Carb tuning questions....

[mrltd]

Uh - no.

If "regular" or "stock" cars didn't need an accelerator pump, then they wouldn't have one from the factory, which they do.

Heck, my 1970 VW Bug would cough if the accelerator pump wasn't working right.

When you bolt on an aftermarket carb, the accelerator pump shot must match the engine. Sometimes, it is close enough out of the box, but sometimes, not.

Mercracer meant that the car more than likely is too rich, and need less fuel on the pump shot. Not that it doesn't need a accelerator pump.


A 390 CFM 4bbl would be more than adequate on a basically stock motor. A 600 is a bit much and needs to be leaned out a bunch. Heck, even my lopo 2bbl car ran great with the right tuning.

Another example is my moms 79 bronco...RHS 400ci motor. 10:1 compressionn, 268AH grind cam, edelbrock 4bbl intake, heads are P&P...It's running a 600cfm carter. It's got enough power to blow a pinion out the side of the nodular housing. It will blow both 33x12.50 BFGs and drag 9000lbs like nothing.

[phayzer5]

a tuned down 600cfm edelbrock fed my hopped up carb swapped engine, while a 360cfm holley fed Stu's 351w

[89GrandMarquis]

Mercracer meant that the car more than likely is too rich, and need less fuel on the pump shot. Not that it doesn't need a accelerator pump.

If you are having a stumble and no black smoke out the tailpipe then you will need to increase the shooter size. If it stumbles and you are getting black smoke from the tailpipe then it will be nessasary to decrease the shooter size.

Yes, I agree with that. Mercracer's comments were not clear to me, since my prior post already said that it may be getting too much, as well. Sorry Mercracer!

It is very hard to try to help someone - at least for me - because I assume that the basics, i.e., timing, plugs, wires, etc. are all in good shape/tune, and we are only trying to fix one problem. With multiple problems, trying to fix a problem via a forum gets mighty tough!

[Mercmarquis]

Alright, I tried messing with it but couldnt get it to work. The timing is set to 14* and works best there. The carb is adjusted correctly as far a floats and mixtures are concerned. The accelerator pump shoots fuel as soon as I budge the throttle so I guess that is OK. But it still stalls and stumbles/backfires when I floor it from a stop with no noticible smoke, but I still have cats for now too. If I floor it from a cruise, it gets up and goes. I'm also gonna be trying an edelbrock, as they seem trouble free.

[gadget73]

Try cutting back on the pump shot. I'm betting its a simple matter of too much carb and not enough motor. That 600 is probably just fine but its more than your lopo needs.

[Mercracer]

Try cutting back on the pump shot. I'm betting its a simple matter of too much carb and not enough motor. That 600 is probably just fine but its more than your lopo needs.

Carb size in this scenario would have nothing to do with the backfiring and gurgling. Too light of a secondary spring can cause stumbling as can incorrect jet sizes. The very first things to check here is vacuum leaks and timing. Setting base timing with the vacuum advance connected to a manifold vacuum source can possibly cause this also.