Holley Carburetor size

hd_cat

Veteran Member
Sep 6, 2013
144
United States
Hi I have a 402BB mild cam and have Holley 750cfm with 72 size main jets the plug keep fouling out will going to a smaller jet fix this or do I need to get a 600cfm if a smaller jet would fix this any recommended size to start with ?


You might consider the following;
- Fuel Pressure 4 - 6psi, most mechanical "replacement" fuel pumps generate more than 8psi, Holley's don't do well with that pressure, it pushed fuel past the needle and seat and runs richer
- Power Valve approximately numbered 1/2 the vacuum level at idle example: 12 inches Vacuum = 5.5 power valve
- Check Primary throttle position, only a small amount of the idle mixture slot should be exposed, beyond that, you're using more fuel than should be used at idle
- Check throttle position Secondaries, use secondary idle screw to slightly open the secondaries, may want to replace the screw with an allen head set-screw
- Install the Power Valve Protector check ball if you have an older carburetor, newer ones have it already
 

FS87LT

Member
Apr 3, 2010
70
DFW, TX
Do go into the Holley Carb Number spec spreadsheet (or the basic Holley online catalog) to determine what jets are in the carb from Holley. Compare that to the 3310 carb items, for good measure.

In the throttle bore area, the outer air bleed holes control the idle fuel calibration. IF you plug it with your finger, it will cause that side of the idle system to go full rich. When it does that, if the idle speed goes up and then down, the basic idle fuel adjustment is too lean. If the idle speed goes straight down, then the idle mixture is on the rich side to start with. This is a shadetree method, for sure, but it was related by a Holley engineer at a Holley carb seminar 35 years ago.

Power valve issues can result. Normal Holley power valves all hit full enrichment at 5.5" Hg, which is really pretty deep into the throttle at lower rpms. The power valves can blow out if there is a backfire through the carb, with very little other real indication other than low fuel mileage, unless you know what you're listening to in the engine's idle behavior. 6bbl set-ups were especially bad about blowing power valves, by observation.

If it's taking a few hundred miles of driving to foul the plugs, the mixture is on the rich side, but probably not too rich, per se. So making sure the power valve is working, the idle air bleeds in the carb are open and not gunked-up with deposits, and then that the idle mixture is set to "lean best idle" at 700rpm range, things should be operating pretty well.

What is the intake manifold vacuum at idle in gear (for an automatic)? Reason I ask that is that I discovered that my Camaro 305 with a 210 degree @ .050 cam and stock converter was pulling 10.5"Hg in "D" at hot base idle (about 625rpm or so). The emissions-spec Holley 4160 I had on the car at that time had a two-stage power valve, which started to open the first stage of enrichment at 10"Hg. Yikes! That was close, as it turned out. So, that is why I'm asking about this.

Of course, making sure the hot base idle timing is enough to have good power and still not clatter at WOT is important and something that you might need to play with to get it "right" for your combination. And that the vac advance is working, too.

I know that many in here like AC spark plugs. Nothing wrong with that, but I've had better results witn NGK (first V-Power) and then Iridium plugs. With a stop at AC RapidFire in the process, back then. There are some YouTube videos which show that the NGK Iridiums (with their fine-wire electrodes) make a much larger flash than other plugs do. More of the spark is exposed to the fuel/air mixture, so better combustion. PLUS all of the newer engines come with ACDelco-marked NGK Iridium plugs in them . . . starting with the Buick 3800 supercharged V-6s in the 1990s. So, get the heat range to match the '86 Corvette iron cyl heads, or the similar aluminum heads, for your type of cyl head and thread reach. PLUS, they last much longer, too.

The GM HEI coil is supposed to be stout enough to fire a .100" plug gap, but allegedly, past about .080" gap, plug wire condition can be an issue. Which means that you should be fine at .040" plug gap or so.

Please advise on how things progress,
FS87LT
 

Harris48cars

Veteran Member
Oct 6, 2021
115
I repair and correct "new" carbs out of the box almost on a monthly basis. QC has changed.

A 3 year old carb is no longer new. Heat cycling, and dry rot take a toll. And if you left the brass sintered fuel filters in the can clog up as well. Any hint of ethanol is hard on parts, especially when things sit, it's worse.

Most "carb" problems are ignition issues, fully or in part in many occasions.
G7, do you fool with Quick Fuel carbs
 

G72Zed

Veteran Member
Sep 8, 2015
3,922
Canada
G7, do you fool with Quick Fuel carbs

Yep, you could say I "fool" with them.....on a regular basis.

QFT Test day 1.jpg
 

G72Zed

Veteran Member
Sep 8, 2015
3,922
Canada
I would check ignition timing first and go one step hotter on plugs

As in one of my earlier posts, and in support of $ Mike quote above, many times "carb" issues are actually ignition issues, or partially to blame.

I'm building my 3rd Holley 750 VS #80508 next week for a 350 sbc 1969 Camaro SS, but it's a 2021 production, so it's a -10. Yours is older as shown by the -1 and julian list date.

Both others on a BBC and stroker SBM. They ran well, but needed a jet change. I also covert the rear metering plate to my mini jet system to make changes quickly on the dyno.

Looking at the pic of the plugs, it looks like the idle timing is rich, and you do not have enough mech/total advance. If the porcelain is coloured the same in the mid sector, and in the base location, you are rich in every circuit.

The last few BBC's I tuned this year required up to 40* timing. Poor chamber shape, excessive quench/squish and low cranking psi & CR ratio for the combo just made it worse.
 

pomartin

Veteran Member
Sep 16, 2019
270
First check your fuel bowl levels. Too high level will enrichen AFR.
Then confirm ignition is set up as desired.
If fuel level is ok chek if low speed air bleeds are clean.
If they are clean then put a thin wire inside all of idle feed restrictors (aboutn 0.01"). This will lean out the cruise and you'll need to readjust idle screws a bit.
If you have adjustable low speed air bleeds there is no need for wire- you can change the air bleed.
 

COPO

Veteran Member
Lifetime Gold Member
Sep 15, 1999
21,710
Ontario, Canada
Holley installed 72 prim jets in their over the counter 750 carbs to keep the BB people happy. I would try 68’s or 69’s with AC R45TS plugs.
 

G72Zed

Veteran Member
Sep 8, 2015
3,922
Canada
FYI, be careful with jetting down on some of the Holley 750's, it's very much based on model design. The original 4779 zinc's with choke were 75 primary's, then went 70's on later ones, the new Ultra DP with chokes are 72's.

All the newer 750 Street HP, UHP, XHP are 75-76 pri-jet, just saying.
 




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