Back fire

[Bulldog90]

I have a fresh frame off restoration with fresh rebuilt engine. Person I bought it from put less than 250 miles on it. Engine has apparently had a back fire issue since it was new - previous owner had a shop that replaced the carb with a Holley 600cfm and as stated back the timing off a little - no specifics.

Backfires under load - with no load no issues and no issues in garage revving the motor. It will do 60 without a single pop as long as you feather the gas. When I got it they had it idling at almost full throttle - I backed the idle back to normal and adjusted the mixture screws to get a perfect idle however I think it is now backfiring more than the less - I don’t know if the high idle was somehow compensating for timing issue.

I have reached out to the company that did restoration in the meantime I can’t believe it is a carb issue since it has had two brand new carbs and same issue. Seems like it has to be timing but I am not mechanic. Any help or thoughts appreciated. Has MSD ignition in it - mild cam

 

[gnpenning]

Which can and firing order? Has firing order been triple checked.

Possibly a lean back fire?

Valve adjustment?

 

[Peachy Bronco]

I had a problem similar to this, turned out to be a loose rocker arm.

 

[pcf_mark]

Which can and firing order? Has firing order been triple checked.

Possibly a lean back fire?

Valve adjustment?

Also make sure you know which way the distributor is turning - it can run but will only be hitting on 4 cylinders and pop out the carb when you try to throttle it.

 

[Johnnyb]

Which can and firing order? Has firing order been triple checked.

Possibly a lean back fire?

Valve adjustment?

Don't forget that there are three firing orders, depending on the motor:

• Stock: 1-5-4-2-6-3-7-8
• 5.0: 1-5-4-2-6-3-7-8
• 5.0 HO/Explorer: 1-3-7-2-6-5-4-8

 

[Bulldog90]

Also make sure you know which way the distributor is turning - it can run but will only be hitting on 4 cylinders and pop out the carb when you try to throttle it.

After discussions here I changed the accelerator pump selector and drastically improved the backfire issue but still slightly there - I am out of carb adjustments unless I start tinkering with jets so will take a look at the timing next. - Thanks for the comments

 

[DirtDonk]

Accelerator pump insufficient, main jet size too small, vacuum leaks, wrong or defective PCV valve, ignition timing, vacuum advance, mechanical advance, probably other things.
All of these can effect how the engine runs, and cause some of the symptoms you are experiencing.
But the symptoms seem to be leaning towards a lean mixture.

Could the carburetor have been tuned for a high altitude application? Maybe the previous owner lived in the mountains, or the carburetor he got was done up for that.

 

[nvrstuk]

Does that model Holley run a "Powervalve" cause I think they all do (not up on newer Holley's)? If so, swap it out and try it again. Holley powervalves usually blow out when they backfire. Usually when they tear you have a rich condition but check it closely because I don't see how if this engine has 250 + miles that 2 powervalves can survive that much backfiring. Also, check the size of it. Different cubes, build and rpms require different sizes. Possibly it isn't even opening due to lack of vacuum which wouldn't allow the valve to open which would could be causing the lean issue. Got some poking around to do.

Back in the day there were some pretty effective Sun Testers that would help diagnose and eliminate most of the other possibilities then you could concentrate on the carb and AFR.

Get a portable O2 sensor and check out your #'s on each bank, verify and dial it in but replace the powervalve first. Paul has a good list to check.

Wishing ya the best.