Idle problem Carter BBD

[Pompis]

Hey dear FABO member I have changed my 1bbl 225 to a Super Six setup. And I took my Carter BBD (Chrysler 1974-84) that came from a 318 and I rebuilt it with parts from Mike's carburetor. The problem is that the idle is too high maybe 900-1000 rpm and I can't make it less. I have checked for vacuum leak twice with brake cleaner and that's not the problem. The idle quality is good and it has power all the way thru when I drive. The throttle valves are shut and I have the throttle stop all the way out. The idle screw setting is 1.5 turn out. Have checked timing and that's good, changed from points to Ignitor yesterday and still it got "high idle". The vacuum advance works too. Can it be something internal in the carburetor that make the idle to high? In the beginning it got a little flooding problem but that was due to a faulty installed gasket on the venturi cluster. The brass float floats. Have set the float little low just for it not should flood. Have adjusted the step up vacuum piston with no cange in idle. Does any one had any idea since it feels like I have tested everything for a week now.

Thanks

 

[moe7404]

that carb has a float level of 1 / 4", from top of the center casting to the top of the float. i have done LOTS of that one and ALL instruction sheets call out 1 / 4". the AFB carb has had a large range of float levels called out. but the most common is 7/32". but 5/16 works best.

 

[KitCarlson]

I have found carbs where the throttle plate has been removed and put in wrong, so they bind in bore and will not close tight. Hold it to a lamp and see how much light you see. Some CAP carbs have small holes to add air, they might be soldered shut.

Throttle shaft wear can also be enough to leak air, but mostly on the linkage side.

Too much base timing will also elevate idle RPM. There may be a leak in a vacuum control that you are unaware of. Examples, choke pulloff, AC control switch, egr if your car has that. .

Idle mixture is often leaned to curb idle 30 RPM, but that will not be enough in your case.

 

[BillGrissom]

The only way to get a fast idle is to let too much air into the engine, so either the throttle plate isn't shutting enough or a vacuum leak. We assume you adjusted the "idle speed screw" on the carburetor, and that the "fast idle" screw isn't still holding it open.

If you want to see what a vacuum leak does, pull the brake booster hose off on a modern car and let it suck air. The idle will increase. The IAC valve will close trying to maintain desired idle rpm, but you will likely overwhelm it. The neat thing is that the engine will still run fine, not stumbling and shaking (or dying) like ours. The engine controller simply adds more fuel to compensate.