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4150 4778 Holley carb questions

General engine tech -- Drag Racing to Circle Track

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Bill Chase
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4150 4778 Holley carb questions

Post by Bill Chase » Mon Jan 13, 2020 10:33 am

I picked up a 4778 700 cfm double pumper from a friend, it was drained and stored properly a couple years ago. I bought a quick fuel basic rebuild kit for it. I hope to simply freshen it up and use it on a budget, but rather "rowdy" street strip toy. 350 l83 crossfire corvette bottom end, probably a set of engine quest 200cc vortecs and a healthy solid roller with springs to match, will be around 10-10.5 to 1 compression and the plan is go essentially run it until it breaks. The car is a 4+3 and has 3:45 rear gears.

Any tips or tricks for this carb, it will be street driven, but only for occasionally cruising and or grudge racing with friends etc, so losing bottom end isnt a concern, I have a 7101 edelbrock performer rpm that I planned to use, along with long tube headers with 1-5/8 primaries.

Any suggestions for baseline jetting or modifications to make this more easy to tune? I have an innovate mtx-l I'll use to tune it, but was hoping for some tips and tricks from those far more skilled than I to establish a baseline jetting and setup???

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Re: 4150 4778 Holley carb questions

Post by Bill Chase » Mon Jan 13, 2020 10:38 am

Plan to mill the divider to match the air gap performer rpm, and can run a spacer if needed, I have no idea what jets are in it, but would like to get an assortment to help tune, I realize this is probably far from an optimal setup, but the plan is simply to have fun, not spend a ton of cash and when it breaks simply replace with better. Last carb I messed with was a qjet for a marine application, and have literally never tore into a Holley. If you good people cannot offer advice, perhaps steer me toward a decent book to read and reference, I can pay someone to do this, but the goal is to learn, as well as have fun in the process.

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Re: 4150 4778 Holley carb questions

Post by dannobee » Mon Jan 13, 2020 11:13 am

Here's the factory cheat sheet on what jets it came with stock.

https://documents.holley.com/techlibrar ... isting.pdf

If it's street driven, you should probably keep the power valve but remember, if it backfires, it'll probably be toast and run really rich. 65 power valve was stock. If you have some small number drills, make sure that the power valve channel restrictions are stock size. There are PV plug kits (and the jets get changed to bigger), but on the street the mileage will be shitty. Really shitty.

If your rebuild kit doesn't have blue gaskets, do yourself a favor and get some. Those old black ones would stick and were a pain to clean up all of the remnants. There are various needles and seats. The cheapo kits don't usually have the open windows. The holley kits are usually better.

There are lots of books out there on these old dinosaurs, but the basics are invert the float bowl to check the float level when off the car. Check the float level via the plug when on the car. Keep fuel pressure around 7 psi. If you change the fuel pump or regulator make sure to recheck float level.

Oh, and make sure that the adjustment for the needle and seats have the gaskets, otherwise they'll leak fuel and could cause a fire.

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Re: 4150 4778 Holley carb questions

Post by jmarkaudio » Mon Jan 13, 2020 11:33 am

The two biggest things would be to make the idle air bleeds adjustable and the PVCR's. You will need a good selection of drills and a couple bottoming taps.
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Re: 4150 4778 Holley carb questions

Post by frnkeore » Mon Jan 13, 2020 2:44 pm

Bill, your description sounds like it could be a C4 Corvette?

I want to eliminate my Crossfire and go with a 4v. I have both a Torquer and Performer manifolds but, if I install the 4v, I don't want to cut the hood.

Do you or anyone else have info on getting 4v on a C4 w/o cutting the hood?

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Re: 4150 4778 Holley carb questions

Post by 1980RS » Mon Jan 13, 2020 5:35 pm

Figure the 4778 700 like the 750 Dominator the difference is the bell shape boosters, easy to change on a Dominator but not on a 4150. Just get one of the Holley 750 HP bodies and swap everything over and have a really good carburetor.

