Starved for fuel or too much fuel?

[72SportsRoof]

My 72 - 351C has a 700 Holley on it, cam (not sure which) & headers. Seems to idle okay and run good when simply cruising but every time I attempt to step heavier on the gas pedal the carb spits and pops. I've had it to 2 shops, 1 said it was the rings (their dead wrong because no oil blow or smoke out of pipes at all) second shop claims it is fuel starved somehow but can't figure it out. Any one here had this issue and if so, what to do?

 

[Hemikiller]

Check your accelerator pump, a weak or inop pump will cause a lean backfire. I've found that the 351C likes an early and healthy shot of fuel. My 800DP has a #37 discharge nozzle and a fast ramp cam.

 

[Mister 4x4]

:goodpost:

 

[Jeff73Mach1]

Blown power valves (an enrichment device in the carburetor) will do the same thing.

 

[TommyK]

Check your accelerator pump, a weak or inop pump will cause a lean backfire. I've found that the 351C likes an early and healthy shot of fuel. My 800DP has a #37 discharge nozzle and a fast ramp cam.

+1 a blue cam a 37 squiter will usually cure most hiccups provided the pump arm is adjusted properly.

 

[boilermaster]

My 72 - 351C has a 700 Holley on it, cam (not sure which) & headers. Seems to idle okay and run good when simply cruising but every time I attempt to step heavier on the gas pedal the carb spits and pops. I've had it to 2 shops, 1 said it was the rings (their dead wrong because no oil blow or smoke out of pipes at all) second shop claims it is fuel starved somehow but can't figure it out. Any one here had this issue and if so, what to do?

72sportsroof, please tell us what 700 holley, give us the list number off of the air horn.                        we can then see just what your carb is supposed to have for accelerator squirter size and pump cam, is the accelerator pump set up properly ? from there transmission type and gearing come into play ,as does tire size.                                                                                                                               Boilermaster

 

[72SportsRoof]

My 72 - 351C has a 700 Holley on it, cam (not sure which) & headers. Seems to idle okay and run good when simply cruising but every time I attempt to step heavier on the gas pedal the carb spits and pops. I've had it to 2 shops, 1 said it was the rings (their dead wrong because no oil blow or smoke out of pipes at all) second shop claims it is fuel starved somehow but can't figure it out. Any one here had this issue and if so, what to do?

72sportsroof, please tell us what 700 holley, give us the list number off of the air horn.                        we can then see just what your carb is supposed to have for accelerator squirter size and pump cam, is the accelerator pump set up properly ? from there transmission type and gearing come into play ,as does tire size.                                                                                                                               Boilermaster

cars still in shop, can't look at carb, took photos of carb and I see the number 83770 in photo I took

 

[72SportsRoof]

Check your accelerator pump, a weak or inop pump will cause a lean backfire. I've found that the 351C likes an early and healthy shot of fuel. My 800DP has a #37 discharge nozzle and a fast ramp cam.

thanks, very informing. I'll mention this to my shop

 

[72SportsRoof]

My 72 - 351C has a 700 Holley on it, cam (not sure which) & headers. Seems to idle okay and run good when simply cruising but every time I attempt to step heavier on the gas pedal the carb spits and pops. I've had it to 2 shops, 1 said it was the rings (their dead wrong because no oil blow or smoke out of pipes at all) second shop claims it is fuel starved somehow but can't figure it out. Any one here had this issue and if so, what to do?

72sportsroof, please tell us what 700 holley, give us the list number off of the air horn.                        we can then see just what your carb is supposed to have for accelerator squirter size and pump cam, is the accelerator pump set up properly ? from there transmission type and gearing come into play ,as does tire size.                                                                                                                               Boilermaster

Here's some photos of Holley

 

[Hemikiller]

83770 is a "770" cfm Street Avenger carb, yours is one of the four corner idle circuit units. Specs show a #31 primary discharge nozzle, which will probably be too small with the volume of the single plane intake. The carb also has to be set up properly with respect to the idle discharge slots, all four butterflies should be open an equal amount, both primary and secondary. If the shop isn't familiar with setting up a four corner carburetor, find someone that is.

 

[72SportsRoof]

83770 is a "770" cfm Street Avenger carb, yours is one of the four corner idle circuit units. Specs show a #31 primary discharge nozzle, which will probably be too small with the volume of the single plane intake. The carb also has to be set up properly with respect to the idle discharge slots, all four butterflies should be open an equal amount, both primary and secondary. If the shop isn't familiar with setting up a four corner carburetor, find someone that is.

