Carb tuning

[cgarb]

Looking for tuning advice on my fathers race car. I have a Fast dual 02 meter on it and right now its showing a rich idle. 7.1-1 when its warm and up to tempature. It acts rich, it starts right up and chugs along even dead cold. Its a very mild 302 ford with a 500 holley 2bbl carb. my dad was having consistency issues with it, (bracket car) and it acts cluttered up when you floor it sometimes and others it takes off clean. It had a power valve, but he removed it and put a plug in to see what it would do. now its cluttered up all the time. Obviously we went the wrong direction there so we are putting back in a power valve. It had a 6.5 power valve, but we think maybe the diaphragm was ruptured and leaking. The 1st time we raced the car the plugs showed lean after we pulled a plug on a 1/4 mile pass. The car is a factory weight 85 Mercury capri with a very mild cam and a set of Flotek 180 heads. It runs 12.40's at 107 in the 1/4 with the 2bbl. I was wondering if the power valve could be sucking back shut with high rpm manifold vacuum? I might order a higher number powervalve to try and see what our idle afr looks like with that. 7.1-1 is pretty rich it seems, I've never tuned with 02 meters before...how bad is that, like a few drill sizes on the Idle air bleeds or way out in left field? I'm hoping the power valve helps that number some.

[rustbucket79]

Left field. 12.5 to 14.5 would be target idle AFR

Have you replaced the float, needle and seat, and set proper fuel level?

You need to monitor manifold vacuum on a restricted carb engine with a power valve, it will absolutely pull vacuum at peak rpm, possibly several inches.

[rebelrouser]

Not saying you can't do it, or that others don't do it, but I don't use power valves on drag cars. I leave the power valves for street cars. Drag car is WOT the whole run, power valve would be open anyway. If it is really that rich at idle, I would be looking at blown power valve, bad floats, leaking needle and seats, excessive fuel pressure. When you take the power valves out and plug them usually you jet 4 to 6 numbers richer. If they idle rich, I take care of it by changing the idle air bleeds. If your carb don't have them, it is not a big deal to drill and tap the carb to install them, done it many times.

[cgarb]

The carb is good, no bad floats or needle and seat that has all been replaced. No fuel dripping from the boosters at idle. I was trying to get an idea of how much of a change I would need to make. At shift point when I play the data log back it gets up to 11.0-1 AFR with plug in and we jetted up 6 sizes from factory 73 jets. We are just trying to clean up the idle number. And will work from there.

[compguy]

What happens to it when you block the idle air bleeds whilst idling? A/F ratio and idle speed/quality?

[Tuner]

Put a short piece of .018" or .020" wire in each idle jet.

[cgarb]

Put a short piece of .018" or .020" wire in each idle jet.

In the metering block? Also, could this be an issue where this carb is designed to run with a PCV valve?

[Tuner]

Put a short piece of .018" or .020" wire in each idle jet.

In the metering block? Also, could this be an issue where this carb is designed to run with a PCV valve?

Yes, in the metering block. The idle jets are on either side of the power valve in the face of the metering block. They are the little brass widgets with (should be) .035" or so holes. Bend the wire in a V shape and put one leg through the jet hole and the other leg through the larger hole that connects to the main well, toward the power valve.

If the metering block is an original 500 2bbl block the carb should run OK with a 10.5 PV and main jet near #70 to #72.

You need to monitor a vacuum gauge when the engine is at WOT and peak RPM to know how high the intake vacuum is climbing to determine if you are nearing closing the PV. You should have no trouble with a 10.5 valve on that engine.

[MadBill]

Looking for tuning advice on my fathers race car. I have a Fast dual 02 meter on it and right now its showing a rich idle. 7.1-1 when its warm and up to tempature...

In your efforts to gain control, presumably you have the idle mixture screws bottomed out?

With a mild cam it shouldn't be an issue, but how much of the idle fuel transfer slots are exposed below the butterflies at idle? If it's much more than the recommended 0.020"-0.040", the mixture screws will lose control and it will run rich. You may have to drill the butterflies* to allow the same airflow with less throttle opening. If so, start small ~1/32", as it's easier to make them bigger than smaller. (*Most guys drill the middle of the blade half nearest the slot, but I'm not sure that it really matters.)

Also, how much spark advance does it run at idle? Sometimes just bumping it up 5 or more degrees (with a matching reduction in centrifugal advance of course) will significantly raise the idle speed, allowing the idle screw to be backed off a couple of turns and thus closing down the slots.

Naturally, setting the idle speed as low as possible will also help.

[cgarb]

Timing is 34 total and 15 degree's advance. Just enough it will start easily when warm. We did put the wire in the idle jets and that helped a good bit. This weekend I will look into the transfer slot situation.