Holley 750 carb adjustment

[Wietse]

Hey all,

I am trying to adjust my carb for some better settings due to hesitation etc.
I've checked and adjusted the fuel bowl levels to the correct level and set the idle screws to max vacuum i could get and still runs good.
Also set both pumps accordingly and they react immediately when operating the throttle.

Now the engine idles good, reacts without hesitation and hangs aggressive on the throttle.
Last problems that remains is the following:

When hitting the throttle hard (not WOT) it takes off like a canon ball but after a few seconds leaving the throttle in that position it cuts out. (no power and the car slows down)
When releasing the throttle and returning it back to a "normal" position for it's speed it just drives normal again.
If i repeat it does the same.
I am wondering if this could something simple as a blocked fuel filter or is there some adjustment i could try to improve this?

I have removed the primary bowl just to check and found a "65" power valve (6.5?) and 2 "78" nozzles.
The best vacuum i could get is 10 Hg VAC with the engine idle in Park.
I do question the gauge a bit as it says 3 Hg already when open to atmosphere so the reading might be not accurate.
The carb is a Holley 750 4bbl with mechanical secondary on a 440 HP engine.
The specs of the cam are unknown, but according me this is a pretty wild cam.

I am an absolute n00b when it comes to carburators and i am hoping someone can chime in and pass on some advice, i am sure there is a lot of knowledge here with setting a carb.

Thx for reading!

 

[PurpleBeeper]

I'm not expert, but it "sounds" like you're running out of fuel. Like you mentioned, it could be a dirty fuel filter OR possibly a fuel pump that can't keep up. Do you have a mechanical fuel pressure gage so you can see what your fuel pressure is at WOT when it starts cutting out?

You might want to get a known-good vacuum gage. It doesn't "sound" like your power valve or jets ("nozzles") are off too, too much.... I think it would take an oxygen sensor to know for sure. IF your vacuum is really 10" Hg, then you might want a 4.5 power valve, but from my limited experience, a 6.5 power valve with 10" Hg would give you a really, really rich idle, which you're not reporting.

Everyone wants to just throw larger jets at Holley carbs, but I learned the hard way that's not "usually" the answer + if it takes off just fine & your spark plugs don't read lean, I'd make a jet change my last option IMO.

Carb experts, please chime in

 

[Wietse]

I don't have a fuel pressure gauge unfortunately, by knowing the materials used when this engine was build i don't think they cut short on a decent fuel pump.
All jets and power valves are the ones that were installed when i bought it and have not made any changes.
To me it does run rich, when revving it smokes a bit and a few weeks ago i had the spark plugs out which were pretty black and greasy.
Regarding the vacuum, due to the gauge failure it only pulls 7Hg which is not much imo, but this could be due to the camshaft.
I did order a new fuel filter but after receiving it i realised it's wrong, i need one with 3 connections. (I need one with a vapor line?)

 

[hashmaker]

once you have established what your idle vacuum is then then your power valve should be half of that. i.e. 12 for vacuum would mean 6 on the power valve. 13 is when you want your 6.5 power valve. In the past for me as RPM climbs if it leans out or acts like it has run out of gas, low float level or jets do need to be bigger because the gas is no longer there to mix with that much more air going down the carb. But knowing your fuel pressure and a new filter is always good!

 

[Wietse]

I think the power valve and jets are standard size for this carb, not sure though.
Fact is, when i hit it, it goes but after a short time it dies.
So everything is there, but guessing it just runs out of fuel..?

 

[hashmaker]

That is what I'm thinking. low fuel pressure, low float level or jets to small to feed it enough gas to do 5k or 6 k however fast your going??

 

[WileE]

They symptom you describe appears to be in the power valve. The accelerator pump circuit supplies the instant burst when the throttle opens, since the air velocity is fast and no vacuum draws from the main metering circuit. Jetting only affects the main metering circuit, and the power valve has nothing to do whatsoever with the idle circuit.

