Thread: Holley 780 Carb

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I have been having a problem with my carb runing rich. 780 holley vac secondaries. I have taken it apart and put it back together several times. New gaskets , new jets, new power valve, new gaskets under carb, just can't stop it from runing rich. I look down the carb while its runing and don't see a flood going on. Pull the plugs after a few miles and they are black. Any suggestions. On a liter not I was talking to my wife about it and she said with a stright face "Run cheaper gas" I about choked.

whats vacuum at idle in gear? did you try a power valve with a smaller number

Try swapping out the spring in the secondary vacuum can??? Might be getting into the secondaries too soon?

Did you properly adjust the fuel level? is the accelerator pump lever adjusted properly?

Thanks guys Mike vac.12" power valve is 6.5 I tryed a 4 was worse is that a clue I'm missing?. Dave installed new. sunset levels all set

Charlie,
I'm not in any way a Holley expert, but I recall the Holley forum talking about power valves and jets -

The purpose of a power valve is for it to add the needed extra fuel when you are under wide open throttle. It is not intended to be an enrichment circuit during normal throttle cruising. If you are lean or rich during normal driving conditions, you need to tune the jets, not the power valve. The reason you check the vacuum at idle and take that reading and divide it in half, is so you do not have a power valve that will open at idle as this will cause an excessively rich mixture at idle. Under WOT the vacuum of the engine will drop to 0 or near 0 and the power valve will then open adding the extra fuel needed during WOT.

Here's the link to the source I used for info - Power valve selection It sounds to me like your jets are too big, and you should be back at the "6" power valve as a starting point (1/2 of your idle vacuum) but you said you installed new jets. A puzzler, for sure.

How many cubic inches are you talking about?

OH yea that may help Chevy 454+.030

Why don't you list the jet sizes, needle/seat sizes, squirter sizes, are the check valves in the accel pump good, how did you set the float heights, and what fuel pressure your runnng???

Plugs are black--is it sooty or oily???

Have you turned your attention to the ignition? Like maybe you are getting about the right amount of fuel but the ignition is too weak to burn it proper? Like maybe the coil is going south?

I ran a 780 years ago and it ran rich too.
The one thing I learned was if that thing back fire's one time, it
will blow the power valve in the older holley's.
Everytime I was messing with the timming or something
when it back fired I had to replace the power valve or it
would dump the fuel in. Loved that carb though, it was a
great carb for racing.
Kurt

plugs black not sooty, but fouled, floats the only way I know how . remove side plug and adjust so fuel dosen't come out...jets are 70 thats what came in it. I just had this thing gone over by a (certified Holley guy???) the plugs are AC CR43 TS / Accel 263 both sets got black and don't fire. Before I can do anything now I have to get another set. this carb has the check valve in it to prevent the P/V from blowing I'm thinking I need a higher # power valve. I was told with to high a # it will get a stumble going from idle to opening the primaries?? If I can't figure it out I'll be taking it back

I set the idle using a vac gauge fuel pump is mechanical about 6 psi.

Sounds like raw fuel. By any chance you have the wrong metering block gasket? Like the one that has an extra notch above the tombstone pv cutout gasket for an accelerator pump crossover tube? My description is complicated, but the powervalve cutout in the gasket is tombstone shaped, some have an extra cutout on the top of the tombstone because some early cargs have a brass tube with 'o' rings for the accelerator pump shot, it joined the metering block to the main body. If you used this gasket then raw fuel will enter the cavity behind the power valve and into the engine via the power valve vacuum port. You can't see it run into the engine. When you have had the carb apart is the cavity behind the power valve wet?

I had it apart yesterday and I thought it was wet but to be sure I'd have to take it apart again. You say I may have the wrong gasket? between the bowl and the block or the block and the carb?

That carb might get an assortment of gaskets included with a rebuild kit. The gasket on top is the one you don't want to use for the metering block gasket. You need the other gasket.
In any event, the area behind the power valve needs to be bone dry. Another very common goofup is the powervalve gasket can get cattywumpus, i turn the metering block upside down and screw the powervaleve with gasket up into the metering block and it stays centered on the power valeve better. So have a close look at that too.