Carter AFB Help- backfiring and stumbling

[Bundy]

I'm having a helluva time getting my Carter 4bbl to run well. I've messed with the primary and secondary screws and I get a stumble when taking off from idle AND at 50 mph and about 4000 rpms when I let off I get a BIG backfire. I was running richer before and it didn't backfire ever, just some occassional dieseling.

Any ideas? Maybe I have the carb tuned right and my timing needs to be retarded a tiny bit?

 

[Redleg06]

Have you changed the metering rods and the jets, or are they as delivered. If not, that should be your first step to bring it online with your engine. Most carbs seem to be set up for a 350-360 sized engine to start.

 

[nrramse]

I'm having a helluva time getting my Carter 4bbl to run well. I've messed with the primary and secondary screws and I get a stumble when taking off from idle AND at 50 mph and about 4000 rpms when I let off I get a BIG backfire. I was running richer before and it didn't backfire ever, just some occassional dieseling.

Any ideas? Maybe I have the carb tuned right and my timing needs to be retarded a tiny bit?

I just had a similiar battle with my Edelbrock (though I didn't get the backfire) - turned out to be a blockage in the carb.

Newell

 

[broncnaz]

recheck timing as well and make sure the vacuum advance is hooked up to ported vacuum.

 

[Bundy]

There is no vacuum advance on this. I rebuilt the carb 6 months ago and cleaned and cleaned and cleaned it again. It has new offroad jets and everything

I don't think it's vaccum- I hooked up a gauge off the carb nipple and it seemed pretty constant under accel and then really pulled under deaccel. I don't know what readings mean though as I thought it was measured off psi and this gauge read off of CM (hg)

600-700 rpm 12-13 CM (hg)
2000-4000 rpm around 16 CM
deaccel from 4000 it spiked somewhere around 25 CM

No idea what this means LOL

 

[broncnaz]

You must have a fairly big cam those are low vacuum numbers at idle and from 2000 up. 25in on decell is normal pretty much any engine will do that.
I would suggest changing out the step up springs. I would try using springs that open at 3 or 4 in of vaccum. Your current springs are probably opening to soon.
Also check your accelerator pump make sure its working like it should ie fuel squirts as soon as the throttle is moved. you may also need bigger accelerator pump nozzle to cover the stumble.

 

[nrramse]

There is no vacuum advance on this. I rebuilt the carb 6 months ago and cleaned and cleaned and cleaned it again. It has new offroad jets and everything

That happened to me too. Full rebuild and new off-road float valves - it ran great for a day then started stumbling off-idle. After taking it apart, I found a rolled up piece of gasket in the one of the primaries.

I also had to buy another accelerator pump because the one that came with the kit just plain wouldn't work!

Newell

 

[Bundy]

I'll check that accelerator pump. I'm sure I have a pretty big cam because it makes that sweet "glub-glub" sound.

I messed with the timing a bit- it was set at around 6-7 ATDC and I moved it to right around 9-10 degrees and although it ran great (throttle response improved greatly) I'm thinking it might be a combo of having the timing too far retarded and the carb accelerator pump.

When I watch the accel pump out the gas it just kinda dribbles like a garden hose, not a spray or mist which I thought i would see and it was a little slow... maybe this could be the stumbling but I dunno about the backfire :-[

 

[nrramse]

When I watch the accel pump out the gas it just kinda dribbles like a garden hose, not a spray or mist which I thought i would see and it was a little slow

I saw that movie too. The new seal that came with the rebuild kit just wouldn't squirt gas, and I ended up buying a whole new plunger. I like my Edelbrock, for the most part, but after finding that rolled up peice of gasket (which I believe came from new gaskets) and replacing the accelerator pump, I'm wondering about the quality of the rebuild kit.

Not sure about your motor at all, but on a stock Early Bronco the timing is typically around 6* BTC.

 

[broncnaz]

The gas should squirt pretty good as soon as the throttle is moved sounds like you need a new pump. Timing can probably be set at 12 degrees maybe even a little more every engine is a little different and since you have a big cam it may need more. Basically you want the timing set where it starts good both cold and hot. many times if you have to much timig it wont start good when hot.

 

[Bundy]

I saw that movie too. The new seal that came with the rebuild kit just wouldn't squirt gas, and I ended up buying a whole new plunger. I like my Edelbrock, for the most part, but after finding that rolled up peice of gasket (which I believe came from new gaskets) and replacing the accelerator pump, I'm wondering about the quality of the rebuild kit.

Not sure about your motor at all, but on a stock Early Bronco the timing is typically around 6* BTC.

I'm gonna take my carter off and check that accelerator pump and plunger... it should squirt much better than it is.

The gas should squirt pretty good as soon as the throttle is moved sounds like you need a new pump. Timing can probably be set at 12 degrees maybe even a little more every engine is a little different and since you have a big cam it may need more. Basically you want the timing set where it starts good both cold and hot. many times if you have to much timig it wont start good when hot.

She runs soooo much better at 10-12 degrees than she did at 7-8... it's like a whole different motor. Good to know on the hot and cold... she actually started much easier after I advanced(?) the timing

 

[broncnaz]

I usaully find 10-12 about right for most SBF's. go more than 12 and hot starting becomes a issue. Timing greatly affects how the motor runs and feels. you can almost always gain some power and even mileage by upping the timing from stock specs. Stock specs are mainly for emissions and the happy medium for the masses.