Backfiring Issues

[4879tool]

Hello everyone my names John and I'm excited to be a new member here. I'm having backfiring issues with my 83' F250 and was hoping you guys could help point me in the right direction to fix it because I'm at my wits end. To give you some background information, I recently replaced the carburetor, intake manifold,  and eliminated all of the emissions junk that was on it. The new carburetor is an Edelbrock performer 750 and the new intake is a Weiand Stealth . I forgot to mention it's the 460 7.5l engine. It will idle just fine but when I go to drive it, it backfires when I accelerate and when I lay off the gas pedal. Pretty much the only time it doesn't backfire while driving is when I'm coasting. I've advanced the timing to 12 degrees  which helped with the lag while accelerating but hasn't helped with the backfiring. At this point I'm not sure what to do and figured I'd swallow my pride and ask for help lol. Thank you in advance for any advice!

[viven44]

Hey there welcome!!

When you say backfire, do you mean "exhaust popping" ? If so, you are running too rich (or at least an imbalance). I went through something similar recently. A few questions.

1) What is your idle vacuum reading ? Any vacuum leaks around the intake and carburetor ? Carb should have a PCV port in the back and front. The unused one is plugged ?
2) Have you compression tested all cylinders ? Highly recommend starting there to rule out possibility of dead cylinders.
3) Are the plugs new ? If not are the plugs fouled possibly ?

[85lebaront2]

Do you still have the air pump on the engine? That system can cause popping in the exhaust, and mine even shot a 2 foot flame out of the tailpipe one time.

[4879tool]

Yes, I guess the better term for it would be "exhaust popping " rather than backfiring. To answer your questions, I haven't checked the idle vacuum to see what it reads, my knowledge on carburetors is limited at best so I will check and get back to you on that one. No leaks around the carb or intake. The PCV valve is plugged in the back and so is the unused one up front. I have not compression tested the cylinders yet, I'll have to stop by my local oriellys tomorrow and pick one up. The plugs and wires are all new and so is the cap to the distributor. Thank you Sir for responding,  I appreciate the help!

[4879tool]

The air pump is gone.

[viven44]

I am still running the "smog" pump on Big Blue 2WD. I swear the exhaust is like a blow torch!! If I get rid of it, will I increase my mpg since parasitic drag on the belt drive is reduced ?? I'm getting a healthy 8MPG at the moment

I'm reluctant to get rid of it because besides the exhaust being hot, its fine otherwise. Also, the "tubes" that go to the back of the head will exist, I suppose them being there isn't an issue if the pump was removed.

[4879tool]

When I bought the truck back in December, the guy who I bought it from had bypassed the air pump by using a different belt. I got rid of mine when I was replacing the water pump because it kept getting in the way. Of course now I'm second guessing that decision because the truck wasn't " popping " before I removed all that emissions crap.

[viven44]

I got rid of all emissions on another 460 recently (except PCV)... that alone should not be causing your problems.. so don't fret that decision.

You should be running a PCV valve though.. without that you won't regulated crankcase pressure well. When you say the PCV valve is plugged, is it the actual valve or the port on the carburetor ? The PCV valve is probably not related to the issue at hand but just curious.

[4879tool]

So on those Performer carbs they have a PCV  port in the front of the carb in between the 2 vacuum ports if that makes sense. That's where I have my PCV running. Now there is also a port in the back of the carb which I didn't need so I plugged it with the threaded plug that came with the carb20240601_185446.jpg

[viven44]

Ok you are good then 👍

[mat in tn]

let's just say that the 460 is a workhorse. cubic inches, stroke, heft, all of what is needed to pull a 1 ton and even a fair-sized dump truck, loaded. an air pump should NOT be the item to load the engine enough to kill mpg.
as to removing all "emissions crap", that's a debate to be opened up at a different time. basically, the engine needs fuel, air and compression to be able to run when told to do so. that's it. the ignition source is doing the telling. get these areas correct and you can have a great running truck. that is not to say that many of the add Ons are without merit. egr can extend an engines life a lot. Vacuum operated timing controls can make the engine work better across wider temps. better starting. some of these add-ons are added as a system and need to be deleted as a system. deleting part may have unwanted results like backfire. I'm not a purist at all and have "tuned" many and resurrect dead trucks regularly. many if not most have pieces missing so I back up to the basics to prove the engine before deciding whether to put stuff back or not.

[viven44]

The parasitic draw off a AC compressor is real though, correct? After I installed the AC compressor, the idle dropped nearly 200 RPM!!

[Gary Lewis]

On a 460????

I can't tell when Big Blue's compressor comes in save for the sound if the hood is open.  I have the 1990+ FS10 compressor vs the FS6 that came on the truck, but that's a bigger compressor so should present more load.  

[85lebaront2]

Gary, the EEC-V system detects the load and compensates. If you log the engine data, you will see the (a) slight drop in rpm as the load comes on and (b) the IAC opening more to compensate for the change. If you listen carefully you will here a change in the exhaust sound as the IAC feeds more air.

Darth's idle speed would drop when he was still carbureted.

[Gary Lewis]

Yes, I have the EEC monitoring the A/C clutch.  But I've watched and there is very little change to any of the parameters.  Haven't tried listening to the exhaust note though.

[viven44]

Is it because you are a free-wheelin cool manual transmission operator ? When I had "Big Red", I couldn't tell when the AC came on.. it may have been a 50rpm drop at best.. but on all the auto trans trucks, be it 302 or a 460, it has been a drag at idle. I'm definitely missing the high-idle dashpot on the stock carb that compensates for this drag... anyway. I just turned up the idle to compensate for this.

[viven44]

I thought you were running a "double stack" Duraspark-2 ?? What does this EEC-V do ? I assume it replaces the duraspark and was necessary for the fuel injection upgrade ? I looked up those ECMs. They are pricey!!!

[Gary Lewis]

Yes, the EEC-V system replaced the DS-II.  Both Bill and I are running 1996 California-spec systems as CA is the only one that had EEC-V with MAF & SEFI.  All other states got EEC-IV with speed density and bank fire.

The beauty of EEC-V is that it is programmable via the OBD-II port.  We used Binary Editor to set ours up.  In his case he has it controlling the E4OD.  In mine I have the ZF5 but I turned off EGR and all other similar systems.