Six Pack High Idle

[EngineerDoug]

Hello,

I have a six pack setup installed on a 493 stroker. This is a set of new generation carbs circa 2020. I am running a mopar 528 solid cam, with 20 degrees of initial timing. I also have 20 degrees of vacuum advance from manifold vacuum (no choice there, because my Promax metering block has no provision for ported vacuum). The car runs great, and develops 16-18 inches of vacuum at idle. The engine loves the 40 degrees of advance...the downside is it loves it too much at idle.

I cannot get it to idle below about 1000 RPM. I have the all six butterflies closed, and no carb baseplate vacuum leaks (can't find any). Switching to the Wagner adjustable PCV valve helped, but find that I have to nearly close the idle flow adjustment on the PCV to get the idle down to ~1000 RPM.

The idle mixture screws work as they should, as the there is no transition slot exposed (butterflies are closed).

The engine is just getting too much air. The end carbs have drilled butterflies from Holley, which I suspect are the problem.

Has anybody else has seen this issue with the newer end carbs?

 

[R413]

All mopar six packs have drilled secondary butterflies. That's not the problem.

If you truly have 40 degrees of timing at idle, try disconnecting the vac advance and see what it does.

Usually culprits of this is secondaries not closing all the way or a vacuum leak. Do you have power brakes? Is the vacuum port on the back of the intake plugged? Don't skip the basics.

 

[EngineerDoug]

Good points!

I will double check the end carb linkages. I have noticed that if the carb mounting bolts are too tight the throttle shafts start to bind slightly. Maybe mine are too loose? As long as the throttles close on their own (once opened manually), is that OK?

Yes to power brakes...I wonder if there could be a small leak there.

Good to know the drilled butterflies are "normal". Thanks.

 

[EngineerDoug]

On the subject of card base gaskets, what is preferred? I've heard debates about thick versus thin - one being more prone to leak than the other.

 

[R413]

I use thin felpro base gaskets, they don't need to be very tight, and the thin gaskets don't crush.

Disconnect the hose to the booster and make sure it's not leaking vacuum.

You need to make sure the end carb close all the way every time. Disconnect the linkage and open and close them one at a time with engine running and see if they stick, hang open or close every time. The linkage just closes them, and that's all it does. It also ties the 2 outboards together.