Hello everyone, I just swapped my 2.8 out for a pretty well built 350. I have run into some carb tuning problems and i need advise/insight because i do not know very much about tuning Holley carbs. I bought the carb (Holley 750 DP mech secondaries, manual choke) off of a co worker that had it on his Maverick track car. I have seen the motor run, i have even drove the car. He blew the motor in it and told me that this carb would be too much for his new 302. I put it on my car the other day to initially start up the new engine for break in, and it runs for like the first 10-15 seconds then stalls.
I have the stock electronic fuel pump that is regulated with a Aeromotive 4 port adjustable fuel pressure regulator. I have the pressure set at 8psi with the engine off (i do not yet have an inline gage hooked up). When the engine begins to stall if it give it some throttle it stalls out even faster. This is making me think that the fuel pressure is not supplying the fuel bowls with enough fuel to sustain an idle for more than 10 seconds. Does this sound accurate? The front float is adjusted like 1 1/2 turns out and about 1 turn out on the rear float. I believe that this is adjusted correctly enough to run. Could the culprit for the poor idle and stalling be the infamous Holley power valve? Does the power valve even play any part in the cars idle? Undoubtably the idle screws/ acc pumps and idle speed is incorrectly set. It would be a great help if anyone could point me in the right direction to get my car running so that i can enjoy the drivetrain that i have spent the last year and a half building. I appreciate any time and advice that is given toward my problem. Thanks -Bob

 

The floats are adjusted by the flathead screws and lock nuts on the top of the bowls, with the engine running or pump on the level should be just below the holes in the side if the bowls. (flathead screws also block these holes.)

I think the screws you are turning are the idle mixtur control screws on the metering blocks. If the screws require a tiny screwdriver then they are not the float adjustments. Some holleys have 2 or 4 idle screws depending on the model.

once the floats are set turn the mix screws al the way in and then back them out 2 turns. Crank the idle way up and start the vehicle. Once running turn the idle down to a respectable level 900 RPM or so and then hook a vacuum gague to the port on the base plate of the carb. The reading should be fairly steady.

Now begin to turn the screws in 1/4 turn turning each the same amount, as the mix begins to lean out the idle will speed up and the vacuum will increase, turn idle back down and repeat process until you have the leanest mix for the highest volume, this should set you straight.

 

To adjust the float level, you adjust the screw/nut things on either end of the carb, on the top of the float bowl.

To do it, loosen of the screw ONLY A LITTLE!! because with the engine running, fuel will spray out. You only loosen this screw off enough to be able to turn the nut. The screw is only a set-screw.

To properly adjust the float level now, you must have the car on a level surface and idling. Now, you will want to have a decent sized rag with you because it can get a little messy. While its running, take out the front sight plug (the small screw in the side of the carb) so you can see the fuel level. Odds are, the level is probably too high, like mine was, and fuel will come out pretty quickly. Now, loosen off the front set screw (as said above) and turn the nut clockwise to lower the float level, and the opposite to raise the level. You want fuel to just barely trickle out of the sight-plug hole. When its there, its right. Hold the nut in place with your wrench and tighten up the set screw. Don't forget to put the sight-plug back in too.

Now do the same to the rear of the carb. There isn't really one setting on where the float should be, because every car is different.

 

8 psi is too much pressure. Lower it, to about 5 or 6. Carbs don't work off of fuel pressure, the way injectors do; if the pump could move enough fuel through the needle valves to feed the motor at 1/2 psi, then that's all you'd need, and you'd gain nothing by setting the regulator higher. In fact, 3-4 psi is usually plenty.

I prefer to set the fuel level to where fuel just barely doesn't quite come out of the sight plug, rather than to where it does come out. I like the primary side to spill when I barely bump the fender, and the secondary side a little lower than that.

It's also possible that your fuel pump doesn't work past the priming stage; i.e. it doesn't come on and stay on after the engine is running. In which case you'd have some sort of electrical problem to sort out before you have any hope of tuning the carb. Looking in the bowl through the sight plugs will tell you that, if the fuel just disappears and isn't replenished.