• Hello there guest and Welcome to The #1 Classic Mustang forum!
    To gain full access you must Register. Registration is free and it takes only a few moments to complete.
    Already a member? Login here then!

Carb Guru question. Updated with after action report.

Sluggo

Sluggo

Active Member
My car is still running a little fat at idle and seems to load up a bit. Other than that it runs flawlessly.

I'm running the Summitt 750 vacuum secondary carb. It has a 95 power valve in the front and a 65 in the rear.
Vacuum at idle jumps wildly between 6 and 11. Cam is a 292/292 512/512

No doubt both power valves are being actuated during idle. The exhaust smell is pretty close to intolerable.

I'm thinking I need to go with 35 power valves front and back.

My concern is, off idle throttle response is very crisp except when it idles for a little too long. I really don't want to have to sit at the light and keep burping the throttle.

Is my power valve choice a good one?
 
Re: Carb Guru question.

Do you have a carb spacer you could try? Other than the intolerable exhaust smell, my car seemed to act the same way when Idling to long.
 
Re: Carb Guru question.

the rear should not be affected at idle, the passage ways are completely blocked when the butterflies are closed (which they should be at idle). You could also play with the needle valves (if summit carb has adjustable ones) to lean the idle mixture before changing power valve. What rpm does it idle at?
 
Re: Carb Guru question.

I'm almost certain I'm running 65F / 35R

And my cam is 292/535. But my vacuum is not bouncing wildly at idle either though.

I hope to test out the new vacuum modulator tonight - weather permitting. I'll check my idle vac reading also.
 
Re: Carb Guru question.

"Fast68back" said:
Do you have a carb spacer you could try? Other than the intolerable exhaust smell, my car seemed to act the same way when Idling to long.
Click to expand...

It has the 3/8ths spacer that came with the carb. My hot soak start problem was cured by blocking the port that runs under the carb.
 
Re: Carb Guru question.

"tarafied1" said:
the rear should not be affected at idle, the passage ways are completely blocked when the butterflies are closed (which they should be at idle). You could also play with the needle valves (if summit carb has adjustable ones) to lean the idle mixture before changing power valve. What rpm does it idle at?
Click to expand...
750
 
Re: Carb Guru question.

I wouldn't have thought that cam should give you a wildly fluctuating idle. I would have expected more of 10-12 inch vac on that. Sure there's not a leak somewhere?

BTW, Craig II is right- you should still be able to lean out the idle mixture with the needles.
 
Re: Carb Guru question.

Fluctuating vacuum readings of that magnitude is almost always a vacuum leak....unless you have the idle set way too low that cam. You likely have the carb too rich to make up for the lean condition from the vacuum leak, thus making the fumes unbearable. The bouncing vacuum readings and fumes, IMO, point to a vacuum leak. Time to block off the vacuum tree and get out the carb cleaner to check for leaks! :pbj
 
Re: Carb Guru question.

Did you block that port off when you swapped from the auto to the manual Sluggo? :beat
 
Re: Carb Guru question.

"70_Fastback" said:
Did you block that port off when you swapped from the auto to the manual Sluggo? :beat
Click to expand...

No. When I put the cam in.
 
Re: Carb Guru question.

"buening" said:
Time to block off the vacuum tree and get out the carb cleaner to check for leaks! :pbj
Click to expand...

My paint job! OMG! maybe I'll just leave it parked and show it to people when they happen by.

You're prolly right. I should not have reused the intake gaskets after I forgot to seal the ends of the deck the first time I put the intake on :craz
 
Re: Carb Guru question.

"Sluggo" said:
My paint job! OMG! maybe I'll just leave it parked and show it to people when they happen by.

You're prolly right. I should not have reused the intake gaskets after I forgot to seal the ends of the deck the first time I put the intake on:craz
Click to expand...
your kidding I hope! :eek:mg
 
Re: Carb Guru question.

IMO, the 6.5 power valve should be fine on both sides.

I haven't used the Summit-branded carb, but, if it's anything like a Holley, my main issues stemmed from too much throttle plate on the primary side, causing invasion of the transfer slot and excessive enrichment of idle fuel/air mixture. This was solved with an adjustable secondary idle circuit and opening of the secondary throttle plates slightly to balance airflow. That modification (long before Holley actually sold a carb with it) made a huge difference in idle quality. Some others were running a lot of compression and a loose torque converter. Probably the best idling engine I ever built was the last 302 (305) which was running 13:1, a Holman-Moody cam and a 4500rpm converter, along with the modified 3310 Holley (780cfm). It idled at 1000rpm clean as a whistle on 112r/m and I could drive around the pits in high gear without loading up. The rpm wouldn't even drop when putting it in gear. I was afraid to ever get rid of that carb and it has been on every engine since and still sits on the car today, idle.

Easy way to check for gross vacuum leaks (I doubt it is an issue but check anyway) is to snap the throttle open, reaching between 5 and 6K and then snap it closed. If the idle speed snaps back, no leaks; if it slowly settles back (high vacuum signal from snapping the throttle closed will exaggerate leaks), then you've likely got a leaker somewhere.

I took a look at that Summit carb. Interesting design; looks like a Holley 4010. Definitely less leak-prone than the Holley's I used, like the 3310. Apparently it uses some Holley parts. I'd have to play with one to see what can be done with it.

Bla, bla.... switch the power valves out to 6.5's (yes, replace the existing 6.5) and try again. Idle speed should be in the 1000-1200 range out of gear. Turn the idle mixture screws in gently until they bottom, then out 1 1/4 turns. Use that as a starting point and adjust in 1/8 turn increments to best idle quality, unscrewing speed adjuster as you go. You can mess with the secondary throttle plate adjuster, but only do it in very small increments, remembering your baseline setting. I did it in 1/8 turns as well and, IIRC, it only took about 1/4 turn to achieve the results I wanted. Remember, your carb doesn't have an adjustable secondary idle circuit.

Post back your results...

Pat
 
Re: Carb Guru question.

Great info Pat,

So, I'll test for vacuum leaks, change power valves to 6.5s, can the 9.5, readjust the idle mixture and set the idle at 1000 rpm.

Did I miss anything?
 
Re: Carb Guru question.

Only other thing I always did is something you can't, running a ton of advance at idle. On the 305, I ran a locked Duraspark and later a crank trigger and the advance was all in at idle. The engines had to crank with the ignition off until they reached cranking speed since the starters of the day couldn't handle the cylinder pressures (long before 'mini' hi-torque starters).

If anything else comes to mind I'll pass it along.... since I last posted, rummaging around, I found this article on the Summit carb which might help you.

Pat
 
You must log in or register to reply here.
Back
Top