Big Block C20 Running Rich...Holley 750

[SavageC20]

I have a Holley 750 ( https://www.holley.com/products/fuel...arts/0-80459SA ) on the 454 in my c20 and it has run rich since I put it on there. To the point where it burns the eyes when the truck is sitting there idling.
I have set the idle mixture screws with a vacuum gauge to achieve the highest reading. I am thinking that I might need to step down a few sizes on the main jets. I know I can take it to a shop and have it tuned but I would rather learn something and save some money.

The carb currently has size 72 main jets. It runs fine at partial throttle and wide open but I know it's too rich. Plugs are black...

Any suggestions on where I should go from here? I don't know much about carburetors on vehicles but I have rebuilt and messed with several on dirt bikes, quads etc so I understand the concepts.

Engine Specs:
-454 bored .40 over
-Stock Crank and Rods
-Flat Top Pistons
-049 Oval Port Heads (Stock Recon.)
-Edelbrock Performer 2.0 Intake
-Stock Cast Iron Exhaust Manifolds
-2.5" Dual Exhaust
-HEI: Timing is set at 14 initial and 35 Total w/ vacuum advance unhooked
-1.72 Roller Rockers
-Hydraulic Flat Tappet Cam Specs:
-Dur. @ .050: 204 214
-Adv. Dur.: 282 292
-Lift: .476 .501
-Lobe Separation: 108 116

[volksworld]

nave you set the float level ?

[SavageC20]

I haven't messed with the float setting but I did take the screw out of the side of the primary bowl while the engine was off and some fuel did come out. Not sure it its supposed to do that or not.

[i82much]

if the idle is too rich, it’s not the main jets - they aren’t flowing at idle.

start by pulling the carb and squaring the transfer slots. clean it out good, put it back on and set the float level so fuel just barely dribbles out. then set the idle mixture screws. chances are you will be good to go.

[SavageC20]

Originally Posted by i82much

if the idle is too rich, it’s not the main jets - they aren’t flowing at idle.

start by pulling the carb and squaring the transfer slots. clean it out good, put it back on and set the float level so fuel just barely dribbles out. then set the idle mixture screws. chances are you will be good to go.

I can remove the carb tomorrow and try that tomorrow. However, the carb really hasn't been messed with too much and it is fairly new. Why would I need to adjust the transfer slot "reveal" if it hasn't been messed with?

[geezer#99]

Any idle adjustment higher can expose the transfer slot.
What’s the idle rpm when you check the timing?

Info here about the transfer slots.


http://www.crankshaftcoalition.com/w...EI_distributor

You might need to open the secondaries a bit so you can close the primary enough to cover the slot.

Float level is good if it just trickles out when you lean on the fender. Check it when idling.

[i82much]

Originally Posted by SavageC20

I can remove the carb tomorrow and try that tomorrow. However, the carb really hasn't been messed with too much and it is fairly new. Why would I need to adjust the transfer slot "reveal" if it hasn't been messed with?

the idle speed screws control how much transfer slot is showing. if those weren’t set initially out of the box or if anyone has adjusted the idle speed screw on either side, they win’t be square. on my carb the secondary idle speed screw is a hex head under the baseplate, btw.

[firedemon]

this guy has some great videos for a beginner holley tuner https://youtu.be/oBOrWUPhYto he is a ford guy but he has some good carb stuff

[KQQL IT]

I had a power valve gasket leaking.

[Wrenchbender Ret]

Your power valve is probably ruptured. It is in the primary metering block.
George

[notsolo]

And you haven't changed the fuel pump recently?

[i82much]

i could be wrong but i thought newer holleys all have power valve blowout protection.

pv gasket could be it, but i would still start with squaring transfer slot. clean the air bleeds and internal metering orifices and needle/seats as well.

[RustyPile]

I've worked with Holley carburetors for many years (more than I care to think about).. Ran 'em on race engines, street cars, boat engines, etc... I find them very easy to tune.. I use a different technique, to establish the transfer slot size, which is very old school.. It involves drilling a small hole in the primary throttle plates.. Holley doesn't speak against that technique.. The reason I use the "drill a hole" method is because it puts all the idle fuel and air flow on the primary "side".. None of the idle requirements are handled by the secondary "side".. Not the idle mixture, not the idle speed, nor does the secondary float level effect the primary "side".. I don't know if my theory is irrefutable or not, I just know it has always worked for me..

Back to the problem at hand. Nobody has mentioned anything about secondary jetting, but they are a part of this picture. I live in East Texas and I don't know how similar the air density, altitude, atmosphere, and other factors compare to where you are in California.. I have a 454 setup in my truck that is very similar to yours.. My Holley is a 3310-3.. The main difference is mine has dual fuel inlets.. Main jets are #71, Power valve is #65, and the secondary metering plate is a #39... Spark plugs have "say so" in the matter also.. Mine are AC #R45TSX.. Gap is set at .048....