Ok i just got a 385 fastburn gm crate motor im sure some of you guys have heard of them or even may have dealt with them before. My problem is that when I got my engine GM performance recommends a 750 holley.....So thats what I got a 750 holley 3310. Well ever since I got it I've had mixture problems.Mostly the carb has been running pretty rich. So I down jetted it and put in smaller squirters.I changed the accel cam and everything but it still runs rich. What happens is at an idle after ive been driving it will idle good and then you can tell its slowly loading up and then it floods out and shuts off eventually. Also if i pop the gas itll stumble but if i pop it again once the carb is cleared it responds perfect...... I tried timing but Ive changed the timing and it just seems to be running really rich. The exhaust fumes are extremely potent(burns your eyes). I wonder how they set up a 750cfm carb on a 350cid engine.....I wonder if you guys could shed some light.Maybe tell me your setup on your high performance 350. Jet number, or cam number, squirter number.....anything.
thank you

 

if you have the money it sounds to me like your just dumpig to much fuel in period holleys are notorious for letting all the fuel the engine wants in and more you might try jumpping down to a 700 or mabey even a 650 cfm carb and see if that solves the problem other than that it sounds like every thing else is right

 

What are you doing for fuel supply/pressure regulation?

The accelerator pump cam wouldn't affect mixture.

 

ya the accel cam was changed because i thought maybe the lag on acceleration was caused by the cam. But I have a holley blue to a holley fuel regulator.My pressure gauge on my fuel rail is reading 7 psi.

 

OK, slow down and get a Holly book, if you like to read the directions last try this.

Put a vacuum gauge on the car at idle, then select a power valve that is appropriate to your vacuum (this is where you need the book, to many numbers in my head already). Carbs only work when the power valve, air bleeds, jets, squirters, and pump shot profile are modified for ..

1) your motor
2) weight of the car
3) driving style
4) atmosphere

It is not easy to get the carbs tuned just right but once you do they work great! And sorry for the general terms used here, after all it took me a good year to get mine tuned to the right numbers on the dyno, and not have it go lean or rich under acceleration, stopping.

 

did you set the float bowls properly?

 

Ya i set the float level on the side to where the fuel is just coming out of the bottom of the hole(which is what most people said was the best way to set it)Ya i guess ill try to find a holley book and all. I just was wondering because even if I look up the 385 fastburn on gm performance website it says use a 750 holley......

 

look for the holley supertuning and modifying book from sa designs. it helped me out. got mine going great. i also have a thread about tuning and accelerator pumps and all. and accordiing to all i have been reading and learning the idle circuit has nothing to do with jets. it is only the idle circuits and transition slots. just off idle is the accelerator pump and discharge nozzle. then the power valve and jet system.

 

Very good point, changing the jets only affect "off-idle circuit" air/gas mixture and they are different. Usually when the car is rich at idle making your eyes burn it is from the power valve opening and letting fuel in. AND another important piece of the puzzle is that if your car has backfired you could have also damaged the power valve.

Idle circuit = small screws on the side of the metering plates.
power valve = operates under vacuum to supply fuel via the horn
jets = cruising air/fuel mixture (secondary circuit)
Squirters = throttle response off idle
Cam profile = throttle response off idle

and on the secondaries, they go both ways....

power valve + small jets
blocked power valve + large jets (I set mine up this way, std hp4150 is the other way)

 

This is all very good info. I guess im gonna look into buying a good book on my 3310. I will definitely look into the power valve. It seems that it could be the power valve because it is rich at idle.

 

The power valve goes through the booster venturi, not the idle circuit. Only way a power valve could affect idle is if the primaries are open too much and drawing fuel through the main circuit.

 

if you just installed the carb, (new or not) there's a chance a little debris from a freshly trimmed fuel line or another source has found it's way into the needle and seat assembly. depending on the size of the debris, all sorts of wierd flooding conditions can occur. easy fix, remove needle and seat assembly counting the turns required to remove it. blast the contact areas of the needle and seat with compressed air or carb cleaner and re-assemble. double check float level.
if you have a dual feed carb, check both assemblies. it doesn't take much time and could solve your problem.
Eric B

 

are you running the stock jets? holley jets their carbs pretty rich to avoid liability from detonation issues. the carb probably came with 72 primary jets originally, you might try backing off to 70's. i wouldn't go much smaller than that for a 385 though

 

It sounds like you need to focus on your idle circuit first. you need to make sure your idle adjustment screws are adjusted properly. This is done with a vacum guage and done to read the highest vacum reading. But there is something that i really think is your main problem. you might want to take your carb off and check to see how much of the idle transfer slot on the main and rear butterflys. you need to have a equal amount showing at both the front and rear which is about 0.010 to 0.020in max showing. You see if you have too much exposed you will bee into your part throttle circuit so adjusting the squirters and power valve jets aren't going to help you at all. The rear throttle blade adjustment is hidden on the bottem of the carb on the passenger side. and expose a little of the rear one like i said and then readjust the front one accordingly. that should give you a base line, then you can readjust the idle screws. Then you can proceed to adjust your accelerator pump. I don't reccommend going with a smaller or larger carb that one should be able to perform beautifly on that motor.