Hi RPM misfire

[Dans 83]

Re: Hi RPM missfire

Let us know what you find after you get the timing light on it.

[VortecRS]

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.

I'm pretty sure I've got the vac adv hook to the carb right. I've had this carb for five or so years on a couple engines and it been on this one for more than 3/4 of those years. If i can get my uncle to look at it he'll make sure the carb is set right as well as the timing thank you everyone for your help

[VortecRS]

Re: Hi RPM missfire

I borrowed a timing light from my uncle today. Disconnected the vacuum advance and I had vacuum leaking so I knew I had it connected wrong especially when the car died cause it lost the timing. Fixed the hoses and left the advance disconnected with the port plugged and fired the car up again the timing was right at or before TDC advanced the timing abit and then the adjustment screws on the carb and took it out for a spin. It is alot more responsive but it still misses. Now it starts missing close to 4500. My plug wires are good and the plugs are new like I said earlier. So now what?

[Dans 83]

Re: Hi RPM misfire

Does the timing seem to bounce around even at low RPM when you put the timing light on it?

[TheSilverBuick]

Re: Hi RPM misfire

A few more things. What are your plugs gapped at? Being an HEI they are kind of known for falling over above 4500rpm in stock form, some more than others. The usual fix is an aftermarket Module and Coil, what kind are you running? Stock?

Also has the problem existed as long as you had the "new" heads on it? Just wondering incase the possibility they flow more air at lower rpm's causing as Rebeldryver suggested a lean out mis-fire because the carb is no longer large enough.

[HoosierGTA]

Re: Hi RPM misfire

Valve springs maybe. While your at it, put a bigger cam in it. Just joking about the cam. See ya wed. .

[smallblockjay]

Re: Hi RPM misfire

Vortec heads don't like a lot of timing, in general, but they do like a little more than TDC. 8) I would think 28 to 30 degrees BTDC total timing would be a good starting point. Vaccum advance needs to be conected to a vaccum port above the throttle blades, not below. If you have it to a port at or below the throttle blades, you have constant vaccum on the advance. This could have been a problem, if the engine died when you disconnected the vaccum hose and the timing was at TDC when you checked it. It could have been at the wrong vaccum port, causing it to be advanced and when you pulled the hose off, the timing fell. With the vaccum advance connected to the correct port, and after making sure the mechanical advance is correct and the correct springs are on the mechanical advance, time it to 28 degrees total timing with the vaccum and mechanical advance all in. Once that's done, if the problem still exists, other things can be looked at and ruled out one at a time.

[VortecRS]

Re: Hi RPM misfire

My distributor is basicly the exact same way it was when GM built it in the late 70's or early 80's so what ever the mechanical adv and vac adv is set to is kinda what I am stuck with now. I think I have the intial timing at 8 degrees BTDC but I have no clue what the total timing is. I'm running the plugs and gap that is recommended for the truck the heads came off of. I'm really trying to think back to before I did the head swap and remember if the miss still happened at high rpm's and I think it did. So maybe the carb is too small and it can't get enough air through it. Idk but hey when I stab the throttle down now it is really responsive. Sunday night driving around crossing a street it really ripped loose. It really hadn't done that in awhile. Thanks again for your guys help

[Dans 83]

Re: Hi RPM misfire

The reason I had asked if the timing was erratic when you checked it with the timing light is because there is a magnet in the base of the distributor and if it is cracked or broken it could be causing a miss.

[smallblockjay]

Re: Hi RPM misfire

That's a good thought. Really, for the money, it wouldn't hurt to replace the dizzy with a new aftermarket anyhow. Or at least a rebuilt stocker with an aftermarket coil and module. I run a factory HEI distributer with an Accel coil and MSD 6AL box and it works well.

[min301]

Re: Hi RPM misfire

I think he's running too lean on the carb, or is low on fuel volume.

[smallblockjay]

Re: Hi RPM misfire

Could be a secondary fuel metering issue, I guess. Forgot what the carb was on this application. If it's a fuel supply problem, there's a problem with the fuel line, like a clogged filter or pinched line, or the float level is not right.

A good distributer is a good thing anyhow.

[smallblockjay]

Re: Hi RPM misfire

BTW

Ya know, it's a funny thing...I'll sit here and take my guesses and offer my advice, and my stroker just started the same symptoms recently. Only difference is, it was running great and just started doing it. I took it out, did several burnouts and banged through the gears, brough it home still running great, parked it and left it for several weeks. Next time I fired it up, it ran fine, but falls on it's face at about 4500. I've played with a few things and haven't got it yet. But I have to admit that I haven't spent enough time working on it to figure it out. I re-wired the MSD box because some of the connections seemed corroded. Next, I will pull the dizzy and take it apart and clean it, make sure there's no corrosion. After that, I will look at the carb, specifically the vaccum secondary diaphram (sp?) and the secondary fuel metering. It runs great on the bottom and will actually do a burnout at slower speed, but it falls on it's face if I push it too hard.

Anyhow, just had to laugh at myself for offering advice on a topic that I haven't solved on my own motor yet. It's a good thing I know everything. ;D

[HoosierGTA]

Re: Hi RPM misfire

Just courios whats a dizzy? Pick-up coil? The carb if I remimber is a 600-650 carter(like a edelbrock)

[VortecRS]

Re: Hi RPM misfire

Just courios whats a dizzy? Pick-up coil? The carb if I remimber is a 600-650 carter(like a edelbrock)

Umm its a distributor and I thought the carb was around that size but I just couldn't remember. The wife had a flat yesterday so we had to drive my car around and the car was running great I could take it to 4500 or so and it would do it with out hesitation. It was great I just hope it will run like that again today. My fuel filter is in a clear housing next to the carb so I'll take a look at it and make sure it doesn't need cleaned. Maybe I need to get some K&N cleaner for my air filter.