Lean idle mixture.

[jim_ss409]

I just fired my engine up for the first time. It started right up and ran perfectly the throttle response was very sharp. The only problem is that it won't idle much below 1,200 rpm. The idle circuit seems to be working as it should but just being a bit too cheap with the fuel. I have to back the mixture screws all the way out and it still seems too lean. These are factory carbs so there is no idle circuit in the front carb. They are on a 476ci stroker with a hydraulic roller cam 238/248 @ .050 110deg. LSA. The intake is the stock dual quad with 1" open spacers. I rebuilt the carbs and the float level should be fine and I've got soft springs in the metering rods. I'm running plenty of initial timing and the motor runs quite smoothly but it just seems to starve out at about 1,000 rpm. I'm wondering if the idle cicuit on the factory carb is too small for the extra cubes and extra cam overlap. I'll check for vacume leaks etc. but I'd apreciate any input or ideas.

[SS425HP]

Jim, the cam in my motor is at least that big. Same LSA, and I'm think at least that much duration. I can idle at 800 if I want. Mostly idle 1100, though. Advance curve is running up the RPM at times. I can pull it down with the clutch, and it will stay down. Give it a little gas, it settles around 1100. Your idle circuit is wrong for some reason. Check the gasket under the primary venturis. I remember Curt Harvey telling me the gasket that "looks right" is wrong. I know mine works fine. And, about 1 1/2 turns out on the screws. Sounds like it could be a vacuum leak. Have you checked your vacuum? Had a problem like yours one time, and found an intake leak.
Also, what gaskets are you using UNDER the carb? Should be only 4 holes in it. Hard to find them. NAPA has them. G7360 is the part number. Check this first.
Good Luck

Fred

[IgnitionMan]

I'd make sure the throttle plates on the primary carb aren't open too far. Thisn is the usual cause of loss of sensitivity on the idne mixthre scrrews, as when the throttle plates expose too much of the idle transfer slot, the engine will now idle off not onl;y the idle mixture screws, but the fuel inteneded to stay in suspension in the slots.

The usual fix is to drill holes in each primary throttle plarte to get more air into the engine and be able to close the idle throttle plates back down to the point the idle transfer slot goes back to holdikng the fuel, not delivering it.

Depending on just which carb setup, you may or may not find some form of cut on the underside of each idle carb primary throttle plate, this is to do what is needed on a stock engine. Cammed up engines would want more air, the hole possibly, to idle down to a reasonable level.

The same spec applies to an AFB that is for a Holley on then idle transfer slots, .040 open with the throttle plates on their regular curb hot idle idle stop, as viewed with the carb off, from the bottom of it.

To richen the idle circuit on AFB's, take the carb top off, remove the primary booster venturii. There will be a brass tube pressed into the bottom of the booster, long, swedged smaller at its lowest end, with a hole in it. This hole is the metering feed hole for both idle mixture screws AND idle transfer slots. Mini drill bit, don't go overboard, and use a HAND HELD DRILL HOLDER. If you go too big, refer back to the smaller size you had recorded, solder the hole up and drill smaller.

CHANGING THE IDLE JET RESTRICTION SIZING IS THE VERY LAST RESORT, ONLY FOR VERY LEAN SETUPS, AND ONLY AFTER THE PROBLEM IS AS FAR AS POSSIBLE RESOLVED WITH THE THROTTLE PLATE IDLE SETTINGS, AND HOLES, IF NEEDED.

[jim_ss409]

Thanks for the input. I'm glad to hear the carbs will work with a bigger cam. I will look at the simple things first like vacuum leaks. I think I got the right gaskets in the boosters but there was about 3 or 4 to choose from so I can check that too. Another thing that makes me suspect a vacuum leak is that the throttle plates aren't open as far as I thought they would be. I don't think I'm exposing much of the idle transfer slot. Anyway, the cams not that big and it runs very smooth with just a bit of choppiness right down to the point where it seem to starve for gas. I'll check out the simple things first and if nessisary I'll try, carefully, some of the mods that the IgnititionMan listed. I'd really like to get everything running as smooth as possible.

[IgnitionMan]

As far as I have ever seen, I've only seen two different booster venturi gaskets, front and rear.