Backfire through carb

[dubs chops]

OK I am in need of knowledge.

I recently replaced the heads in my 383 with a set of Eddy RPM heads. I also added new plugs and wires, as well as a used 750 Mighty demon carb which I went through and replaced worn pump and gaskets.

I have gotten the car to idle good but anytime I give it gas quickly it will backfire through the carb and die if I don't immediately let off.

I put the old carb back on and got the same problem. Right now I am running open headers and have 76 primary jets and I think 80 secondary jets. Any ideas? I am worried I may have a valve being stuck open.

Put it at TDC and the rotor was point at #1 so I know it is not 180* out. Its not drive-able at all as it pops and bogs almost all the way through the rpm range.

 

[Mike67]

Timing...Thats the only way i know of it to backfire, intake valve is open when spark ignites...

 

[threewood]

What cam, lifters, pushrods and rocker arm assemblies did you use? Possible something isn't playing right snd holding a valve or two open?

 

[dubs chops]

What cam, lifters, pushrods and rocker arm assemblies did you use? Possible something isn't playing right snd holding a valve or two open?

this is my thoughts using all of the factory lifters, rockers and pushrods, I measured the preload on the lifters though and thought .040 was good to go.

 

[dubs chops]

maybe a compression test is in order next?

 

[Coronet 14]

Vacuum leak at carb or intake

 

[Mike67]

Threewood makes a great point, especially since you said you used all of the oe valve train; pushrods might be too long now. Did you check your pushrod length?
I don't know the difference in the mounting heights of the pedestals for the rocker shafts, spring height/valve length or your gasket thickness between stock heads and the RPMs, but there very well could be a difference thats causing your issue.
I'm sure someone will reply that has that knowledge.

 

[PurpleBeeper]

Threewood makes a great point, especially since you said you used all of the oe valve train; pushrods might be too long now. Did you check your pushrod length?
I don't know the difference in the mounting heights of the pedestals for the rocker shafts, spring height/valve length or your gasket thickness between stock heads and the RPMs, but there very well could be a difference thats causing your issue.
I'm sure someone will reply that has that knowledge.

Great point Mike67... I didn't think of that one....makes a TON of sense....pushrods now too long with new heads. Otherwise, my 2nd guess is the timing is off. Coronet 14 makes a good point too since you changed the carburetor. Maybe you've got the carburetor "fat" to keep it idling with a vacuum leak?

 

[Dreamcatcher]

I had a 426 wedge that did the same thing. Seems the cam went on break in. No ZDDP when I fired it the first time. There should be 0 valve lash with hydraulic cam and lifters. When I checked mine after it started back firing through the carb it had .035.

 

[dubs chops]

Great point Mike67... I didn't think of that one....makes a TON of sense....pushrods now too long with new heads. Otherwise, my 2nd guess is the timing is off. Coronet 14 makes a good point too since you changed the carburetor. Maybe you've got the carburetor "fat" to keep it idling with a vacuum leak?

What should the vacuum be at idle. It is the Mopar purple .474 lift cam

I had a 426 wedge that did the same thing. Seems the cam went on break in. No ZDDP when I fired it the first time. There should be 0 valve lash with hydraulic cam and lifters. When I checked mine after it started back firing through the carb it had .035.

if I cant find anything with plugs or vacuum im going to take the valve covers back off and start going back through the valvetrain

 

[Dreamcatcher]

With that cam I would think about 10 to 12 inches of vacuum. But it should be steady no matter what it is. Often valve issues it should bounce around.

 

[vintage chromoly]

Is your firing order correct? IE. Plug wires routed and landed on the correct cylinders.

 

[Dreamcatcher]

One thing more. What carb you running? If it uses a power valve they will cause issues like that when they go bad.

 

[lewtot184]

i'd start with the timing being too late. hopefully the cam is in correct. .040" probably won't be enough pre-load. also, look at the accelerator pump arm to make sure it's properly adjusted.

 

[moes]

a bad ignition coil or a clogged fuel filter will sometime cause a back fire through the carb

 

[Ranger16]

You never did say what your timing is set at.

 

[lewtot184]

some of those holley type carbs have only one hole to feed the accelerator pump nozzle. basically the gasket can be put on backwards.

 

[Xp29h]

I was thinking with 383 the harmonic balancer slipped and timing mark is off but when timed it would look right but be off??? Just thinking....

 

[dubs chops]

Timing is at about 13* right now. Farther advanced seems to run Better. I have a checklist already made before I posted thread.

1. Check firing order on distance,
2. Check for vacuume leaks
3. Check for valve issues.


The carb is a 750cfm mighty demon.

 

[Kern Dog]

With that cam I would think about 10 to 12 inches of vacuum. But it should be steady no matter what it is. Often valve issues it should bounce around.

Sorry, I have to disagree on this.
The Lunati solid cam I had in my 493 caused the idle vacuum to bounce around a LOT. It went from 8 to 12, 7 to 11, 9 to 13.... The heads were fresh. The rings seated fine. A fat cam results in some choppy idle quality.

Set the initial timing to 18 degrees BTDC and see how it runs. The 484 cam needs MORE initial timing than a stock cam, LOTS more. The cams that cause rough idle are the type that require MORE initial timing to idle better. One common practice is to set the timing where the engine has the highest vacuum. Attach a guage to a port that has vacuum at idle and try twisting the distributor around to see what it tells you. You may find that the engine idles best with 14 to 20 degrees of initial timing. This improves idle quality and acceleration, BUT....You then have to do something about how much TOTAL timing the distributor delivers. Most V8 engines will run best with a total timing of between 32 and 36 degrees. The problem is, when you set the initial timing to 14 degrees for a better idle, the internal mechanism of mechanical advance then adds another 26-28 degrees of "Centrifugal advance" resulting in the total timing advance around 40 degrees. Yes, that DOES result in engine spark knock.
Sounds crappy, I know. The "fix" is to somehow limit the amount of centrifugal advance so that once it is up to speed, it goes no further than say....34-35 degrees. Several ways to do this. If you have a stock 1972-78 Mopar distributor, google "Mopar distributor advance slots" for how to limit the amount of advance. Other distributors will have different methods to fix them.