My 75 f250 390fe Holly 4bbl vac sec, has a bad stumble under load. I can rev it all I want at idle and it's fine. When I go to take off when its warmed up it acts like it's still cold and stumbles real bad. If you ride the clutch until you get some rpms it's fine and has plenty of power. I thought I had it fixed twice now. First I added a bigger pump nozzle and high flow pump nozzle screw. I then saw my timing was too retarded. I advanced it to 10 degrees and it made a huge difference in throttle response. I have an AFR gauge and it says my idle screws are 13 when warmed up. If I rev it really fast there is a very brief lean spot. Vac advance is on ported vaccum.

 

Do you mean take off as you normally would or take off hard to where the secondarys would come in ?

 

Take off normally

 

Is your power valve matched to your vacuum reading ?

 

That's what I was wondering. My vaccum reads 11 but I have the stock 6.5 power valve.

 

Where I wont receive parts till next week could I richen up my idle screws to make it more driveable for now?

 

11 is a little low if it's a stock build, do you have a performance cam ? have you checked for vacuum leaks ? the 6.5 is a little much for your vacuum but it should work. the power valve is really mostly for WOT but it's good to have the right one.

The idle mixture screws won't help you off idle.

Are you trying to fix a problem on a known carb or is this a new installation ? if your jetting is way off it could cause problems. what do your plugs look like? what CFM is it ?

It sounds like an accelerator pump issue to me, but you say you upsized it. are you sure it's adjusted properly and can you see it shoot gas in when you open the throttle.

 

It has a mild 270h cam. Holley 600cfm vacuum secondarys. All stock except for accelerator pump nozzle and screw. You can see the AFR gauge go lean for a millisecond when you hit the throttle real fast at idle. I haven't watched the gauge while under load yet. The plugs in general look rich.

When you go to take off in gear it falls on its face and backfires through the carb. I have adjusted the accelerator pump so there is no play and it squirts a decent stream everytime you press the gas. If you ride the clutch to build rpms then you can floor it and it will have no hesitation. It's in that idle transition period that it's bad. Can I pull the idle screws and blow out the idle circuit with carb cleaner like i had to do with my Edelbrock? The Edelbrock would act like this when it would sit a while and restrict flow in the idle circuit.

 

A 270h will have a little lower vacuum. a 600 isn't overkill .

I'd look harder at your timing . what is your total all in at 2500 ? retarded timing can cause what you're talking about. set your total at 34 and see what your initial is. drive it and see if that makes it better. if you don't have a dial back set your initial up to 15 and see if it helps.

The other possibility is your accelerator pump is working but not good enough. do you have a slight clearance between the arm and the actuator ? you say you've already upsized it so that may not be an option .

 

If your getting back fire I would look at the distributor for full advance. Your vacuum advance needs to be dialed in or the distributor has problems.

 

It sounds to me like your idle speed screw is turned out too far, so your transfer slot isn't exposed. You need a little exposed, so there is no interruption in fuel delivery coming off idle. Turn the idle speed screw until the primary butterfly leaves a square at the transfer slot or use a feeler gauge to measure the slot exposure per the holley tuning manual. Once you have that set don't touch it. Get everything else right before coming back to the idle speed screw. Don't use this screw to chase lean or rich idle conditions.

I would also check there isn't too much slack in the accelerator pump arm. If the gap between the arm and the throttle linkage is too large the accelerator pump will come in late. [NM. It sounds like you have the pump arm dialed in]

 

I took the carb off. The transfer slots were too far exposed. Almost fully exposed. The idle air bleed vents also were dirty. I am going to fully clean everything and put it back together. I will square the transfer ports. Should I drill a hole in the throttle blades? I could not get the idle correct without opening the screw. Or should I test it again first?

 

I'll bet you or who ever has been tuning that carb previously was chasing a lean condition cause by vacuum leak around the carb, or they quit using the idle mixture screws and hopped over to the idle speed screw to add fuel. Since your transfers were already on full there has been no extra fuel until the accelerator contribution made it to the party. Air may be getting in other places besides the venturis, maybe through worn throttle pivot bores or leaky base or spacer gaskets. If you spray a little carb cleaner around the outside of the carb while running and the idle speed jumps you probably have a leak. Start with all new gaskets and check for play in the throttle pivot. If it wiggles you may need to get a new throttle plate off another carb. Since your blades are fairly open at idle, it's likely throwing off your ported vacuum signal and will affect your timing if your distributor is using ported vac advance so you will need to adjust timing again after that carb is sorted out.

I would never drill the throttle blades. That's a permanent modification compensating for stuff that was adjustable from the factory.

 

Well the engine acts like a brand new motor. I set the transfer slots to be barely exposed. I have the timing around 17 on ported. I set the idle screws for best vacuum which was around 11-12 at 600rpms. I take off and there's no bog. None at all. It will turn the 33s like nothing. I do think I have a small vacuum leak as was suggested, because my idle screws are 3 turns out and read 15afr.