Summit 750 vacuum secondary carb... anyone tuned one
 

I have a 461 big block Chevy, Summit RV cam, Summit 750 vacuum secondary carb, Edelbrock Performer 2.0 manifold, Hedman mid length headers, Summit HEI vacuum adv. distributor, timing set at 33 total, and manual transmission. I have questions about the carb (SUM-M08750VS)... I just got the engine running, and has a couple hours of run time on it. I bolted on the carb, and the only things I have done to it were set idle speed at 750 rpm, and both mixture screws turned 1 turn out from bottom to get the best idle. The fuel level is between the bottom and middle of the sight glasses on both floats. It starts good cold, but starts hard when warm, and has a stumble just off idle before it pics back up and feels responsive. It cracks good above 1200-1500 rpm. I took a vacuum reading on the full time manifold vacuum port at the front corner of the carb, and it idles at 13 to 14 inches of vacuum. I see that the carb comes with a 9.5 primary power valve, and a 6.5 secondary power valve. Should I change the primary power valve to 6.5 to be half of my idle vacuum reading? What about the secondary power valve? Any other advice to tune the carb better? My searches haven't turned up much info on tuning this carb.

Re: Summit 750 vacuum secondary carb... anyone tuned one
 

Leave the carb alone for a bit.
Let’s get your vacuum higher first.
You have 33 degrees total timing.
How much of that is initial timing?

Re: Summit 750 vacuum secondary carb... anyone tuned one
 

I started with about 10 degrees. It probably has changed a bit from adjusting it a bit to get 32-33 total. I need to re check that.

Re: Summit 750 vacuum secondary carb... anyone tuned one
 

You should be safe with 16 initial at your elevation.

Re: Summit 750 vacuum secondary carb... anyone tuned one
 

Thanks, I'll check it tomorrow after work and see where it's at.

Re: Summit 750 vacuum secondary carb... anyone tuned one
 

I checked timing, and it was retarded quite a bit... the distributor must have move some when I initially set it. I disconnected the vacuum advance, timed it at 36 degrees total, then checked the vacuum reading. It was much more responsive too. I had 18 inches of vacuum, but the idle had come up some with the timing change. After I reset the idle to about 750 rpm, I had 16 to 17 inches of vacuum. The motor is a lot more responsive, and starts a lot easier. I didn't drive it yet, but the hesitation off idle is gone just by reving it.

Re: Summit 750 vacuum secondary carb... anyone tuned one
 

If you’re at 36 total, what’s your initial timing?

Re: Summit 750 vacuum secondary carb... anyone tuned one
 

13 initial

Re: Summit 750 vacuum secondary carb... anyone tuned one
 

You’re getting there.
You could leave it alone or try a few more degrees.
Just to see what it does.
You might gain a little more vacuum.

Re: Summit 750 vacuum secondary carb... anyone tuned one
 

Ok, I appreciate the tips. I might try to go 2 more degrees and see if it likes it. It starts and shuts off nice now, and seems to be running good. Thanks!

Re: Summit 750 vacuum secondary carb... anyone tuned one
 

At least now with better vacuum you don’t need to get into the carb.

Re: Summit 750 vacuum secondary carb... anyone tuned one
 

Yes, thanks!