So i have a 68' 390 In a 1977 F250 and when I start to accelerate from about 1000-2500 RPM or so the engine sputters, almost stalls, and will usually right itself above that. Also it's very weak until about 3000 then it goes back to pulling hard. I rebuilt the edelbrock 1406 after it sat for awhile and the fuel went pretty bad ( engine ran like hell) and it was leaking fuel out the side. The float level is set per edelbrock specs, i did however use the same needles and seats. The accelerator pump runs dam near the same on all 3 settings which i found odd. When i engine brake it backfires same with just standard downshifting (normally it just cackles since i have pretty crappy old cherry bombs) When cruising in 4th if you give it a little gas it goes just fine, so it's from take off till 3rd. Possibly the timing might be off since I'm sure it should be 6BTDC but i havn't checked ( since I'm sure it would affect the engine all the time) At idle the vacuum pulls about 18 with the needle not moving more than 1/4, so it's idle screws are set to the best vacuum pull. So today I'm going the check the plugs for fouling, timing for any deviations , is there anything I'm forgetting to check? or is there something that i need to do like change the accelerator pump or? I know I've answered some of my own questions but it feels like I'm forgetting something and any suggestion would be appreciated

The engine is a 390 with rebuilt heads with slight work (cleaned up mostly, hardened valve seats) 255DEH cam, 428 PI alum. intake. And flamethrower ignition

 

I have heard the edelbrock carbs are sensitive to fuel pressure , do you know what fuel pressure is and what fuel pump you running ?

 

Time to replace the accel pump diaphram? The rubber may be dried and not pushing any(enough) fuel which will cause it to stumble and lay over til it starts pulling fuel through main metering.

Turn up the timing a few degrees. Most run around 10* if it doesn't rattle under acceleration.

 

Bad diaphragm is my best guess.

 

Might have the wrong size or a defective power valve, if that carb has one.
puttster

 

Looks like the accel. pump was bad and needed alittle adjusting, the arm was bent off a bit. Had to remeasure the plunger top and get it closer.

 

And thanks for the replies guys

 

Just FYI, Edelbrock carbs don't have power valves. They use a different system to richen the mixture when the manifold vacuum drops below a pre-set level. Metering rods (step-up, to be more accurate) attached to a plunger, held down by vacuum, and different stiffness springs to adjust the vacuum at which they come into play.

 

good news! the problem returned.... ugh... So i wonder if it might be a combo of fowled plugs and maybe my linkage is bent off a bit and not opening secondaries all the way....and it now backfires through the carb hmmm.... the pump is shooting a good stream down the carb, and also on another note, my vacuum guage has a tiny bit of play while at idle (back and forth rapidly over a small area...less than .2 ) It stumbles again on take off or while pressing down on the gas in any gear for a bit. It clears up alot better and i made sure that the floats, pump etc are all within edelbrock specs. Not so sure on vacuum leak as spraying around with starting fluid yeilds no surges.



So is there something I'm missing here? I've replaced the pump diaphram again twice now, and they're all sealing to the wall and not cracked, or warped....

 

Have you actually checked the timing and vacum/mechanical advance?

 

Yes, it's currently 8BTDC and it will wind up till around 30 at WOT

 

Sorry to say I'd take a step back. Do a compression test and a leak down. It's easy to blame rough running on the carb, I did. When I finally started to dig I found engine problems. I'd want to put aside valve problems, sticking lifter, cam lobes (have you been running Zinc in your oil?). Sometimes higher rpm can mask a problem. It's worth a look, it is prudent to look for serious engine problems first. I hope it is the carb but don't assume it is.

Good luck, Dan