Carb Guru question. Updated with after action report.

[Sluggo]

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?

[Fast68back]

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.

[tarafied1]

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?

[70_Fastback]

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.

[Sluggo]

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.

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.

[Sluggo]

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?

750

[70_Fastback]

I'm also ideling faster due to my auto.

[apollard]

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.

[Sluggo]

I'm wondering if I dont have a vacuum leak.

[buening]

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!

[70_Fastback]

Did you block that port off when you swapped from the auto to the manual Sluggo?   :beat

[Sluggo]

Did you block that port off when you swapped from the auto to the manual Sluggo?   :beat

No. When I put the cam in.

[Sluggo]

Time to block off the vacuum tree and get out the carb cleaner to check for leaks!

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

[tarafied1]

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

your kidding I hope! 

[Sluggo]

your kidding I hope! 

I only had one set  :scar

What?

[tarafied1]

nothing  :doh :bur should work fine

[David67]

 :nta

[camachinist]

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

[Sluggo]

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?

[camachinist]

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

[apollard]

That's the article that led me to get the Summit carb. All the jets, valves, etc are holley.

Sluggo, if you find that leak, and follow the rest of Pat's advice, you'll get that vac up and you'll forgive her for the cam episode 

[Sluggo]

That's the article that led me to get the Summit carb. All the jets, valves, etc are holley.

Sluggo, if you find that leak, and follow the rest of Pat's advice, you'll get that vac up and you'll forgive her for the cam episode 

I've already forgiven her. She runs like a scalded arse ape other than the loading up and the melted  :ep thing.

I can't imagine how insane it would be with a 3:70 out back

[camachinist]

IIRC, idle vacuum readings were generally in the 12-14 range in gear. IMO, that's a good number to shoot for, but don't worry if it's anything over 10. I don't know the rest of your combo but it's probably low compression so overlap reversion and the relevant vacuum signaling is always an issue at idle. I'm assuming, if it were close to 'stock', we wouldn't be having this conversation LOL...

Good luck!

[Sluggo]

IIRC, idle vacuum readings were generally in the 12-14 range in gear. IMO, that's a good number to shoot for, but don't worry if it's anything over 10. I don't know the rest of your combo but it's probably low compression so overlap reversion and the relevant vacuum signaling is always an issue at idle. I'm assuming, if it were close to 'stock', we wouldn't be having this conversation LOL...

Good luck!

Here's some info:
351w stroked to 427 10:1 compression, cast iron heads, performer intake, summitt carb, Lunati bracket master II camshaft part 00061 using 289 firing order, manual transmission.

If I can get a steady 10 I'd be thrilled. Oddly enough the power brakes work pretty decent.

[camachinist]

With those cubes, that cam and 10:1, that thing should idle like stock. That's a little cam for a big engine. I mean that in a good way. This idle issue should be easily resolvable. If not done already, do the vacuum leak tests that I and others suggested, first.

Also, as a baseline, borrow a known-good carburetor for comparison. If this issue has been around since installation, there might be something wrong with the carb.

I offered the same advice to Tom (Sac69) at Knotts when he was complaining of a miss in the mid-throttle cruise range. Baseline it, then work the problem.

Maybe I should start a business as a fly-in engine consultant. I'm sure Aunt Toni would love that  :lol