tuning issues got that sorted out now a month later I filled up with 91 gas 100% NO CORN like I normally do and within about 15 miles it started backfiring and then stalled took forever to start back up and then ran like crap all the way home left it overnight started it back up ran fine for about 10 miles gave it a full throttle run then did the same thing. Tried different carb, its not a carb issue cause new carb did same thing, vacuum checks out, fuel pressure checks out...whats next
I dont think its a timing issue cause it works ok when idling in garage and when you first start up, could it be just bad gas??? should I just empty tank and try new gas or am I missing something

JMD

My first guess is an intake leak, probably from the intake manifold to the crankcase...

I have found that the 298/302/351 engines with "big port" heads and/or "big port" intakes can be a bitch when trying to seal an intake to a head.

Some combinations leave very little room for error with regard to intake gasket placement, the sealing margins can be very, very thin.

emttrevor

well thats what I was afraid of, I guess Ill pull the intake and take a look this weekend

rtintwo

My first thought is vapor lock. Runs good cold, drive it a few miles and once its warmed up it acts like its starved for gas. Spring is almost here and temps have been warmer.

JMD

For sure my answer is just an educated guess, based on a few experiences with sealing up some of these engines. My son had a TOUGH time trying to seal a stock 80s-90s fuel injection manifold to his ProComp heads. It took him 3 tries to get it right, and on both failures we were looking for the problem to be somewhere else.

Another thing that led me to my conclusion is that everything was cool till the manifold was changed, then the problems appeared. I would ordinarily look for the carb to be flooding out, or for there to be a vac leak, vac hose off, etc, but since the carb was changed out, this stuff SHOULD be cool.

It might be a good idea to run a vac gauge on it before changing the manifold out. Assuming it is known what vac the engine should pull.

Oh yea, I almost forgot, some of these intake leaks can change intensity with temperature as the parts expand and contract.

67 evil eleanor

"stammering, farting, backfiring and stalling issue"......Yeah, it happens to all of us as we get older. As far as the car I would lean toward a spark issue. And then there may be other issues like with the fuel. I would attack the simple things first like new gas then move into the more costly issues.

emttrevor

well I redid the intake and its almost like it made it worse... I can idle and rev it up to 3000rpm in park but as soon as I put it in gear it stammers and backfires and finally dies.. I have checked and rechecked the timing changed springs and wieghts etc in ditributor same thing checked tdc all seems ok plugs wires are in correct places im about ready to pull the heads maybe cracked but no smoke or anything like that

emttrevor

YEH!!!! its fixed ended up being a bad coil...replaced and all is well..

JMD

Funny thing about internet diagnostics....

I used to answer just about every question like your with "check the coil"...

After being wrong about the coil 99,234 times, I moved on to a new thing to be wrong about.....

Sorry about the manifold.....

mr_velocity

I was going to say check the ignition. MSD boxes will fail if the battery connection to them becomes loose. For whatever reason they don't like that, I always carry spare boxes with me.

I lost a coil once in the boat, started running like crap so I stopped. When I pulled the coil wire off, coil was mounted horizontally, oil just ran out. Of all the spare parts I carry that is one item I didn't have. It was a long ride home on one motor

Glad it was an easy fix.