Cant get rid of backfire

[cuda lover]

Hello,

I have rebuilt a 73, 340 engine, which has a mopar performance  cam (P4120231, Cam ID D.C. 284-108, duration 284, overlap 68, design lift 0.484, rec install cntr line 108). The heads were ported with the short block I bought built for power (450 hp per engine builder).

When it getting idling the best with timing at 22 degrees advanced, with a 14 degree limiter plate in the distributor and a new electronic box from 4secflat, i get a steady 7-8 psi of vacuum it is still backfiring (a little more than occasionally).

I have made sure the headers are sealed well and there are no air leaks (had some backfiring due to header leak earlier in process).

I have an edelbrock 1411 carb, which I put in metering rods and jets to lean it out (combo 17 in the owner's manual, by the char and used the 3 psi springs), but it still seems to be running rich. I also drilled holes in the throttle plates (worked my way up to 1/8''). The mixture screws work better but still getting the backfiring.

For next step I was going to do a compression test to make sure all the valves are closing properly and not hanging up.

I was thinking of getting a new carb which might allow me to adjust air and fuel mixtures for idle, primary and secondary circuits since I can seem to get the idle circuit to lean out enough to not back fire. No back firing when the primaries kick in, or at least I don't hear any.

any other recommendations, or recommendations on a tunable carb?

[cudabob496]

plug wires may not be routed correctly, or timing off.
or dist out 180 degrees

[cudabob496]

backfires can also hurt power valve

[734406pk]

Hello,

I have rebuilt a 73, 340 engine, which has a mopar performance  cam (P4120231, Cam ID D.C. 284-108, duration 284, overlap 68, design lift 0.484, rec install cntr line 108). The heads were ported with the short block I bought built for power (450 hp per engine builder).

When it getting idling the best with timing at 22 degrees advanced, with a 14 degree limiter plate in the distributor and a new electronic box from 4secflat, i get a steady 7-8 psi of vacuum it is still backfiring (a little more than occasionally).

I have made sure the headers are sealed well and there are no air leaks (had some backfiring due to header leak earlier in process).

I have an edelbrock 1411 carb, which I put in metering rods and jets to lean it out (combo 17 in the owner's manual, by the char and used the 3 psi springs), but it still seems to be running rich. I also drilled holes in the throttle plates (worked my way up to 1/8''). The mixture screws work better but still getting the backfiring.

For next step I was going to do a compression test to make sure all the valves are closing properly and not hanging up.

I was thinking of getting a new carb which might allow me to adjust air and fuel mixtures for idle, primary and secondary circuits since I can seem to get the idle circuit to lean out enough to not back fire. No back firing when the primaries kick in, or at least I don't hear any.

any other recommendations, or recommendations on a tunable carb?

Wow! Nice 340 build! These Edelbrock carbs are ok for a stock to mildly modified engines. Your 340 with 7-8 InHg at idle may be too much for this carburetor making it difficult to impossible to tune. Was the cam installed with a degree wheel or "straight up"?

[cuda lover]

Wow! Nice 340 build! These Edelbrock carbs are ok for a stock to mildly modified engines. Your 340 with 7-8 InHg at idle may be too much for this carburetor making it difficult to impossible to tune. Was the cam installed with a degree wheel or "straight up"?

Timing straight up.

What carb do you recommend? i was thinking of the demon 625 street carb with vacuum advance. I think it might have control of fuel air mixture separately for idle, primary and secondary.

[cuda lover]

plug wires may not be routed correctly, or timing off.
or dist out 180 degrees

rechecked wires multiple times. Timing rechecked multiple times.

[73440]

Verify the distributor cap is in good condition , no cracks.
Make sure the 5 and 7 plug wires are not touching, adjacent in the firing order  a misfire is possible.
Is it backfiring when idling back thru the carb or when driving and you let off the gas thru the exhaust ?

[cuda lover]

Verify the distributor cap is in good condition , no cracks.
Make sure the 5 and 7 plug wires are not touching, adjacent in the firing order  a misfire is possible.
Is it backfiring when idling back thru the carb or when driving and you let off the gas thru the exhaust ?

have only been idling because can't get tuned.

Backfire mostly though the exhausts (both sides), occasionally through the carb. but only at idle, backfire seems to go away when over 2k rpm.

