[skullandpistons]

I hate for my first post to be a cry for help but here goes. Haha. I have a 350 with the hei distributor. The thing is the plugs, the wires, the cap, the rotor, and the ignition coil have all been replaced. It doesn't miss or back fire when its idling or you just barely give of throttle. Well it does miss at idle but you wouldn't notice unless you looked for it. But if you try to drive it normally the it starts missing and backfired alot. There is a whistling noise that decreases in pitch with rising done which makes me think there's a vacuum leak. Or of could be an intake manifold leak. Could any one of these cause a miss fire/ backfire?

[prumora1]

try checking your vacuum advance. also if think there's a vacuum leak try spraying around the engine while running with wd-40. it will find it when it sucks in the fluid and changes the engine idle.

may want to check for a burnt valve as well.

[vinniekq2]

If you have a heat gun,you can compare the cylinders.when you find the problem area you have something to work with.
maybe a vacuum leak,maybe 5&7 have a cross fire in the wires,,,

[skullandpistons]

The wires aren't mixed up can the spark jump even if its a brand new cap? And also how do I check vacuum advance? I now those wires in the distributor can wear out over time from being flexed by the vacuum advance. Is that what your talking about?

[jaw22w]

I like to check for vacuum leaks using starter fluid. Spray at carb base and the head to intake manifold mating surface. Engine will speed up if you have a vacuum leak. Starter fluid won't leave a mess like WD-40.
To check amount of vacuum advance, disconnect vac adv hose from distributor and plug. Rev motor until all mechanical advance is in, then check timing. Reconnect advance hose, rev motor until all advance is in. Check timing again. Subtract the first timing number from the second number. That is your total vacuum advance.

[Jimbo17]

Try adding one quart of Marvel Mystery oil just to make sure it is not a lifter sticking every once and a while.

This happened to me with my 350 and after changing plugs,wires, distributor, vacuum hoses and a few other things a friend of mine told me to try the Marvel Mystery oil and I never had the problem again!!!

The funny part is it feels just like an electrical problem and it only did it every once and a while.

I spent a few hundred bucks and I learned a good lesson.

I would also suggest a few cans of dry gas just to make sure it's not moisture in the gas tank which would also give the engine a miss feeling.

Jimbo

[bigblockragtop]

Sounds like a vacuum leak, you can use either method above, you can try to choke the carb with a rag if the idle goes up usually it is a vacuum leak if it dies immediately then maybe not. You can also use a propane torch UNLIT putting the torch around where you think it might be leaking and the propane will change the idle. If you can hear the whistle try to see where it is coming from, brake booster, manifold etc.

[68NovaSS]

Originally Posted by skullandpistons

I now those wires in the distributor can wear out over time from being flexed by the vacuum advance. Is that what your talking about?

No, Vinnie's talking about the #5 and 7 spark plug wires crossing each other, touching.

[skullandpistons]

I'm going to try the oil and hunt for the the vacuum leak. Thanks for the help. I'll post what I find out in the morning.

[41'srfun]

Originally Posted by skullandpistons

I hate for my first post to be a cry for help but here goes. Haha. I have a 350 with the hei distributor. The thing is the plugs, the wires, the cap, the rotor, and the ignition coil have all been replaced. It doesn't miss or back fire when its idling or you just barely give of throttle. Well it does miss at idle but you wouldn't notice unless you looked for it. But if you try to drive it normally the it starts missing and backfired alot. There is a whistling noise that decreases in pitch with rising done which makes me think there's a vacuum leak. Or of could be an intake manifold leak. Could any one of these cause a miss fire/ backfire?

I had a similar problem recently on my daily driver. After changing the HEI dist. cap and rotor, new spark plugs and new Acell plug wires it didn't help it one bit. I was readjusting the timing last weekend and just for the heck of it after i got the timing readjusted, I started moving the timing light to different plug wires and watching the light. When I got to the #7 wire the light was flickering real intermittent. I pulled the #7 wire off and replaced it with one I had in my"junk" box and the miss went away. This was a new set of plug wires so I didn't expect anything to be wrong with them, but I did have one that was bad.

[skullandpistons]

My problem.seems like it is more than one cylinder. I could be wrong. Ite also not a steady miss it very erratic and seeks to only do this at higher speeds or there's a load on the engine. Like it'll go through 1st and 2nd "alright" but when third or fourth kicks in it starts missing and backfiring. I'll check the wires again tomorrow.

[oldbogie]

Originally Posted by skullandpistons

I hate for my first post to be a cry for help but here goes. Haha. I have a 350 with the hei distributor. The thing is the plugs, the wires, the cap, the rotor, and the ignition coil have all been replaced. It doesn't miss or back fire when its idling or you just barely give of throttle. Well it does miss at idle but you wouldn't notice unless you looked for it. But if you try to drive it normally the it starts missing and backfired alot. There is a whistling noise that decreases in pitch with rising done which makes me think there's a vacuum leak. Or of could be an intake manifold leak. Could any one of these cause a miss fire/ backfire?

Yes a vacuum leak would make a lot of noise at idle because the manifold vacuum is high so the suction pulling on the leak is very strong. This sound would diminish as the throttle was opened because this results in reducing the manifold vacuum thus the leak rate.

At idle inspite of the added forces to draw more outside air its influence on mixture is somewhat less because the idle mixture tends to be richer than cruise so to some extent the problem as it causes miss fires is more hidden as well as the lower power on the crankshaft at that point results is much less effect of one cylinder dropping out being not so noticable.

At cruise it will be more noticeable because the mixture is leaner at that point so the improper fuel fuel mixture will be more likely to miss fire. Additionally, the power on the crank is higher so a miss fire is more noticable by the force not applied.

If it's rather a one cylinder miss fire instead of random over all the cylinders it probably shows the leak is local to a port opposed to leak at or under the carb. or something feeding the plenum like the PCV valve.

Bogie

[skullandpistons]

Alright guys. I've found the problem. It's and intake manifold leak at cylinder 2. I was spraying around with carb cleaner and when I got to that spot it bogged down. Thanks for your help.