BigDreams92

iTrader: (1)

Like the title says, I'm backfiring through the carb at higher RPM and with quick snaps of the throttle. The popping through the carb seems to increase with RPM. Is my timing retarded too much or is it fuel delivery related? I set it with an adjustable timing light at 10 degrees advance (I think, not entirely comfortable with the adjustable light). I didn't have this issue for the first few hundred miles, but I've never really tried to get down on it till now. It's a fresh Vortec Headed 383 fueled by a Quick Fuel 750 DP with mechanical secondaries and proform main body.

Tobias05

what kind of dizzy you running, and what does your mechanical and vacuum advance look like?

BlkBird408

Also might try getting the ignition module tested.

crazynights

iTrader: (3)

10 degrees, I'd move that up to 12-14...but if I was you, I'd set total timing more than initial timing.
I have a vortec 383 and mine is 34 degrees at 4000rpms.

BigDreams92

iTrader: (1)

It's an Accel HEI Vac Advance set up (SuperCoil etc.). Cap and rotor are new, new wires, etc. I set the timing with a adjustable light, and if I set it correctly base timing was 10* and hooked vac line back up and revved it up to just over 2k and was at 34*. I'm just thinking maybe it's way retarded b/c I screwed it up by not being sure on how to use an adjustable timing light.

This is what I did, and if it's wrong, be sure to me:
I disconnected and plugged the vac advance line. I set the timing light@10* and moved the dizzy until the marker on the balancer hit the zero mark on the timing tab, then I changed the timing light to 0* and rechecked initial timing and the marker was now crossing at 10* on the balancer. Plugged the vac advance line back in and put the timing light at 34* and revved it up, full timing hit the zero mark on balancer at about 2300rpm.

Sonix

iTrader: (1)

No, you leave the vacuum advance line disconnected. You want 34* WITHOUT vacuum advance. Actually i'd go for 32*, vortec heads don't need a schwack of timing. This will give you ~14* base I imagine. You can back off your idle screws to lower your idle speed back again. Your base timing is what it is, your WOT timing is what you want to control (without vac advance!)

With vac advance you'll have around 45* timing under low throttle cruise conditions, and that's just fine.

Rev it up to 3000RPM or so. Find the point where timing no longer advances, should be around 2800RPM. If its way low of that, use stiffer springs in the dist, if it's still advancing at 3500RPM, use a lighter spring combo. You want to be all in by around 2800RPM.

Then you can see if it still backfires. I'd look into the pump shot being too small or something.

BigDreams92

iTrader: (1)

Thanks Sonix, would this scenario explain both the quick throttle backfire and the high RPM backfire?

And to clarify, the vacuum advance line was hooked up at this point, but I was just seeing where I seemed to hit full advance, I set timing with the vac line plugged. However, I didn't test much higher than where I hit 34*, I'll rev higher and see if it's perhaps a spring issue.

Keep in mind, that on the quick throttle hits, the RPM hasn't even tipped over 1500 when it backfires. Are the springs allready compensating that much b/c of how far I've got the accelerator mashed?

Sonix

iTrader: (1)

Well, i'd setup up the timing curve first off, then deal with any carb issues. They seem to disappear once the ignition is all sorted out

Yea, i'd leave the vac line disconnected for all timing matters. Unless you're specifically checking your vacuum advance amount.

Once timing is bang on what you want, then drive it around and see what the symptoms are, chances are they've changed/fixed.

Tobias05

do you know the max mechanical advance for that Accel HEI unit?

BigDreams92

iTrader: (1)

Unfortunately, most of the distributor is unknown as it came with the motor when I originally bought it as a 350. Took the cap off this morning and springs seems to be moving fine, but they do look a little grody. Didn't have much time to tinker with it, had to put it back on so I could get to work.

BigDreams92

iTrader: (1)

Just to let everyone know, it turned out to be a timing issue, I was only at about 4* base timing. Not sure if I was using wrong mark, or if the dizzy bounced, but it's running beautifully now.

Sonix

iTrader: (1)

Rock on! Glad it was so simple. Thanks for following up, I hate not knowing the punchline of a story.

My84CamaroZ28

Mine does it with just a little gas you can hear it and can hear the compression