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View Full Version : Holley 750 Jet Sizes?



smbarcelow
07-28-2008, 01:10 PM
Can anybody running a Holley 750 could tell me what size metering jets you're using? I've got a Chevy ZZ4 350 making around 350 HP. The carburetor is currently set up with .058" primary metering jets and .080" secondaries. She seems to be running very rich at low and high speeds. Mid range seems okay, but that may be due to the relatively low load. (I'll know more later in the week when I get a chance to pull a couple plugs and see what they look like.) Anyway, it would be very helpful to know what others with a similar setup are using. I don't know how this carburetor cam setup from GM Performance Parts. If you happen to know the accelerator pump discharge nozzle sizes and cam colors, that would also be helpful. Thanks in advance.

VetteLT193
07-28-2008, 01:25 PM
I think GMPP sets the ZZ4 up with a 750 so it can turn in the mid 6,000 RPM range... I know of a lot of Corvette and Chevy guys who butt heads with GMPP regarding that choice, most of them swear by a 650. Other guys swear that the 750 is great. Usually the guys that don't like the 750 are having problems similar to yours.

with that said, if you want to keep the 750 your low and mid will probably need to be cut back, I wish I had more specifics for you. good luck! You might try some of the classic chevy forums, they are heavily used in older Chevys

BUIZILLA
07-28-2008, 01:37 PM
what is the carb # ???????

58's in the front of the auditorium is goofy small

80's in the mezzanine are porky big...

mrfixxall
07-28-2008, 02:05 PM
like buiz said we need a carb #,,mine had 72 primary and 78 secondary which are differand now..holly reccomends that you only go up or down 4 jet sizes.

leave the squirtwes and cam alone for now unless you have a boug then you will need to change them..my stock ones were 28 now i have a 37 in their now,btw i also have a 355 but more hp then yours.

smbarcelow
07-28-2008, 02:12 PM
mrfixxall - When I took the carb apart, I found that the larger discharge nozzle was in the primary side, which struck me as odd. If memory serves, the 31 was up front and the 28 was in back. I switched them around when I put it back together.

BUIZILLA - The list number is 4779. I believe it's a 4150/4160 series.

VetteLT193 - I'll try the classic Chevy guys and maybe run it by my Pontiac friends at The Formula Source, too.

Thanks for the help so far, guys. I knew I'd get good feedback here.

BUIZILLA
07-28-2008, 02:20 PM
4779 is a std 750 double pumper for a car.... nothing special... doesn't belong on a boat that's fer sure...

smbarcelow
07-28-2008, 02:31 PM
What advantage does a marine carb have over a street carb? Sorry if it's a dumb question, but you don't learn if you don't ask questions.

VetteLT193
07-28-2008, 02:34 PM
4779 is a std 750 double pumper for a car.... nothing special... doesn't belong on a boat that's fer sure...

I agree. Car parts make boats go kaboom.

I have to strongly suggest updating the carb to a marine version. Something like a Barry Grant Marine Demon... You also want to check your ignition system.

Or, at the very least, get a blower that will suck the living hell out of the engine compartment.:bonk:

VetteLT193
07-28-2008, 02:37 PM
What advantage does a marine carb have over a street carb? Sorry if it's a dumb question, but you don't learn if you don't ask questions.

J tubes. the vent tubes are bent back in towards the intake. that's the major noticeable thing to tell a marine carb from a regular carb. there is also a few more things to prevent fuel from coming out of the carb. It is mainly a safety during flooding, but who hasn't flooded one before :bonk:

VetteLT193
07-28-2008, 02:39 PM
Also, you are going to want to check:

starter
Ignition
distributor
alternator

any more I missed?

If it makes a spark, and on a marine gas engine, it needs protection.

mrfixxall
07-28-2008, 02:50 PM
what vett said,also matine carbs also have no vacume tubes which could cause an explosion in the bilage if the tubes are not blocked off properly if you get a backfire..

i run a proform carb on mine (auto ) but i motifyed it for marine by installing the j tubes and removing and plugging the vacume ports. it was laying around and new in box so i used it..4 cornes idle circuit and screw in air bleede jets:).

80 in the primarys is huge for that engine! drop it down to 76 and put the 80 in the secondarys and try it..

MOP
07-28-2008, 02:53 PM
Yes and no! There is a vent tube from the fuel pump that dumps any raw fuel in case of diaphragm rupture into the engine so it does not end up in the bilge. That tube can go one of two places, a barb on the carb or a barb on the flame arrester.

Added note the tubing is special as it closes of in case of a fire!

smbarcelow
07-28-2008, 03:00 PM
Thanks, guys. I usually open the engine hatch when I first start it and always run the blower before re-starting or when cruising slowly. It sounds like the differences all related to explosion prevention. Are there performance differences, too? My dad used a street engine in his jet boat, but the hood was removed so the risk of explosion was negligible.

By the way, the 80's are in the secondary and 58's are in the primary. Sounds like the 80's may still be too big even on the secondary side, which is what I suspected.

