Issue losing power with backfire

[JohnnyT]

Hey just looking for some advice. Had my engine rebuilt with about 500 miles since. Has been sitting for about a year.
Before it was parked I started to notice when pressing on the gas while in drive the car would backfire and have no jam. Idle in park seems fine.
This year I haven’t been able to drive it due to this problem. Took off tank, it’s clean and polished on the inside. Just took apart carb (Edelbrock 650) it’s clean of debris; however has a solid covering of carbon (black as black). Pic below shows section of carb wiped.
Any suggestions on next steps or where to troubleshoot? Thx in advance for your time and help.

 

[Don C]

A lean condition is the most common cause of backfiring. The carbon on the choke and throttle plates and the venturies is probably due to the backfiring.
Did you thoroughly clean the carburetor? After setting for that long varnish in the internal passages is a likely culprit. Take it apart and soak it in carburetor cleaner. You can get carburetor cleaner in a small bucket that has a basket for small parts. Blow it clean with air, make sure when you blow into one orifice that the air/debris comes out someplace else.
Vacuum leaks can also cause lean conditions, so check all vacuum lines, vacuum canisters, and brake booster. Check around the intake manifold gaskets and carburetor gasket. Check the PCV system. If equipped, check the EGR system.

 

[JohnnyT]

Great, thanks for your detailed reply. I haven’t finished putting the carb back on. But will throughly clean and blow the passages. Once I get the tank and carb back on with some premium gas I will check in.
What do you suggest for trying to understand where vacuum lines go and which ones belong/are needed?
Thx again

 

[Steve McMahan]

Post pictures of your engine bay with the air cleaner off, paying attention to where the small vacuum lines go. Also, do we know your engine I’d and information like that? California cars were different than others I believe.

 

[will e]

So it drove fine for a while, then started to backfire when getting on it and then you parked it for a year or so and it still has the same backfire issue?

 

[Don C]

Great, thanks for your detailed reply. I haven’t finished putting the carb back on. But will throughly clean and blow the passages. Once I get the tank and carb back on with some premium gas I will check in.
What do you suggest for trying to understand where vacuum lines go and which ones belong/are needed?
Thx again

You can find most vacuum diagrams in this thread:
https://7173mustangs.com/threads/vacuum-hose-diagrams.23844/#post-244049
The vacuum connections that are necessary are to:

• Distributor
• Power brake booster
• Vacuum modulator, for automatic transmissions, it may run through a vacuum tree on the firewall.
• Vacuum cannister for air conditioned vehicles
• Flapper on air cleaner snorkel
• Ram air, if equipped

There are others for emissions controls, but are only necessary if required by your state DMV.

 

[kcmash]

Mine was doing that, I did 2 things.
1) Get a blue wire condenser. They are much more robust than a black wire condenser. Its a Standard Ignition part.
2) I replaced the coil. I had been using an original coil.

Not sure which one fixed it. But my backfiring is gone.

Kcmash

 

[DAverkamp]

Hey just looking for some advice. Had my engine rebuilt with about 500 miles since. Has been sitting for about a year.
Before it was parked I started to notice when pressing on the gas while in drive the car would backfire and have no jam. Idle in park seems fine.
This year I haven’t been able to drive it due to this problem. Took off tank, it’s clean and polished on the inside. Just took apart carb (Edelbrock 650) it’s clean of debris; however has a solid covering of carbon (black as black). Pic below shows section of carb wiped.
Any suggestions on next steps or where to troubleshoot? Thx in advance for your time and help.

Have you verified your timing? What type of cam do you have?
Have you pulled the valve covers to check out the complete valve train?

My experience with sudden backfiring was shortly (500 mi) after my (2V replacement) rebuilt long block 4V went in, my FT cam washed out the #2 exhaust lobe and she backfired upon peddle to the metal as well but idled fine with a “clickity click” inside the right bank valve cover. Since yours has been sitting so long have you pulled the valve covers to ensure the Vtrain is all together?

 

[71ProjectJunk]

First thing I would check is the operation of the accelerator pump on the carburetor. Most of the time this is caused by insufficient accelerator pump shot. The accelerator pump plunger may be a bit bad and just not pushing the amount of fuel you need, or you may just need to adjust where the rod is on the pump arm. Usually Edelbrock carburetors have 3 positions on the pump arm. Usually the middle one works fine, but if you need more pump shot you can go to the lowest one. Look down the carburetors primary side with the engine off and the choke open so that you can see the pump shot. Open the throttle all the way. You should see 2 heavy streams of continuous fuel coming out of the pump nozzle. If it the stream of fuel is not continuous, if it just spits out fuel like a hose that you turn on and off, or the stream of fuel is very weak, then you probably need a new plunger. If the accelerator pump checks out to be good, then check all vacuum hoses, to make sure you do not have a vacuum leak. If you check all vacuum hoses and find nothing wrong, check your timing, it might be a bit retarded.