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Re: 4150 4778 Holley carb questions

Post by Tuner » Mon Jan 13, 2020 7:32 pm

1980RS wrote:
Mon Jan 13, 2020 5:35 pm
Figure the 4778 700 like the 750 Dominator the difference is the bell shape boosters, easy to change on a Dominator but not on a 4150. Just get one of the Holley 750 HP bodies and swap everything over and have a really good carburetor.
Don't you think he should get aluminum heads and use a roller cam too?

There is nothing wrong with the 700 Holley whether it has skirted boosters or not, not all of them do, only the early ones. Later units (from 4778-3 or -4 on, I forget) have the same size venturis as early, 1-5/16" Pri x 1-7/16" Sec, but with standard straight leg boosters.

Chances are good a 700 body from an early production run will be a more accurately made casting than a newer one. When Holley let the dies wear out the core shift produced sloppy main bodies with venturi inducer and exducer (upper and lower halves) out of concentricity. Because a sloppy casting usually has a different amount of deviation in each bore, the offset venturis cause the strongest booster signal to wander from corner to corner as flow changes. As you might guess, identical and consistent booster signal is considered desirable, unless you have a reason for different metering pressures, such as to correct a distribution anomaly. In any case, non-concentric castings don't flow as well as a proper concentric casting.

If close examination of a carb body finds the upper and lower venturi sections are offset, such that the venturi exducer wall encroaches into the flow below the correct minor diameter location which is necessary for the venturi to have a properly symmetrical vena contracta, it would justify examining a new body to see if a better casting is available.

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Re: 4150 4778 Holley carb questions

Post by F-BIRD'88 » Mon Jan 13, 2020 7:38 pm

A prime candidate for a holley/proform HP 750 carb body upgrade.

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Re: 4150 4778 Holley carb questions

Post by MadBill » Mon Jan 13, 2020 10:26 pm

Bill Chase wrote:
Mon Jan 13, 2020 10:38 am
... If you good people cannot offer advice, perhaps steer me toward a decent book to read and reference...
By far the best advice I can offer, regardless of any of the good suggestions you receive here, is to thoroughly study at least one of the numerous good books on Holleys. Dave Emanuel, Mike Mavrigian and Des Hammill have all authored very good ones, but my first choice would be David Vizard's "How to Super Tune and Modify Holley Carburetors" which in addition to the basics, covers a number of quite advanced mods and tuning tips.
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Re: 4150 4778 Holley carb questions

Post by rustbucket79 » Mon Jan 13, 2020 10:49 pm

If the carb hasn't been molested, just do the rebuild, put in the stock jets, and tune with the AFR gauge. ( a cam with lots of overlap can give you a falsely lean reading at idle)

If the passages are full of crap, it's messed with, or just a lemon, you might never be successful with that carb.

Both the base Edelbrock and Holley Brawler series of carbs are excellent alternatives and budget friendly.

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Re: 4150 4778 Holley carb questions

Post by 6.50camaro » Mon Jan 13, 2020 11:08 pm

I would start with a 70's or 72's for the primary side and 80'sor 82's on the secondaries . This may be a little rich but should be safe (not to lean). I trimmed the excess off the throttle plate screws and used the wicking type loctite to secure them . Pink accel pump cams and 31 primary squirters 35 secndary worked good for me for best off the line reaction . Good luck . Dan

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Re: 4150 4778 Holley carb questions

Post by Bill Chase » Tue Jan 14, 2020 3:35 am

1984 crossfire corvette with l83, top end is essentially garbage, but it does have heavy trw pistons that are forged, cheap fun with minimal cash outlay is the goal. As near as I can tell the intake and carb I have will fit without hood cutting, although fitting a breather may be a challenge.

frnkeore wrote:
Mon Jan 13, 2020 2:44 pm
Bill, your description sounds like it could be a C4 Corvette?

I want to eliminate my Crossfire and go with a 4v. I have both a Torquer and Performer manifolds but, if I install the 4v, I don't want to cut the hood.

Do you or anyone else have info on getting 4v on a C4 w/o cutting the hood?

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Re: 4150 4778 Holley carb questions

Post by nabx22 » Tue Jan 14, 2020 1:19 pm

I have an 85 that is pretty much done that way.I used a 7530 team g intake with a 750 double pumper and a nitrous plate and the hood still closes with a low pro air cleaner

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