Six Pack on center carb only?

[Frustration]

I am planning to run my 69 Road Runner in a bracket series this summer. The car has gone as fast as 11.07 on it's only track time with the new combo. I do not have a roll bar, but would like to run it at a dial in of 11.50 or slower. Is it ok to unplug the outboard vacuum lines in order to run solely on the center carb? Will I risk damaging the engine? Thanks in advance.

 

[66Satellite47]

Limit the throttle opening on all carbs.

 

[lewtot184]

i'd try putting black springs in the vacuum pods. they'll take some starch out of it.

 

[PurpleBeeper]

I like Lew's idea. Stiffer springs make the outer carbs open later/slower.

I would also suggest you look at an idea by 66satellite, but I would be inclined to limit how far just the outer carbs would open. I don't think you'd break anything running on just the center carb, but I also don't think you'd be able to run 11.50 on just the center carb. You might slow the car down slowly retarding the timing a few degrees too or stiffening up the advance springs in the distributor.

You'll just have to make some track runs & tune it to see what gets you consistently at 11.51 & take good notes about temperature, track conditions, etc so you don't "break out".

 

[lewtot184]

the center carb is very small. there may be a way to use a small screw as a limiter in the semi circle section of the center carb that connects the end carbs. setting the timing back few degrees with the black springs may be the simplest route.

who says 6paks don't make power?

 

[66Satellite47]

The black springs probably won't slow it down much at all. Can you temporarily move the cable mount so that you can't get full throttle opening?

 

[lewtot184]

I've track tested springs. blacks are killers.

 

[66Satellite47]

I've track tested springs. blacks are killers.

OK, but didn't slow mine down much, no where near 1/2 second.

 

[68 HEMI GTS]

You must have found some et, didn’t it run around mid 11’s when you put it together?

 

[Frustration]

You must have found some et, didn’t it run around mid 11’s when you put it together?

Yes. It ran 11.55. I changed to a 9.5" Dynamic Converter and it picked up big time. Best to date is 11.07 @ 123 throught the pipes on street tires. I haven't touched the jetting yet. Pretty sure it will run in the tens in the cool spring air.

I forgot to mention in my original post...the center carb is 500 cfm.

If I were to limit the throttle opening of the center carb, wouldn't that eliminate the outboard carbs from opening? I'm pretty sure the vacuum can only pull the outboards open when the center carb is at WOT.

This is not an every weekend bracket car, but I'd love to be able to disconnect the vacuum hoses on the outboards and slow down enough to be legal. I don't care if it runs 12.0. I doubt it would slow it that much though. Thank you for the responses. Spring is on it's way!

 

[747mopar]

Never ran a six pack but am familiar with how they work and the affects of different springs ratings in vac sec 4 barrels, I'd try what Lew suggested first and go from there. Got a picture of the throttle linkage?

 

[68 HEMI GTS]

I wouldn’t limit the center carb. You could build an adjustable stop for the outboards. I would just make something that mounts on the front carb, if you limit that it’ll also limit the rear carb. Should be pretty easy. Adjust till it runs 11.50’s! I would also put the stiffest springs in the outboards.

 

[66Satellite47]

You can add a "bleed" valve in the vacuum line to bleed off the vacuum that actuates the end carbs so there isn't enough to pull them open all the way. A small tee fitting with a needle valve, close off the vacuum supply, no end carbs, limited vacuum supply, limited opening.

 

[dvw]

Modify the secondary vacuum pod covers with a screw in the center. Screw touches the diaphragm and limits secondary opening. I've pulled my opening back for 20+ years to run index.
Doug

 

[lewtot184]

Frusration, how and where are you pulling vacuum off the center carb (assuming an aftermarket 500cfm 2bbl)?

 

[Cranky]

Might try getting off the throttle at the 1000 foot mark and see what your ET does.....

 

[Frustration]

Might try getting off the throttle at the 1000 foot mark and see what your ET does.....

Not consistent enough, but thanks.

 

[Frustration]

Frusration, how and where are you pulling vacuum off the center carb (assuming an aftermarket 500cfm 2bbl)?

I'll post a photo after work today.

 

[Cranky]

Not consistent enough, but thanks.

OOoo k but that seems to work pretty well for a lot of bracket racers ie sand baggers. I would start with backing off on the timing a bit and I like to run my carb/carbs a bit on the rich side. Changes in the weather seems to not affect them as much as being on the money or a tad lean. If that doesn't work, then do limiters.....easy stuff first. A simple adjustable stop under your loud pedal can be effective too.

 

[66Satellite47]

Frusration, how and where are you pulling vacuum off the center carb (assuming an aftermarket 500cfm 2bbl)?

Not sure what frustration did, but on my 500 conversion we drilled the 500 in the same location relative to the venturi as stock, pressed in a piece of 1/8 brake line, inserted a fishing line sinker, then drilled the sinker with the same size hole as the stock center carb had. Worked perfect.