Fresh 496 Running Rich. Ideas?
 

Hi everyone. Hoping you can help. 1963 GMC with a freshly built 496 (Mark IV) that is running really rich at idle and I’m out of ideas. Plugs are gapped at .045. It is hard to start and won’t start unless the choke is fully open. Even then, it takes several cranks and doesn’t run smoothly until warm. The idle mixture screws are out only 5/8 of a turn and that keeps the AFR at idle around 13 (with 1.5 turns out it’s running at 10 AFR if it will even start). The engine does die if I turn them all the way in. I have tried initial timing from 10 up to 22. 22 is where it is now but that seems excessively high to me. It was at 10 initial/34 total at the dyno and I originally changed nothing when installing into the truck. Timing was read with vacuum advance properly plugged. On my second set of plugs and wires just to rule those out. HEI is properly wired and receiving 12V. Checked for vacuum leaks and found nothing. Any ideas for what else I can check? Is this just the nature of the beast? Defective carb? I’ve searched around this site and the internet in general and I feel like I’ve tried everything. :(

Heads: AFR 3610-1 265cc heads
Cam: Comp XE274H, Int/Ext @ .050 230/236, Lobe separation 110
Compression: 10.06:1 static, 8.21:1 dynamic
Gas: Pump 92
Fuel pump: Mechanical @ 5.5 – 6 psi
Plugs: Autolite 3924 gapped at .045
HEI: MSD Pro Billet
Carb: New Holley 4150 Street Avenger 870cfm, 6.5 power valve, 79 primary, 83 secondary
Vacuum at idle: ~12-13

Let me know if I’ve left any important info out. I can post scans of engine build/dyno runs if that helps.

Thanks!

Ryan

Re: Fresh 496 Running Rich. Ideas?
 

If you can turn mix screws in like that then it tells you the primary blades are too far open.
Open up the secondaries enough to allow you to close the primary enough to make the transition slot look square.
That might help.

What intake? Is it an air gap type?

Re: Fresh 496 Running Rich. Ideas?
 

Thanks, I'll take a look at the transition opening later today.

Intake is not an air gap. Edelbrock Performer RPM dual plane.

Re: Fresh 496 Running Rich. Ideas?
 

Good choice on the intake.

Info in here on the slot.

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

Re: Fresh 496 Running Rich. Ideas?
 

Do you have the vacuum advance hooked up to full manifold? that will help you close up the primaries and pull more from the idle circuit.

Re: Fresh 496 Running Rich. Ideas?
 

If it was running fine when dyno was done I would think that the carb settings
were fine. what are you using for fuel delivery?

Re: Fresh 496 Running Rich. Ideas?
 

Took carb off and inspected the transfer slots. The primary wasn't too far off "square" so I adjusted the idle screw until it looked square by eye. The secondaries were not exposing their slot at all. After doing some research online it seems like they should idealy match the exposure of the primaries, so I adjusted to match the primary transfer slot exposure.

Bolted on the carb, adjusted the idle mixtures to 1.5 turns out and reset the choke to normal starting position to get a normal baseline. No luck starting so I slowly started leaning things out to where they were before and still no luck. Choke fully open and idle mixture screws 5/8 out.

Is my adjustment to the secondaries counter-productive and actually creating a richer mixture?

Re: Fresh 496 Running Rich. Ideas?
 

That's what I was thinking. No adjustments were made between the dyno and installing in the truck, so I'm really surprised and frustrated to have this issue. They did mention something about it having trouble idling at the dyno, but we were able to break it in and pull several runs. They didn't mention it idling really rich after they adjusted it to idle and on my best run the AFR readings from 3,944 to 6,010 rpms show a min of 12.87 and max of 13.39.

For fuel I have a mechanical Edelbedrock fuel pump. 110gph regulated at ~5.5-6psi through 3/8 fuel line.

Re: Fresh 496 Running Rich. Ideas?
 

With one pump of the loud pedal Cold to set the choke and a second pump for a little more fuel from the accelerator pump it should fire right up.
You got a strong shot into the primaries from the accelerator pump?

Re: Fresh 496 Running Rich. Ideas?
 

At 13 your not that far off of an idle AFR, especially with your cam. One simple thing to look at is the boosters at an idle. Are they drizzling. If so, you may have a too high float level or something else wrong with the carb that the engine is pulling fuel out of the boosters.

Re: Fresh 496 Running Rich. Ideas?
 

Have you thought to check the float levels in the bowls?

There is a removable plug in each fuel bowl to determine fuel levels...remove the plug and if fuel runs out the float height is too high...

See image below...