Tuning a 493 inch Six Pack

[EngineerDoug]

Hello all,

I've got my new 493 stroker RB motor running, installed, and in final tuning stages. Here are the particulars:

Stock six pack setup
440 stroked to 493 inches
1 7/8" headers
A727 with 3000 stall converter
Trickflow 240 heads
9.8:1 compression
Cam is solid flat tappet, custom ground, very close to Mopar 528 cam on wider lobe centers
Mopar electronic ignition with vacuum advance, connected to ported vacuum
Initial advance set for 18 degrees
Mech advance limited to 18 degrees
Running the Wagner adjustable PCV valve

So I have gone over all the six pack tuning guides, and have the carbs pretty well dialed in for mixture. The engine will idle at 1000 rpm, pretty smooth with about 15 inches of vacuum.

The problem is the ported vacuum. The throttles are open enough that I get some vacuum at this port. So the idle tends to vary because the vacuum advance forms a positive feedback loop. The idle rpm depends upon the added vacuum advance, and the amount of vacuum depends upon the idle rpm. So I get this chicken or the egg dynamic between the two = wandering idle speed.

If the carb is set up properly the throttles should be closed enough so there is no signal at the vacuum port, but how can I do this? I've given some thought to upping the initial advance to 20 degrees; this might allow me to close the throttles a bit more. Any other suggestions on how to get the ported vacuum covered?

Thanks.

 

[Challenger340]

Very difficult to obtain correct A/F's and EGR's let be TP openings on a six-pack setup without monitoring those parameters ?

IMO, Wheel or Engine Dyno with the latter being best option, and sometimes it can take an 8 hr day all by itself.

 

[493 Mike]

Try closing the throttle plates in the primary carb and adjust the secondary carbs throttle plates more open to maintain your desired idle speed. That will stop the excessive air flow in the primary carb that is triggering the vacuum advance.
Mike

 

[EngineerDoug]

493 Mike - yes I had that same idea. Just didn't know if that was "accepted" practice. I notice the secondary butterflies already have small holes in them to give some airflow.

I will try it out; thanks.

 

[Midnight_Rider]

Is the vacuum cannister on your distributor adjustable?

If it is, and you can't get things completely dialed in with carb adjustments, there may be some relief there. I'd probably throw a Mityvac on that advance can regardless to see exactly what it's doing, if it's leaking down, etc.

 

[EngineerDoug]

Yes the vacuum advance is adjustable. My next experiment is to adjust it all the way CW. This might stabilize the amount of advance at ~15" of vacuum.

 

[33 IMP]

What Mike suggested is exactly what i had to do to make my sixpack happy.
Thats what i was gonna suggest, Mike beat me to it.
I adjusted the secondary opening and idle screws to get about 600 rpm without any opening of the primary at all, then used the primary to get the idle speed and mix i wanted.

 

[EngineerDoug]

Cool! I know that a Holley four barrel has a small set screw to limit the closure of the secondary butterflies.

So it sounds like the way to do this on the six pack setup is adjust the linkage rods. Hopefully 1 turn will not be too much air. I will try it.

 

[33 IMP]

Cool! I know that a Holley four barrel has a small set screw to limit the closure of the secondary butterflies.

So it sounds like the way to do this on the six pack setup is adjust the linkage rods. Hopefully 1 turn will not be too much air. I will try it.

I used the throttle blade stop setscrew, under the base of the carb. Opening up the secondary throttle blades also used to be a way to keep the secondary throttles from sticking, due to lack of use.
I seriously doubt that will be a problem in your case.

 

[EngineerDoug]

Is there a set screw on the outboard six pack carb?