I can recheck the 5 and 7 again, but I know they are not touching, I have them in a loom.

[73440]

https://www.carshopinc.com/product_info.php/products_id/115660/HP1555

This is the book I have been using to try to clear up my Carter tuning issues.
I put an original rebuilt Carter on the 440.

A leaking float or high float level are listed as causes of mixture too rich .
An incorrect carb gasket listed for engine runs rich.
Does this carb have the solid fuel filter in the carb at the fuel inlet ?

The change to leaner metering rods and jets you did should help.

Ensuring the choke is working correctly important.

Backfiring is about the only trouble not referred to in the troubleshooting section in the book.

[73440]

https://quadrajetparts.com/carteredelbrock-afb-avs-carburetor-parts-c-299_130.html

I have bought parts from this firm.

[734406pk]

I would recommend a compression test first. Record and post the readings. Are you using the original stamped rocker arms?

[Beekeeper]

Sounds like the short block was professionally built but the thing that first came to my mind was if the cam was accidentally installed one tooth off. If so, it could run but run extremely poor with lots of backfiring. Probably not but something to keep in the back of your mind. Worst case scenario, cam timing could be verified with the engine in the car.

I'd be tempted to back off the initial timing to around 15 degrees to see if that improves your problem. It only takes a minute to test then it's one more thing to rule out. From your description, it sounds like it's worse with less initial timing and 22 is what it takes to keep it running. If so, maybe you have something going on other than carb issues.

Another thought thats easy to check. I helped a friend install his freshly built 440 last year. When we fired it, it was behaving exactly like yours. Kept falling on it's face and didn't want to stay running. Lots of backfiring as you cracked the throttle. We checked the voltage going into the coil and found it was only 8 volts or so. Once we tracked down that issue and got proper voltage to the coil, it ran amazing. I never imagined that poor voltage to the coil could do this.

I'd start with this last thing since it only takes seconds to check.

[Beekeeper]

I have an edelbrock 1411 carb, which I put in metering rods and jets to lean it out (combo 17 in the owner's manual, by the char and used the 3 psi springs), but it still seems to be running rich. I also drilled holes in the throttle plates (worked my way up to 1/8''). The mixture screws work better but still getting the backfiring.

I was thinking of getting a new carb which might allow me to adjust air and fuel mixtures for idle, primary and secondary circuits since I can seem to get the idle circuit to lean out enough to not back fire. No back firing when the primaries kick in, or at least I don't hear any.

Final thought for what it's worth...it's been my experience that backfires are more a result of lean conditions rather than rich. You have taken extra steps to lean it out but maybe you're adding to the problem. That carb may not be a good one for your engine set up but I would think you should at least be able to get it idling okay with that carb.

In order to be certain you have carb issues, I'd put the richer jets and rods in for test purposes and see what happens. If it really is too rich, you will see it on the spark plugs.

[xtopfuel]

I say pull the number one plug, bring engine to TDC and make sure distributor is at number one sounds like TDC is off to me

[cuda lover]

Sounds like the short block was professionally built but the thing that first came to my mind was if the cam was accidentally installed one tooth off. If so, it could run but run extremely poor with lots of backfiring. Probably not but something to keep in the back of your mind. Worst case scenario, cam timing could be verified with the engine in the car.

I'd be tempted to back off the initial timing to around 15 degrees to see if that improves your problem. It only takes a minute to test then it's one more thing to rule out. From your description, it sounds like it's worse with less initial timing and 22 is what it takes to keep it running. If so, maybe you have something going on other than carb issues.

Another thought thats easy to check. I helped a friend install his freshly built 440 last year. When we fired it, it was behaving exactly like yours. Kept falling on it's face and didn't want to stay running. Lots of backfiring as you cracked the throttle. We checked the voltage going into the coil and found it was only 8 volts or so. Once we tracked down that issue and got proper voltage to the coil, it ran amazing. I never imagined that poor voltage to the coil could do this.

I'd start with this last thing since it only takes seconds to check.

I did check the voltage to the coil since I was switching to the 4secflat electronic control box. I was not 12 v but was 11. The guy at 4secflat said not ideal but OK. I am not sure how to trace the wiring through ignition switch etc all the way back (battery is 13 volts).