 

[miller]

They symptom you describe appears to be in the power valve.

I tend to agree with WileERobby. I'm not a Holley guy, but my limited experience on 'em, even if the power valve is the right #, doesn't mean it's any good. One backfire, and their junk, and needs to be replaced.

 

[eric383]

Also don't forget the acc pump squirters can be changed to a larger size if the carb still has the factory squirters. Also there is a bunch of good tuning info on Holleys web site.
https://www.holley.com/blog/post/how_to_adjust_the_accelerator_pump_on_holley_carburetors/

 

[Wietse]

Thx guys, good info.

To confirm on how i understand the story of the accelerator pumps.
When you open the throttle more to accelerate, the pump(s) shoot a dose of fuel in the intake to feed the engine, during the process of burning that fuel the intake vacuum increases which pulls the right amount of fuel from the main metering circuit to maintain engine power/speed.
What about the power valve his function/operation?

To carry on with the story:
I removed the fuel filter and blown it dry with air, after some tapping on the workbench i noticed some shite came out and continued tapping the filter housing till everything was out, see picture. (no the lighter was not in there but just for reference)
After i reinstalled it and as a precaution i installed the new inline K&N fuel filter i ordered previously.
As i need a fuel filter with 3 connections, which i don't have at the moment but will order a new one.
It already had an additional inline filter which was clean so i am sure all dirt stayed inside the first filter.
Now, while i am at it i am installing my new alternator as well, once done i will give it a try and see what happens.

 

[Wietse]

So did some test driving and it looks like the filter caused it. (no wonder with that much **** in it)
It does run pretty crisp for being adjusted by an amateur
Although i am little confused with the 4-corner idling screws.
As per most instructions i've seen you screw them all the way in, then back out 1-1/2 or 1-1/4 turn.
With a vacuum gauge connected you monitor the vacuum and back out the screws 1/8 of a turn at the time to get the best vacuum reading right? (or screw them in??)
After them set you can adjust the idle rpm if required.
Is above correct? As with adjusting yesterday i started at 8-9Hg and after backing off the screws a few times i ended up with 10Hg and it did not increase after so i left it at that setting.

 

[Cornpatch MO]

Good heavens! What is all that crap in the filters from??. That confirms my thought on your problem --fuel starvation. Are you using ethanol in a possibly dirty fuel tank or old gas lines? especially if any are rubber. That stuff will break crud loose and plug fuel filters.
#88 jets should be plenty on primaries, and may be a little rich
Unless it is a 20+ year old Holley, the power valves will not blow out with a back-fire.
Power valve--engine vacume closes the valve, which blocks a portion of the circuit. --punch the throttle=engine vacume drops-power valve opens up , and supplies additional gas. The number on the power valve is the amount of vacume at which the power valve closes--drop below that vacume and the power valve opens...It's actually a fuel economy thing..............MO

 

[Wietse]

Yeah it was a lot indeed, i found small parts of rust, splinters of wood and steel wire.
The steel wires look like coming from a small wire brush, but my thought also was from a filter/screen so thats why I installed the K&N fuel filter as wel in case that filter is damaged.
Probably the fuel cap was left off during a restoration job and allowed all that **** inside.
When i pull the engine this winter i will replace all fuel hose anyway to make sure they are good and might drop the fuel tank as well to clean it out.
I am not using ethanol, pure pump gas 98 octane.
Thx for the explanation of the power valve, now i know

Guess i will strip the carb as well this winter and clean it out, i did find a small amount of black sludge inside when i had it open.
Just need a gasket kit, trouble is according me it is a 4150 model but i can't find any model number on the carb itselft.
It is not stamped where it supposed to be?