Thanks man...looking into it and appreciate the advice.  :thankyouyellow:

 

[jpaz]

I had that same carb and I was very disappointed with it. The butterflies need a hole drilled in them to get it to run correct and depending on you engine set up it needs to be re jetted. Just ask Stanglover, he’ll tell you all the problems with this carb. I think it’s one of the worst carbs Holley ever made. I could never get it right. So I changed it out for a Quick Fuel 735 and what a huge difference! I couldn’t believe how much better my engine ran after I got rid of the sa carb. Every set up is different, so maybe you can get it to work with your engine better than I could. Good luck and I hope you can figure it out.

 

[boilermaster]

83770 is a "770" cfm Street Avenger carb, yours is one of the four corner idle circuit units. Specs show a #31 primary discharge nozzle, which will probably be too small with the volume of the single plane intake. The carb also has to be set up properly with respect to the idle discharge slots, all four butterflies should be open an equal amount, both primary and secondary. If the shop isn't familiar with setting up a four corner carburetor, find someone that is.

72stanglover, tommyk and stanglover give pretty good advice, I believe I have the blue cam and .031''nozzle.  everything was great until I put the nitto drag radials on the car, which were 1'' taller than the bfg's I was running, mild 408 c , toploader, 3.50 posi and 275 60 r 15 nittos.       after I got things sorted, I went back to the white cam, and sure enough, baby had caught a cold.  if I remember right the blue adds 4 to 5 cc's over the white cam. changing the cam without changing the nozzle  ( larger )will give more pump shot (add cc's) changing just the nozzle will keep the cc"s the same but shorten the amount of time the nozzle. sees fuel.                         I suggest you make sure the pump clearance is perfect , then proceed to change cams until the bog gets better, if it does not get better with an increase in volume the bigger shooters will probably not help.                                                                                                                Looks like you are running the same intake as me, where is your carb spacer to dissipate some of that heat.                                                                                                                                   Boilermaster

 

[72SportsRoof]

83770 is a "770" cfm Street Avenger carb, yours is one of the four corner idle circuit units. Specs show a #31 primary discharge nozzle, which will probably be too small with the volume of the single plane intake. The carb also has to be set up properly with respect to the idle discharge slots, all four butterflies should be open an equal amount, both primary and secondary. If the shop isn't familiar with setting up a four corner carburetor, find someone that is.

Thanks man...looking into it and appreciate the advice.  :thankyouyellow:

Well after all the carb jarb talk, turns out whoever owned the car before me over-tightened the lifters, did not set them correctly and destroyed the cam, all went bad in the 351c and had to replace cam and all associated with it along with machining the head! After tons of diagnosis by 3 different shops, tons of money and work I get it back tomorrow.  Live and learn...I know what a bad cam sounds like and acts on a motor now. Bummer man!

 

[Stanglover]

 Well, that sucks! 

Awhile back, when I first had my 71 4V motor rebuilt at a reputable shop, the builder decided to install a high volume oil pump. Long story, but the result was it took out the cam and all the main bearings. Before that was diagnosed, I could not get the car to run right, It had a Holley 670 SA on it when I bought it, swapped it out for a custom built 650, then back to a new 670. At the local tuning shop, the Guru drilled the primary plates and what a difference on the idle circuit!. But I still to this day get a lean backfire or a "bog" when I jump on it. Tried pump cam swap and different squirters as well as upping the power valve. To get to the end of this story, the motor was rebuilt again under warranty for the most part, but I lowered the compression to 10:1. Timing is also a BIG part of it and I learned how to rebuild an recurve for optimal timing on MY engine. The only thing I have not tried is a larger 50cc pump, but I am actually trying to decide if I want to swap out for a Quick Fuel HR 735 as I feel it needs a bit more air. Before I do that, I want to get a fuel/air ratio reading, do a cylinder pressure check and go from there. The one thing I do like about the current 670 set-up is I can get really good fuel mileage if I cruise around 60 mph. That will almost certainly drop if I up the cfm and jets with the bigger carb.

No matter what carb you use and as mentioned, you need an intake insulator gasket at least 1/4" thick under the carb. I use a 1" thick fiber plate, but then I have the stock manifold, but with the heat cross-overs blocked off. Still, there's a lot of heat under the carb.

Good luck and keep us posted on the results.

Geoff.