Backfiring SBC not sure why???

I read up here and on Holley's site but I'm getting to be stumped. Hopefully someone here has had a simlar experience. I have a SBC with a Holley 600 carb (brand new). I replaced the spark plugs and the plug wires and it's got a HEI dizzy. I've checked the vacum, and timing, seems good from my cheapo timing light. I'm running a mild edelbrock cam and stock heads/manifolds.
The problem,
The car starts fine, and idles fine, and as long as you stay in the primaires runs fine. As soon as the secondaries open up it stumbles and backfires. At idle with the hood up and the air cleaner off if I step on it it backfires quite a bit. If I ease into the throttle no backfire. This happens at speed and in the garage.
I replace the secondary pump nozzle on the carb twice (from a 25 to 28 to 32) and it didn't change. I'm not sure what to look at next, carb, timing, something else!?!?
I read that the jets or the acclerator pump spring or any number of things can be tweaked on the carb but on a sbc with stock heads, a mild edelbrock cam and stock manifolds I figured the out of the box carb would be fine.
Any ideas?

 

with the engine off do you get a nice big squirt of gas down into the carb when you pump the throttle? and does it look even on both sides?

 

Yeah I get a nice a stream on both sides of the nozzle when I pump the throttle lever.

 

float level right...? recheck the timing...? my nephews was doing this ....ended up the distibutor was loose....also where are you pulling the advance vacuum from ....? brandon

 

Crosswired? It happens all the time Stafford

 

It almost sounds like a timing advance problem. Try checking the timing just before and after it starts backfiring, see if it goes off the scale. Just a thought.

 

5 and 7 are cross wired.

 

my guess too..

firing order should be
1 8 4 3 6 5 7 2

I had the same problem once.. the car ran pretty good but would backfire / miss carry on etc.. under load..

the result of a 3AM wire install

 

Make sure you have ACTUALLY got some vacuum to the advance cannister under acceleration....Then check the weights under the rotor to see they are free and moving properly

Then make sure the module mount plate is moving when the Vacuum cannister is pulling it, to advance the timing.....
OR try another distributor....

 

Maybe get back to the basics here?
Eliminate the possibilty of a mechanical problem,,,run a compression check.
It "could" be a valve spring problem at a certain RPM.

 

Did you just replace the carb or is this a fresh motor? Or a new cam?
I'd do as everyone here has said but I'd also double check your valve lash settings. Intake valves may be on the tight side.

Josh

 

I have had this problem. reset the lifters. then check the timing. always do the hard one first. it makes the easyone seem easy!

 

Thanks for the replys guys. I'll check the wiring, I did just replace them but did it one at a a time. The carb is new, the cam is new, the shortblock is stock with new bearings. I'll recheck the valves they may be tight, I adjusted them all a while back. Compression is ok.
Now the adavanced, when I was checking the timing every so often the mark would lurch off the indicator, I figured it was my cheapo timing light. I'll have to get a buddy and check the advance as well.
Thanks Guys

 

Try this if you think your timing lite is giving bogus results. Fire it up, and undo the vac hose to the dizzy. You should hear the motor lose a little rpm. If it does then hook it back up, loosen the dizzy hold down and back the timing up til the motor stumbles and back fires , then start sneaking up the timing til it clears up, then kick a few mores degrees in for good measure. Drive and see if it pings any. Alot of Chevy hold downs, espically the tawian chrome jobs will not hold it no matter how tight you get them, the timing will back off due to dizzy gear against the cam. Also, the balancer may have slipped on the rubber ring giving bad timing marks, seen this alot also. One last thing, it you have access to a hand held vac pump, hook it to the dizzy and pump up to 15 inches of vac, it should hold it, if not your vac diapram is shot. Hope this helps

 

Thanks Bobby, I'll add that to the list. I checked the dizzy hold down, it's an oem and pretty tight I couldn't move the dizzy by hand. I actually had the vac. line wrong, the holley has two ports and a buddy switch them, that helped the idle a lot so I think I got that fixed. After I check the valves and wire order I'll recheck the timing.

 

I'd check the timing sounds like it might be too far advanced

 

it could be a bad carb. Your chances of getting a brand new carb that is faulty is quite likely...

New Holley carbs (vacuum secondary 600s) are coming with no provision to adjust the float level.... its just a plugged boss now.

 

Well for an update, I checked the sparkplug wires, all are in the right spot. Here is a bit more info, if I just pop the throttle it's a mild backfire/popping. If you stand on the throttle FIRE out the carb! Now it's a new holley and I believe the power valve is protected, right??? I am starting to lean towards timing or valves. Funny thing is it idles pretty smooth and fires right up. Toinght I hope to readjust the valves...

 

Maybe a flat cam lobe..
Just a thought.

 

The cam is a brand new Edlebrock but at this point I guess it could be anything!

 

I've found that Holleys usually come out of the box pretty close to being set up properly. Jetted a little rich if anything. Your problem sounds like an ignition or valvetrain problem to me. Very hard or impossible to pin down frm my desk in NJ. If it was mine...I would start with the basics...check the dist. and timing. Make sure it's advancing, plug wires and firing order are OK, etc.
I would then check the valve adjustment and cam. You mentioned that the cam was new, and that's why it could be suspect. Cams DO go bad...and they will usually do it during the break-in or very soon after(You followed the break-in procedure, right?). Pull the valve covers and spin the engine over. Observe the rockers in action-if one or several are not moving as much as the rest of them STOP-you have a lobe(s) wiped and you're going to have to tear it down. Another clue that a lobe is gone is when you have the excessive lash showing up after an adjustment. In any event, if everything passes muster I would carefully adjust the valves again. While the covers are off inspect the springs and rockers. You should be able to find your problem. Take it one system at a time and it will make itself known. Let us know what it was...

-Scott

 

Yes, Holley carbs made in the last 10 or so years have the check valve installed already.

 

Yea Root probably nailed it. I did that on the Pusher last year run me ragged and I've been doing this forever.

Check your vacumm at idle and make sure you're running the right powervalve. it should have a 6.5 from the factory and your powervalve should be about half of your idle vacumm.

But i think you're going to find that carb probably isn't the problem its most likely ignition.

 

What about spark plug gap? Maybe one is closed? Could be cam lobe my sons was doing that and we took the valve cover off noticed the rocker arm hardly moving lobe was rounded.

 

I am following this one...Had the same problem with the 40....All i change was the dizzy to HEI,put a Ron Francis hot start kit on it, and had the carb rebuilt.

Went through everything and it still does it when you drive at 60+ mph. Cools down and it is fine.....

 

It sounds like you're not getting any advance when you step on it. Put the timing light on the balancer and see if the mark moves when you rev the engine. If not, you're igniting the fuel too late at high speed and causing the combustion to escape out the eshaust valve...aka...backfiring.

 

Can a bad cam lobe cause you to run very hot? what about poor fuel air mixture?; To far advanced, not enough? not trying to highjack the thread, just trying to pin point some areas that might be releated. I'm getting some small backfires up in the carb but more importantly running very hot!

Jon

 

Id pull the spark plugs & valve covers & spin it over....(no ign)
Take a good look at everything when its moving,& inspect the springs.

 

Alright Guys,
For closure on this thread, a buddy, Buckle, let me borrow his HEI out of a good running 305. Swapped out my HEI and that fixed the problem, idles smoother, and when you hit the pedal she goes like stink. Good thing I added traction bars to the rear end now!
Thanks for all the help on this one!

 

ahhh a cracked dist cap tween 5-7 ?????