SBC Timing Problem
 

I just finished putting a 383 into my 84 Chevy truck.

At idle, if I rev it, it will stop running, unless I let of the gas quickly. I have to really ease into the throttle to get it to rev up from idle in neutral.

When driving, if giving it gas quickly, it will hesitate and pop through the carb. It has a big hesitation off idle when getting on the throttle, and pops and snaps.
It doesn't have a smooth throttle. I think it is advances way too much, too early.

What can I check? The carb was just built specifically for this engine, so another reason I am thinking it is timing related.

- Travis

I have traced most of my backfire conditions (through the intake or exhaust) due to a lean condition.

It makes sense to think an intake backfire is due to too much advance. But with normal advance and a really lean condition you will get backfire through the intake. The charge is hotter and lean so it will fire off quicker than a cooler more fuel saturated charge. The A/F ratio is lower so it doesnt take much to fire the charge off. Prematurely if its very lean. To the point where with a mild or common advance will lite it off before the valves are closed.

With an engine that is not leaned out, say even rich you can advance the ignition ALOT without getting backfire through the intake. You make get alot of preignition (pinging) but the flame front wont get burning fast enough because its fat with gas so it wont cumbust out the intake valves. The flame front is much slower due to the heavy fuel charge.

With a light fuel charge it will ignite faster, and if the intake valve is still open a lil you will get backfire.

Do you have any exhaust backfire (after fire)? Usually a lean condition will also backfire out the exhaust too.

I dont think Ive ever had a backfire out the top if I had plenty of fuel available even with a really advanced ignition. And Im talking way advanced. If I advance it too much with plenty of fuel I just get preignition. JR

I've had a similar issue once with my 350. It turned out that my accelerator pump was not working smoothly as it should, give that a shot.

ABSOLUTELY it is not getting enough accelerator pump shot. Do not eliminate the carb because "it was built specifically for the engine"...

From hearing your guys thoughts and talking with a few people, I am starting to think it is the carb too. This is the second time I have heard that it might be lean.

After the holidays I am going to take apart the carb and see what jets it has. It was recommended to me to start with stock jets, and then adjust accordingly from there.

It doesn't backfire through the exhaust, only through the carb. It actually caught fire at the top of the carb, a small flame that burned, until I put is out real fast.

Just my .02 worth....

Backfires don't occur because the carb is lean (or fat either for that matter). Backfires are timing related where the intake valve is partially open when you have a charged cylinder lighting off. If the intake valve is closed - then there could be no "back fire" thru the carb.

Lean conditions usually present themselves as "misses" -- bucking - or pinging/knocking... surging.

I would be checking your spark plug wires... to make sure you have them right on the cap and the plug they go to. Check your timing - and that doesn't mean just checking the timing setting with a light - it means to make sure that your timing pointer and balancer are actually at zero when at TDC and that the motor is physically at TDC when they say that's where you are.

The other thing that can cause you're condition is a valve that is sticking and or a valve lash that is keeping the valve from seating.

I had essentially the same problem, but only when the truck had a few passes on it in the day, re-timed it, checked the carb a hundred times, drove me nuts for 2/3 of the summer, it turned out to be a sticky lifter that wouldnt bleed down.

Ok, I will double check timing and make sure the wires are correct in the cap at TDC. When I do get the engine revved up, by slowing giving it gas, it does run smooth. Just any quick kicks of the throttle or anything, and it wants to die.

I will take a video of it when I get a chance next. Appreciate the help so far.

Travis -

Is your vacuum advance hooked up? If so to which port on the carb? You'll most likely want it to the PORTED vacuum not the manifold vacuum.... ported will have NO vacuum at idle. Manifold will have vacuum at idle.

I'll also ASSume that you timed it at around 12 or 14* INITIAL timing - with NO VAC ADVANCE - that the vac advance was disconnected and the port on the carb was plugged. ALSO if you have a dial back light -- make a map of your timing advance curve. You let the car idle to get the "initial" - then hold another 500 rpms on the engine and check timing - add another 500 rpms and write that down etc. Until you see no more MECHANICAL ADVANCE.... do all of this with the vacuum advance disconnected and plugged. Mapping this will tell us the CURVE in your distributor. You'll need a dial back light to make this easy... as you'll just dial back to zero on the balancer and then read the timing in the light.

Also -- give us the vacuum readings the motor makes. Have you adjusted the idle screws (on the carb) to give you highest vacuum and highest rpms? :cheers:

Travis,
What carb & # number you running?

What dist? MSD?

Where is you initinal & total set.

Do the springs allow the advance to move off idle?

If you are running a Holley and had backfires you may need to replace the Powervalve.
Jeff