Help please,
Just put a "remanufactured" 2bbl carb from Champion Parts on my '69 F-100. (Actually 2 , the first one had a broken screw in choke attach hole, second also had some linkage problems.)

Now it backfires through the tail pipe. Did it twice on 3 mile drive.
My son drove to a fastfood restaurant and some busybody was going to call 911 because she thought she heard a gunshot.

Any Suggestions?

Richard

 

try re-timing the engine

 

Thanks for the suggestion read 3rd paragraph.
I have a correction! after I put the third rebuilt carb on. This is slightly different than the old one that was on when purchased 2 months ago..

The engine is backfiring through the Carburetor not the exhaust.
It did not back fire with the old carb. just hung up and would not idle down. Why would changing the carb cause backfire??

I tried to check the timing and can NOT find any mark on the harmonic balancer. There is an indicator mounted overhanging it.

Thanks,
Richard

 

Sounds like the new carb is running too lean. A lot of the time, if the accellerator pump stroke is adjusted too short you won't get enough gas when you hit the pedal, especially if you hit it hard. is this when it backfires? The timing may also be advanced too far which enhances this carb problem.

Also, some remanufactured carbs are not rebuilt too well. I've found busted pump diaphrams or breached leather in plungers...even in brand new carbs! If it idles fine, and cruses fine, but backfires when you hit it, check this stuff out.

 

Polkat and others- thanks

I found the very faint timing marks after a thorough cleaning. Set the engine at 8or 9 BTDC at idle of 700rpm.( that's as low as I could get it.) Shows about 30 BTDC when revved up, IF it doesn't stall out or backfire.

Engine now runs good at idle and cruise. Just can't get past the initial acceleration. Easing on to the pedal usually works but stomp on it will definitely backfire or stall.
There is SOME gas being pumped in with acceleration pump.

?? How can I adjust the accelerator pump throw or amount of gas squirted in? There are 2 holes in the arm and the pin is now in the hole furthest out.

? The remanufactured carbs have had two configurations on horizontal surface to the rear of the choke plate. One is smooth while the other has a 1 1/4 inch indention that is shaped like the outside of a vacuum diaphram. Both had the same symptoms. Both are Motorcraft. Does it make a difference?

Glad for more suggestions!

 

Joseph, Polecat and others,
Thanks for the suggestions.

Found the marks set it at 8 BTDC and still backfired, hesitated.

Remanufacturer and others sure it was the acceleration pump.

I tried accelerating with the choke partially closed and it did not backfire. It backfired more as I opened the choke plate. Thus agreement that it was getting more air than fuel.


Investigated and found there is some adjustment on the acceleration pump rod. The end pin was in the 2nd of 5 small holes in the connection arm at the carburetor. I moved it out to the 5th or furthest from the pivot point. That I believe increased the stroke distance.

At the pump lever arm I moved the pin to the innermost of two holes and I think that started the pump earlier.

Now it does not seem to hesitate.
Will verify it on the road once I get the gears changing.

Thanks to all,
Richard

 

Right! You've got the most storke out of the pump now. The next thing to do now is make sure the choke is fully open when the engine is hot. If that's happening the only other thing might be dirt in a jet, or a float adjusted too low. Good luck.

 

Almost there but NOT-

with the most stroke on the accelerator pump I still get hesitation but much less than before and little carb backfire.

Hesitation seems to be even less if I close the choke plate about 15% by adjusting the manual choke cable connection. I guess that is limiting the total airflow at open throttle. Which might be making the pump amount sufficient.


Do you know about the adjustment of the two screws on the front of this motorcraft carb. Could that be part of it since it's close.


Discouraged now - Do I just need another carburetor that always pumps more gas on the accelerator pump? Is there such a thing. I don't think I need more total airflow- just more pump?

Help again please.

Richard

 

Richard,

The 2 screws should be the idle mixture screws. Try adjusting them and see what happens. Just remember to keep up with where you start so you can go back if needed. I can't see you needing to adjust them more than a turn and a half max from their current position in either direction. Who knows, this may help.

I would also suggest checking into the float level. I only know how to adjust a Holley, but there has to be a way to adjust yours.

Good luck,
Lee

 

Thanks Lee,

The idle is good so I doubt that I will adjust much.

I have found out that there are about 6 venturi opening sizes on the 2100 carburetor.

I measured the one I replaced and found it to be larger than any of the ones listed in the chart I had. I will take off the remanufactured and measure it this weekend. It might be too small.

I hate to take apart a remanufactured store bought to try to adjust the float. From motorcycle repairs, I thought the float only controlled the amount of gas that went into the bowl. Didn't know it controlled the output.

I may just buy a Holley with adjustable accelerator pump, but locally the 600cfms are all 4bbl and I need a 2bbl. From this forum I have heaard that the 4 to 2 converter plates are not such a good idea.

Thanks again,

Opinions and input still appreciated