Holley 390 CFM Accelerator Pump

[37fatfender]

I have a fuel leak coming from my accelerator pump, so I am going to replace the diaphram. I haven't taken the carb off the engine yet, so maybe I am ahead of my self. The engine runs well but the plugs are always black. I went online and there are to sizes to choose from a 50cc and a 30cc, which one do I use? I am leaning for the 30cc, the engine is a 276ci with a Schnieder cam and headers.

[scicala]

You want the accel pump diaphragm for 30 cc. The 50cc diaphragm is too tall for your carb. As far as the black plugs, take the front bowl off of the carb and see if it is wet from fuel on the vacuum side of the power valve on the inside face of the metering body. If so, the power valve is ruptured and needs to be replaced.

Sal

[Terry,OH]

Most likely the accel pump housing is warped, some sandpaper on a glass plate works well to get it flat again, if that is the problem.

[37fatfender]

Sal and Terry thanks for the advise. Whats a good power value to use ? They are differents specs also, Right?

[19Fordy]

37: Is your Holley a 4150 or a 4160? What number is stamped on the choke plate? For instance, 8007. The Holey rebuild kit sheet will tell you what part # you need.
look here:
https://documents.holley.com/techlib...al_listing.pdf

and here:
https://www.google.com/search?q=what...hrome&ie=UTF-8

[backrodebob]

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the power valve your looking for is a 6.5! just changed mine!

[37fatfender]

The air horn on my carb is stamped 8007 and below that is 1559. So i need a rebuild kit for a 8007, what is the significance of 1559? No 4150 or 4160 on the carb.

the rebuild kit appears to be 37-720 and it is a 4160 series carb. Thanks 19Forty

[37fatfender]

Shame on me ! ...... what's leaking is the Vacuum Secondary Diaphragm, not the accelerator pump...... !

[scicala]

Originally Posted by 37fatfender

Shame on me ! ...... what's leaking is the Vacuum Secondary Diaphragm, not the accelerator pump...... !

The 1559 number is a date code. Your carb (8007) is a 4160 model. That is never stamped or cast on the carb. The 4150 model had a secondary metering body similar to the front one.

That is very odd and almost impossible to have fuel leaking out of the secondary diaphragm, since fuel can't get into it. Been doing Holley carbs for over 50 years, and never saw any fuel in the diaphragm housing.

Sal

[37fatfender]

Hmmmmm.... Will let you know.
Thanks, Sal

[Bored&Stroked]

Sal is 100% correct (as usual!) the secondary diaphragm only sees vacuum and it actuates the secondaries on the outside . . . no fuel anywhere near it.

The accel pump has a small rectangular diaphragm inside a much smaller rectangular housing on the bottom of the fuel bowl.

As noted, you may have a blown power-valve . . . that will cause things to be really fat and rich. Did the carb every work correctly - are these recent issues?

[4dFord/SC]

A noticeable smell of raw gas after the engine is shut down can be an indicator of a blown power valve.

[37fatfender]

I pulled the card today, Yes, it is leaking from the Accelerator Pump area. I am waiting for the rebuild kit before I tear it down. Long ago when i put it together I marked the fuel bowl with a marker and noted "50", I am assuming that's the jet size I installed, any thoughts on the "50's" jets?

[scicala]

The original primary jet size for a 8007 390 Holley is #51. I would stick with that. Also, a #50 is only .001" smaller (2% smaller).

Sal

[37fatfender]

Spark plug photo!

[Crankster]

Are those spark plugs the standard heat range? Were they removed after a highway "chop"?

Getting the correct "pump squirt" will help, as will correct jetting and power valve. It helps to know what engine manifold vacuum numbers are at steady cruise and on slight hills &c.

There is some seemingly contradictory information on how to or what power valve # to select which I attribute to the intended audience. Drag racing or semi-performance engines versus stock stock street applications.

It's important to realize that engine vacuum is the only thing holding the power valve closed. The power valve should remain closed at all times other than demand for power, acceleration, under load etc. If the wrong value is selected, or it is defective, it will constantly dribble fuel.

There is also a correction factor applied for altitude or local elevation, a PV suitable for Iowa or Kansas is not going to work for high country Colorado. Plugs are really difficult to read with modern gasoline and will ordinarily look pretty lean when they are correct. The old spark plug reading color picture charts aren't really that useful anymore.

Make sure, the power valve is not open during flat ground cruise, not even a little. Then, jet up or down as necessary.

[19Fordy]

37fatfender: What engine are you running? Is it stock? Do your sparkplugs have enough reach?

