Hi RPM misfire

[VortecRS]

I've got a 350 with vortec heads, edlebrock eps manifold, AFB carb and HEI distributor and when running through the gears about 3500-4000 it starts to missfire. I've checked to make sure the coil was getting plenty of juice and found battery voltage at the coil all my plugs are new and the plug wires are only a year old. Any ideas on where to look or what to look at to get this thing running good track season is starting soon and I want get it running good

[TheSilverBuick]

Re: Hi RPM missfire

What size carb? And is timing set right? I just recently had the mechanical advance on my HEI sieze up and when I put a timing light on it with the vacuum advance disconnected it didn't advance at all. With it connected I would get upwards 25* total timing full advance which was still far short of where it needed to be. Basically the engine would start missing right in that range. My new limitation is throttle linkage is preventing the secondaries from cracking more than a 1/4" and that shows up around 4,000rpm at full throttle.

[shaun callaway]

Re: Hi RPM missfire

if you rev it up to that rpm range in the driveway will it do it, if it does what is your timing doing? do you have a knock sensor?

[TheSilverBuick]

Re: Hi RPM missfire

if you rev it up to that rpm range in the driveway will it do it, if it does what is your timing doing? do you have a knock sensor?

VortecRS's posts says running through the gears, same here.

[shaun callaway]

Re: Hi RPM missfire

if you rev it up to that rpm range in the driveway will it do it? if it does what is your timing doing? do you have a knock sensor?

VortecRS's posts says running through the gears, same here.

silver

i can read that is why i asked the question he did not state that he tried that.

[VortecRS]

Re: Hi RPM missfire

if you rev it up to that rpm range in the driveway will it do it, if it does what is your timing doing? do you have a knock sensor?

With it out of gear it will still do it but it will be at a higher rpm range and not as much. Also the amount it misses is depending on how hard i'm pushing the gas down. If I slowly raise the rpms it won't miss but if I floor it it will. I do not have a knock sensor. I'm not sure what size carb I have.

[shaun callaway]

Re: Hi RPM missfire

do you know what the timing is doing when it missfires

[TheSilverBuick]

Re: Hi RPM missfire

Sounds to me like either the timing is off or a weak spark failing underload. Check that the mechanical advances look good and move freely. Even so another check would be to try and advance the initial timing some (as long as your engine isn't sensitive to detonation under load) and see if the mis-firing moves up in the rpm range or disappears entirely, if so then timing is probably your problem. If the problem just recently popped up the ignition coil could be weak or the plugs excessively gapped (or worn, but you said they're new). Doesn't hurt to check for burned plug wires against the exhaust either just to be sure the mis-fire isn't being caused by an arc to the exhaust.

[VortecRS]

Re: Hi RPM missfire

Im not sure what the timing is doing. I've been bumping the timing up for a while now. I put these heads on almost a year ago and boy I had the timing way too retarded but I was that way because I don't have a timing light and I was trying to play it safe. So I've been advancing it since then just a little here and there and it has been getting better but recently this missing has gotten a lot worse so I'm not really sure what the problem is.

[TheSilverBuick]

Re: Hi RPM missfire

Too far can do the same, especially if the vacuum advance is pushing. As a crutch when I found my problem to get by until I had time to pull the distributor I turned the initial advance up in steps testing it. Eventually it got to the point of surging while cruising (too much vacuum adv.) and it would still miss above 4,000rpm at WOT, so I backed it off until the cruise surging stopped. Some numbers from a timing light would be good. It'll probably be a giant red flag or say start looking else where.

[VortecRS]

Re: Hi RPM missfire

If i can remember correctly when I had the distributor out when I was swapping heads the mechanical advance worked. So I need to get a timing light then.

[TheSilverBuick]

Re: Hi RPM missfire

Recommended. If you don't have timing tape on your balancer you may want to get it or just spring the extra cash for a fancy timing light with the knob (I forget the proper name for it) to see what your timing is doing outside idle.

[shaun callaway]

Re: Hi RPM missfire

dial back would be the name

i have used white out and it works pretty good to.

[VortecRS]

Re: Hi RPM missfire

I've got silver mark on my black balancer so knowing where the crank is at is not an issue. My uncle has a light so I might see if i can borrow it or just see if he can help me out with this. He has helped me in the past with getting things tuned right.

[Scott Liggett]

Re: Hi RPM missfire

misfires can be from a lean out condition at higher rpms. Not enough fuel to feed the air. Also, check the vacuum advance if you are using one. Pull the hose off the carb and suck on it. If you are sucking air, it's bad. The vac advance line should be attached to a ported vacuum source. No vacuum being pulled through it at idle.