BUIZILLA
07-28-2008, 03:07 PM
try 72 & 76, 6.5 PV up front and 4.5 PV in the back office, if it's a dual plane intake I would stagger jet it a tad... drop down 2 sizes on the short runners

get back to us

smbarcelow
07-28-2008, 08:02 PM
The secondary metering block does not have a power valve. I'm wondering if that in combination with the fact that the secondary accelerator pump was hydro-locked due to garbage in the needle and seat caused a previous owner to overcompensate by using gargantuan secondary jets.

mrfixxall
07-28-2008, 09:05 PM
Ya go a tad richer when you eliminate the secondary PV..
80 ain't a tad though.. :nilly:
Holly reccomends 10 jet sizes:)

smbarcelow
07-28-2008, 09:16 PM
Good to know. This has been very helpful. I didn't have a chance to work on it at all tonight. Pulling a couple plugs will tell me a lot. At this point, I'm assuming that it's rich. Thanks for all your help. I'll keep you posted as I learn more.

smbarcelow
08-01-2008, 07:34 AM
Correction: The primary jets were 68's, not 58's. I left them alone and swapped out the secondary 80's for 75's. I found out that new Holley 4150's part number 4779-2 come with 70's in primary and 80's in the secondary, so it wasn't far at all from stock. (It's entirely possible that, when this carb was new, they were getting 68's and 80's.) I pulled one of the plugs and it was black, but not the worst I've ever seen. I'm planning to pick up a new set of plugs, too. The carb is going back on tonight so we'll see how she does.

smbarcelow
08-18-2008, 07:03 AM
After swapping out the secondary 80's for 75's, I still had a misfire. I decided to look at ignition since it seems like I've spend enough time on fuel. Upon pulling the HEI distributor cap off, the electrodes are all loose. Since of them wiggle what looks like 30 to 40 thousandths. The plug wire connector on the number 6 electrode was coming apart from the piece inside the cap and actually pushing the electrode out of the correct plane. (Not enough that the rotor would not pass by, but it kind of paints the picture of a messed up cap.) I had turned back full spark advance from 36 degrees to 32 degrees, having purchased a timing light with advance. Last run only gave me 4,500 RPM's. I'm not sure if that was due to timing or jets, or maybe a combination of the two. Anyway, I bought a new cap and put the 80's back in the secondary metering block. The 80's worked well this spring, so I think I've determined that the cap was responsible for most of the trouble. I'll probably end up setting full advance to 34-36 degrees, too. I'm putting her back together tonight, so we'll see how she does.

BigGrizzly
08-18-2008, 08:19 AM
I was going to stay out of this but you gave me some Key information. You said it worked fine this spring. That does not designate a carb problem it is electrical type failure. The only time I have seen a misfire and be caeb fuel related was someone came in and a jet had come out. "I never touched the carb" he said as I looked at the buggered up clutch screws. As for carb sizes, on a marine application, most people run a carb that is too large in CFM. In a boat the 6,000 RPM thing is not a smart idea with an outdrive. The larger carb usually covers up tuning issues, and mistakes. I have a 351 Cleveland in boath my 1971 Mach I and my Corsican. The engines are the same except for the carb. The Mach uswes the 750 and the boat the 600. The other difference is the RPMs I run The Mach goes to 6,000+ and the boat to 5,500 RPMs. The cams are the same but they are degreed in slightly differently. Yes I have swapped the carbs around and in the boat it makes no difference at all up top only down in the mid range.

smbarcelow
08-18-2008, 09:31 PM
Well, she runs MUCH better now. Thanks Randy and everybody else who weighed in on this one. I was encouraged when I saw the loose electrodes on the old distributor cap. After running her tonight, it was evident that she was missing even in the mid-range where I thought she was okay. She's much smoother now. I was only able to get 4,700 RPM's out of her (with one passenger and trim set at the middle stop), so I think I'll give a little of that timing back. I'm guessing I'm about 200 RPM's lower than I should be under the same conditions. With 34-35 degrees and the trim at the top stop, I think I'll be back up to the 5,000 I was getting this spring. Anyway, thanks again for the advice and feedback!

smbarcelow
08-26-2008, 09:56 PM
35 degrees full advance got me back to 4,900 RPM's with one passenger and the trim at the middle stop. (We found that it was 28 or 29 degrees before tonight's adjustment; not the 32 degrees we thought we had. Must have been that the old cap was really messing up the timing light.) Anyway, I think 5,000-5,100 RPM's with the trim all the way up should be doable. I'll know after this weekend. I'm hoping to be able to update the top speed in my signature, too. It sure would be nice to get an even 60 MPH out of her. :cool:

mrfixxall
08-26-2008, 10:49 PM
Whats your sparkplug gap? ive found on certain engines with higher compression will blow the spark out at higher rpm's.. .035 works the best,some people set the gap a little higher with msd ignition,if you have it set them at .040.

smbarcelow
08-27-2008, 09:23 AM
I think it's 40 thousandths, if I remember correctly, but I'll have to double check. I'm running the stock HEI distributor. That's definitely worth looking into. Thanks for the advice.