View Full Version : back firing carb

Power mad
10-21-2000, 06:25 AM
I have a 73 C20 454 that I rebuilt the engine, rebuilt the heads (spings, valves, etc ) It has an Edelbrock gear drive,Performer cam, and 750 manual choke carb. I rebuilt the distributor (Accel super coil, etc, etc). It does'nt backfire all the time. (just when I'm trying to show off) Sometimes I mash the loud pedal and it slams you . next time it backfires out the carb farts and sputters then takes off like abat outta hell . trashing the air cleaner in the prosess. I want to put this in my K5 blazer . but not acting like this . Any thoughts would be apprecieted .

10-21-2000, 10:06 PM
I'd bet on a sticking accelerator pump. That backfire, cough and puke sure sounds like it's going lean sometimes.

Power mad
10-22-2000, 12:17 AM
Thanks Alan for the info. The carb is new so the pump should'nt be bad yet. Although I have wondered if the carb is to small. I did have block punched .40 over, so that bumps the C.I. to 463. Some articles I have read says that a 750 cfm will support an engine this size. Others say 850 cfm is needed. I do know that carbs are to spendy to just throw one at it and see if it cures the problem. So I think I'll try fatting up the jets

10-22-2000, 11:42 PM
If your carb will support the engine to high RPM it is not too small, at least not for street use. And a "small" carb is not the cause of poor performance off the line.

What happens is that when you bury your foot in it off idle air velocity goes to almost zero in the intake and carb. Engine speed is low and throttle blade is wide open, engine is not pumping enough to keep the justs drawing fuel. The accelerator pump sees the low vacuum and squirts raw gas into the throat to help get past that flat spot.

A smaller carb will see more vacuum over the jets and not have as much of a bog problem. The other side of that coin is that you run out of flow capability at high RPM, when airflow is at a premium.

Enter the spread bore carb, small primaries to keep velocity and vacuum up and provide good performance at part throttle, while allowing high airflow at high RPM, due to the huge secondaries.

I'm not familiar at all with E'brock carbs, are they mechanical or vacuum secondaries? I know that on the Q'jet the opening of the air valve was critical, too early and you bogged, too late and you lost time.

I don't know how to check it, but I'd still suspect erratic accelerator pump operation or possibly a problem in the vacuum advance, not ******ing timing under low vacuum conditions. That might account for your backfire, but I's think of fuel first and that you are geting lean backfires.

For what it's worth, we ran a 600 Holley on a 460 Ford in a 1 ton truck. I never tried to make real revs with that engine but the 600 would carry the 460 as high as I dared turn it.

Chuck Smith
10-25-2000, 03:32 PM
The Accel vacuum advance is adjustable. They are not "set" where you want them right out of the box. They need to be adjusted. This is done by removing the vacuum line to it, and inserting an allen wrench (3/32" I think). I forget which way is which, so look at the literature you got with the Accel guts for the dist. to determine which way to turn the screw.

Also, depending on how radical the cam you put in is, vacuum may be low because of that as well. Like Alan was saying, vacuum plays a big part. They do sell vacuum reserve cannisters to help that problem.


John DiMartino
10-27-2000, 06:40 PM
Alan is on the right track,lean misfires and backfires are common on non stock combinations.For what the carb costs new i wouldnt pay anyone to rebuild one. most guys do not know how to set them up correctly,and the motor never runs to its potential.The accelerator pump may not be giving its full shot of fuel that it is suppossed to.For now,roll into the throttle instead of mashing it,and get someone who's good at tuning those carbs.Have the timing checked too,it might be a little late.

Power mad
10-28-2000, 04:35 AM
So far I have adjusted the pump rod so that it will give the biggest shot of fuel .No change, but as I was saying it comes & goes . So tomorrow I'll go through the electrical and ignition system, and see if there is something that I have been overlooking.