[37fatfender]

19Fordy,
The engine is a 8ba, Merc crank, 276ci, Schnieder cam 355 lift. Holley 390cfm carb. Edelbrock 952 4bbl intake with a 1 inch riser. Note I run the carb backwards to accomodate the 40 gas peddle linkage. The head are early version Edelbrock 32-48 modified water passages for the 8ba block. The plugs I have in it at present are NGK FR4, which are comporable to Champion RC12YC that Edelbrock recommends for use in these heads. I believe the reach is correct, and if I remember, which is getting harder to do these days........ I opened up the plug area before I installed the heads.

[19Fordy]

37: Thanks for the feedback. That FR4 Plug has a 3/4 inch reach which is good.
On my 239 cu. in 8BA with Motor City Flathead early style heads. I am using NGK VR4 2635 plugs that also have a 3/4 in. reach. Also running a 390 Holley with #51 jets.
No problems. Wonder how "my" plugs would work in your engine.

Take a look at this video and see if it might help you. I replaced the slotted screw with an allen head screw so it can be adjusted with the Holley carb on the car.
https://www.youtube.com/watch?v=b8ROjrGAH2I

[5851a]

Just out of curiosity what distributor are you using with the 390 Holley?

[37fatfender]

5851,
The dizzy, is a SBC with Pertronix Electronics installed. No vacuum advance.

[19Fordy]

5851 and 37: The distributor I am using is a Mallory dual point distributor with mechanical advance that I converted to electronic with a Pertronix kit. It ran fine with dual points but, I decided to try an electronic unit 5 years ago when installed an electronic Mallory Unilite kit. The Unilite modules kept burning out so I finally installed the Pertronix kit which is what I have now.
You can see the part number info. in the photos.

Pertronix does make a kit for the stock 239 Load-o-Matic distributor.

I realize this won't help as your using a SBC distributor.

[37fatfender]

19Fordy,
Ditto on the Uni-lite......they had me on the side of the road twice.
Are the plugs your using VR4 or GR4-2635?

[19Fordy]

37: The plugs I am using are NGK GR4-2635. I made a mistake earlier.

[BUBBAS IGNITION]

tUNE WITH VACUUM GAUGE INSTALLED ..

crankster said
It's important to realize that engine vacuum is the only thing holding the power valve closed. The power valve should remain closed at all times other than demand for power, acceleration, under load etc. If the wrong value is selected, or it is defective, it will constantly dribble fuel.

A 6.5 power valve would remain closed as long as vac is above 6.5 , then as throttle is opened and vac goes below the 6.5 setting the power valve would open adding enrichment ........check with vac gauge , pretty easy to get the perfect setting allowing engine to run really well........

[19Fordy]

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Mr. Linder, Thank you for that very clear explanation.

QUESTION:

I have read that the power valve should be selected based on the vacuum at idle. Is that true?

If that's true would using a #9 or 10 power valve be correct if the vacuum reading at idle is 19 in?

Thank you.

[Crankster]

The problem is carburetor instructions are a little bit vague or don't fully explain what is going on with a power valve. A mechanic's vacuum gauge plumbed into the cabin is really useful to get a handle on how the engine handles different load and power settings and terrain. A 6.5" power valve is a good safe choice for many engines, but a performance engine with a lumpy camshaft, especially at any sort of altitude, would quickly run into problems. If you know what vacuum your engine pulls when cruising along on flat highway, that is really useful for selecting a close cut power valve #.

A defective power valve, or a # mismatched to that particular engine configuration can really skew things, eapecially reading plugs. Trying to select proper jetting for example. It's also possible to get defective power valves right out of the box. Testing with a Mityvac before installation is a good plan, to weed out duds. Like many other reproduction auto parts the quality isn't always there compared with brand name or NOS. I haven't much luck with the ersatz no-name power valves holding up to today's crap gas either. A year if lucky.

[5851a]

Thanks for answers on the ignition systems.

[37fatfender]

I have a Vacuum gauge mounted under my dash. I get a pretty consistent 17-19 in. at idle. I dont think the power value is my problem but i will replace it when i do the rebuild. Still waiting on the kit.

19Fordy.... any particular reason you use the plugs you do?

[aussie merc]

you only have a 390 not a 600 i would definatly steer clear of a 50cc pump that a lot of fuel in a short time you may need to change the cam and/or adjustment if its stumbling but i dout you would ever need 50cc on that carb

[19Fordy]

37: I use the NGK GR4-2635 because they were the ones recommended
by Mark Kirby (Motor City Flathead) in 1999 when I bought the heads from him.

[37fatfender]

Thank you, Everyone for your help.

I finally got to rebuilding the carb. The Accelerator Pump diaphram was leaking and the small orange rubber check valve was in pieces. Probably from our modern ethanol gas.

It's back together and fired right up.

Thanks Again All.

[19Fordy]

That's great news. Be sure to add STABIL to your gas.