Hey, i just put on a Edelbrock 750cfm with elect choke yesterday and its been popping and backfiring like mad. Can anyone help me with tuning this thing properly? I have a 1976 small block 350 and an eledelbrock intake manifold. thanx :mad

 

Drop a note to Lars. He's the carb guru here! :yesnod:

 

Stinger -
Popping and backfiring up through the carb is an indication of one of two things:
1. Lean carb mixture. Usually caused by vacuum leaks.
2. Timing not set correctly.

Did you install the manifold at the same time? In other words, have you removed and re-installed the distributor also? I always tell people that 90% of the carb problems I see are ignition problems... Most often, new carbs out of the box are set pretty close to the correct mixture, so you should not have significant tuning problems with a new carb - if you have to change jets by more than 2 sizes on a new carb, you probably have problems that are not related to the carb.

The first thing I would do is to check and correct your timing. Verify that your initial timing is in the 12-18 degree range, and that your total mechanical timing is set right at 36 degrees.

Once you are certain that the ignition is set up right, start tracking down vacuum leaks. Common source for vacuum leaks on aluminum manifolds is at the manifold gasket - be sure you use the Edelbrock manifold gasket sealant to eliminate vacuum leaks at the head/manifold interface. Then, disconnect all vacuum hoses from the carb and plug every single port except the PCV - be sure the PCV is hooked up to a good PCV valve. Also, make sure all the vacuum port holes in the manifold are plugged. This baseline should produce a good-running carb. Tag me back if you continue to have problems.

 

Lars, when you say "initial timing" what are you referring to?

 

Initial timing is the timing you read at slow idle with the vac advance disconnected. Total timing is the timing you read when you rev it up and the centrifugal advance is completely pegged out (also with vac advance disconnected). Total timing, then, becomes the sum of the initial timing plus the amount of centrifugal advance built into the distributor. Of the two numbers, total timing is what matters: you set the total timing to 36 degrees, and initial timing ends up being whatever it is. If the centrifugal advance curve is the right length, and if the distributor is in good shape with all its parts intact, the initial timing usually ends up being somewhere in the range indicated. If it's not, you need to do a little distributor work to get the initial timing into a range that will make the car idle well while maintaining the 36-degree total.

 

So let me get this. for initial timing, I let car simply idle and with adjustable timing light, adjust light till the timing line registers at the zero mark. The reading on the timing light will be the initial. Either way, I should be @ 36 degrees @ 3000rpm with vac advance disconnected right?

 

Couple of questions for you. Was the car running ok without the poppoing before you switched carbs? If so, and you didnt mess with the timing, I doubt that is the main cause. Just by changing the carb, it shouldnt have effected the timing to a point where the timing itself is the problem. 2nd question, is it popping out thru the exhaust, or out thru the carb?

Lars explains how to set total timing and he does a good job of it and knows his stuff. However, the idea of doing this can sometimes be confusing to some. If your unsure of how to do the way he described, another choice to get you back on yur feet would be to begin by setting the timing to the factory setting first, then get the carb right, then come back to tinkering with the advance curve and total timing figures. The factory setting for yurs should be 12 degrees BTDC at idle with vac hose disconnected.

 

Is you engine stock? if so that carb is pretty big for a stock 76 sb, I would think that a 600 - 650 would be more in line.

 

It is popping through the exhaust and it was popping a bit with the origional carb that night.. Im thinking it is timing too...... The engine is not stock, a 4bolt main and about 270hp.

 

This might sound way out in left field but here goes anyway. Change the plugs. Dont just inspect them, change them. I had this exact problem when I changed carbs and maybe it was coincidence. But if you have even one plug thats not firing, first that cylinder isnt hittin, then the charge for that cylinder, since it cant get to ground thru that plug, will find the next easiest route to ground. Many times that charge will jump posts inside the dist cap to an adjacent cylinder. When this happens this adjacent cylinder fires out of time. If it fires when an exhaust valve is open, you get that popping from the exhaust. If it fires when an intake valve is open, you can get backfire or flames out thru carb. Like I said I chased this popping sound directly after changing carbs once before. couldnt figure it out. My bro came over and after listin to it for 10 seconds he said change the plugs. I did so and the problem was solved.

 

I had a similar situation recently (mis-fire). I had my headers re-coated by Jet-Hot. Had to remove the spark plugs. I took the opportunity to replace with fresh plugs. Well, I dropped the #6 spark plug on the floor twice. I must not have re-gapped because after I fired the engine, it ran rough and was popping through the exhaust. I had had a real challenge installing both #4 and #6 so I started at #2 and worked my way back just to be safe. #6 not only had two cracks in the white insulator but the metal 'gap' was firmly flattened against the spark plug tip. I replaced with a fresh one, fired the engine and it ran fine from there.

 

Stewart -
In answer to your last question above: yes, that's correct.