Holley 750 "stumbles" on WOT

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Lift&Move

Lift&Move

FABO Gold Member
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Hello All,
I purchased a nice used carb from a fella a while back.
It was on his 68 RR, 440 auto.
The facts are these :
Worked perfectly when new, but over time when stomping it to the floor from a dead stop, it would hesitate for about 1 or 2 seconds then WAHHHHHH
He replaced it with an identical new carb and his problem was solved.
I have a new kit for it, but would like to know the source of this particular symptom for future reference.
Thanks
 
With the crappy ethanol based fuel we are running these days, it's a wonder we don't see carburetors rotting from the inside out. If the car isn't run to operating temps almost every day, the fuel absorbs moisture from the air. I suggest running a fuel stabilizer, at least that way the fuel doesn't turn to s**t if it sits for a few day between outings.
 
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Demonracer said:
With the crappy ethanol based fuel we are running these days, it's a wonder we don't see carburetors rotten from the inside out. If the car isn't run to operating temps almost every day, the fuel absorbs moisture from the air. I suggest running a fuel stabilizer, at least that way the fuel doesn't turn to s**t if it sits for a few day between outings.
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I agree with you...maybe buy the ethanol free fuel from Buccee's, but its pricey
 
Too little info to say with any certainty what the problem was. But it doesn’t matter. Proper rebuild and tuning and the problem will go away.
 
I have rebuilt many over the years, with 100% success, but would like to identify the root of the problem to boost my knowledge.
What gets me is, he said it would start, idle, and run perfect under normal driving conditions.
It is not a dbl pumper, electric choke, vac secondaries
 
Have you check the spring and diaphragm for the secondaries? The diaphragm may be getting a weak spot in it, or the spring maybe too heavy. Just throwing out ideas, I don't normally fool with VS carburetors.
 
If it is a VS carb (3310?) and it’s truly an instant bog when going hard to WOT from a stop or slow roll, it’s in the primary side of the carb and likely squirted/pump related.

But don’t change anything during the rebuilt.
 
Yes, it's a 3310...sorry for not being forthcoming with that.
Thanks for your insight, I will focus my attention on that too!
 
Lift&Move said:
when stomping it to the floor from a dead stop, it would hesitate for about 1 or 2 seconds then WAHHHHHH
He replaced it with an identical new carb and his problem was solved.
Click to expand...

Tough diagnoses only because he claimed no other changes besides changing carbs.
Always begin with correct fuel level.
The jets and accelerator pump inlets are low and at the bottom. However it is possible the fuel level in bowls were low.

Next, check for contact between pump arm and pump cam at idle. This is something that someone could change when adjusting idle speed with the stop screw and not think it was a change. The pump must begin to move when the throttle opens. The 1/10 clearence in the specs is when the throttle is fully open. The clearence at full open is to be sure the spring is not be bound up. It provides some cushion from overtravel.
(Pictures here)

After that I would say check for pump check valve (probably an umbrella type - and these do go bad, especially older ones when used with newer fuels, its not just the eth, its other components too).

Finally, before you set this up for your car (or his) make sure the primary side throttle blades at slow idle are in the acceptable range of .020 - .040" transfer slot showing.
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Mattax said:
On an old school Holley 4150/60, greater than .040" of transfer slot exposure at slow idle would generally cause problems as the throttle was opened.
The sidebar Urich and Fisher's Holley Carburetors and Manifolds covers proper throttle to transition slot relationship. (pp. 121-2 in the 1987 edition [Holley Part No. 36-73, HP Books])
The section is called Special Procedures for Wild Camshafts
I've linked it above but the links to copyright material are often temporary.
Click to expand...

make a note of how many turns or (1/4 turns) of the idle stop screw to reveal .020 and .040 of the transfer slot under the throttle blades. Write that down! Now you have a way to know too much and too little without taking the carb off the intake while turning. :)
 
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Secondaries do nothing until the air velocity in the primary side is moving fast enough to call for secondary opening.
So from dead stop the problem is either accelerator pump circuit or idle/transfer circuit.
Seperate the two by slowly accelerating from dead stop. Hesitation when very slowly opening the throttle indicates idle/transfer circuit is not responding. Fix that first.
 
Possible the throttle adjustment screw is "loose" and working it's way in closing the transition slot?
 
Thanks everyone!

This site is so cool!
It's one thing to install a kit and fix the problem, but not knowing what the actual problem was is like a splinter in my brain.
 
Testing with different secondary springs on 3310s has produced a similar WOT bog on some engines with good signal to the carb for me. You “can” make them open too early and the primary accelerator pump can never cover that big of a lean spike so tuning becomes an issue. Start with a stock colored spring in the dash pot and disconnect the secondary linkage when first tuning. Get the primary side functioning perfectly then bring in the secondary tuning.
 
Lift&Move said:
Thanks,
All good advice!
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Actually no. Not all good advice. At least not for what you described. This is the problem with a vague problem description. Can you discern one from the other?

Alternatively, what might been poor advice could be good advice if there are other details that you have missed.
 
Accelerator pump cam. Buy the kit and try cam (pink) with more initial fuel response. Helped my big block immediately.
 
MechNerd said:
Accelerator pump cam. Buy the kit and try cam (pink) with more initial fuel response. Helped my big block immediately.
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It worked fine before, then it didn’t. Do you think the original white that worked before is worn-out?
 
Since it needs a kit anyway, my advice is build it. Get all of the adjustments dead on for a good base line and go from there. I wouldn't put different cams, pump shooters or anything in it yet. Build it back STONE STOCK. That way, you have a base line. You said it ran good for a while and then started acting up, so I see no reason to change anything just yet.
 
Lift&Move said:
Hello All,
I purchased a nice used carb from a fella a while back.
It was on his 68 RR, 440 auto.
The facts are these :
Worked perfectly when new, but over time when stomping it to the floor from a dead stop, it would hesitate for about 1 or 2 seconds then WAHHHHHH
He replaced it with an identical new carb and his problem was solved.
I have a new kit for it, but would like to know the source of this particular symptom for future reference.
Thanks
Click to expand...
Most likely when you stomp it the vacuum drops dramatically in the intake manifold causing the fuel to return to a liquid state then as it hits the floor and walls takes time (longer) to get to the cylinders as fuel on the walls and floor travel slower than fuel entrained in the airstream.

I would suspect it is a pump shot issue so start there. I would also check the IFR and IAB sizes to make sure its rich enough on the Tslot. The Tslot fuel is quicker to respond to rapid opening of the throttle blades. You can actually make the Tslot circuit rich enough so as to not need the pumpshot circuit but that creates other problems.
 
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