I think this is the one:
https://www.holley.com/products/fuel_systems/carburetors/hp/street_hp/parts/0-82751SA

 

[PurpleBeeper]

once you have established what your idle vacuum is then then your power valve should be half of that. i.e. 12 for vacuum would mean 6 on the power valve. 13 is when you want your 6.5 power valve. In the past for me as RPM climbs if it leans out or acts like it has run out of gas, low float level or jets do need to be bigger because the gas is no longer there to mix with that much more air going down the carb. But knowing your fuel pressure and a new filter is always good!

This sounds reasonable.

 

[PurpleBeeper]

Thx guys, good info.

To confirm on how i understand the story of the accelerator pumps.
When you open the throttle more to accelerate, the pump(s) shoot a dose of fuel in the intake to feed the engine, during the process of burning that fuel the intake vacuum increases which pulls the right amount of fuel from the main metering circuit to maintain engine power/speed.
What about the power valve his function/operation?

To carry on with the story:
I removed the fuel filter and blown it dry with air, after some tapping on the workbench i noticed some shite came out and continued tapping the filter housing till everything was out, see picture. (no the lighter was not in there but just for reference)
After i reinstalled it and as a precaution i installed the new inline K&N fuel filter i ordered previously.
As i need a fuel filter with 3 connections, which i don't have at the moment but will order a new one.
It already had an additional inline filter which was clean so i am sure all dirt stayed inside the first filter.
Now, while i am at it i am installing my new alternator as well, once done i will give it a try and see what happens.

View attachment 495958 View attachment 495959

Holy sheep **** that's a lot of junk in the filter!!!! I strongly suggest that you buy 4-5 fuel filters & swap them out every week for a month to catch all that garbage. I think you've found your problem.

 

[mentor70]

may want to see whats in the gas tank too...damn

 

[Wietse]

It still chokes and powers down sometimes when the secondary supposed to come in.
I can feel the extra build up of resistance in the pedal, which must be from the secondary mechanism.
Once i go there at some occasions it still does the same, no power and only option is throttle down to normal cruising speed and he is picking up again to normal.

Guess first thing will be order a rebuild kit for the carb and check the secondary system for blockage or a failed component.
Would a stock fuel pump keep up with this kind of carb?
What is the standard carb CFM rate for the 440 4-barrel?

Fuel supply from tank must be there now, either the fuel pump is not keeping up once the secondary has kicked in or there is something wrong inside the carb on the secondary side.
I get the feeling that for a short use of secondary (like 2-3 seconds) nothing happens but after that it is powering down, not sure if this is consistent though.
By using the primary there are no issues at all.

One way i want to go and get it adjusted by someone with experience with this but to be honest i want to sort this myself and learn troubleshooting and adjusting carbs myself.

 

[Cornpatch MO]

It could be your jet size-primary and secondary together may be to large in total. I once had a 327 chebby I traded for. Had a nice cam in it and aluminum hi-riser. Holley carb. It ran terrible --huge bog when the secondaries kicked in. I checked the numbers and found out the Holley was for a 427 Vette! I found the correct jet sizes for a 325 horse 327cu inch and installed them . That made a HUGE difference ..ran like a stripped zz ape !
Also, if you have that fine black crud in your fuel bowls, my bet is it will need to be cleaned and kit installed............................MO

 

[Wietse]

I decided to go ahead and open up the secondary metering block.
I found the power valve to be a blank dummy??
Jets are size #84.
Did not find any dirt or blocked channels.
Only abnormal thing i found was that with the current fuel bowl setting the needle is squeezed that tight in it's seat that it gets kind of stuck, with knowing the fuel pressure will be very low as the primary is also taking max fuel from the line it might not be able to open it up and run the secondary fuel bowl dry.
I backed it off a full turn (increasing the level) and will build it all back and see what comes out.

 

[Wietse]

When building it back noticed as well that the float was touching the housing, limiting it's movement down.
Due to this the needle valve will not fully open and feed enough fuel to the bowl.
See the scratch mark on the housing, just had to bend the float arm a bit to straighten it up with the housing.
Guess this issue is solved once it's back in.