Carb idle question

[wilson1970]

When engine is cold it will idle at 800. In gear it will idle at 800, after driving and at temp engine RPM increases to 1200 in park. Any thoughts?
Carb is a 750 8779 D/P with chokeless center section adjustable air bleeds are for Primary 36/36 inner and outer, Secondary is 73/73 inner and outer. 72 84 for jets.
Engine is 355 SBC with .520/.540 .236/.242 @ .050 110 hydraulic roller, 10.3 compression, RPM Air Gap, MSD billet distributor and 6AL.
Thanks

[ProPower engines]

When it heats up it gets looser. Ya know less friction. The oil is thinner and flows easier and its sealed better.
It all ad's up to higher idle speed.
And the carb can maybe use a bit of tuning on the idle circut as well to clean up the idle speed as well or you may have a small vacuum leak causing the issue. The advance will also increase some as the speed goes up helping the higher idle.

If the tune is good check the AF R at idle and see if its rich or a bit lean you may be able to adjust the fuel curve some
to help it. if the advance curve is good maybe back up a couple degrees to slow the idle.

These are just some of the many ways to correct minor issues that are worth checking

[JDR Performance]

If the air bleeds are as described they're incorrect. The 73's should be on the outside, 36 inside on both sides. You might have more issues than that if it's just a main body transplant.

[Ks Fats]

"adjustable air bleeds are for Primary 36/36 inner and outer, Secondary is 73/73 inner and outer. 72 84 for jets." Sounds like someone has mixed up the air bleeds; normal spread would have the High speed (inner) at 36 and the Idle (outer) at 73. 36 on the high side sounds a bit high but there are a lot of variables that enter into that. The idle air bleed is usually driven by the size of the Idle feed restrictor and other variables. The base line airbleed set up might be available on the Holley site; looking through my notes I have nothing on that particular carb. ( I see JDR has responded)

[wilson1970]

Yes you are correct. I mis spoke. Any direction on where to go? Purchasing an A/F meter would be first.

[77cruiser]

Do you have mechanical timing curve? If so when does it start?

[F-BIRD'88]

Recurve the distributor so that the timing does not change from park to idling in gear idling.
Change in load -rpm is changing the idle timing.

Suggest a 10 deg mechanical curve. Limit.
(Physically limit the mech curve travel to 10 deg)
Then:
Slow it down with 1 med and 1 light adv spring so that the mech advance does not start till 1000+ rpm.

24-26 initial base +10 deg. = 34-36 deg at max mech advance.

Connect vacuum adv to ported vac on the carb
Limit that to max 10-12-15 Max at high man vacuum.

You can fine tune the idle air bleed size to get the idle/off idle transition AFR shift correct, using the AFR gauge.
#73 pri idle air bleed. Will be .066" to .070" when done.

[6.50camaro]

Does this carb have 4 corner idle adjustment or just the 2 front adjusters . Dan

[PackardV8]

The varied questions and suggestions offered thus far are all valid possibilities. However, I will no longer attempt to chase carb issues without an O2 meter. Fifty years, I thought I had a fair handle on them, but only after beginning to use O2 meters did I learn how much was going on in there and what was the most likely fix.

[1972ho]

http://www.quickfueltechnology.com.au/c ... s-AUST.pdf

[MadBill]

For this issue, I often use the A/C solenoid from an older carburated GM vehicle. It can be controlled by a toggle switch and adjusted to the desired 'cold' idle speed while using the idle speed screw for the hot idle.

[Boz-Race Engines]

sounds like your ignition curve may be starting a little early as it warms up idle goes up so does timing hence a higher idle again when hot, get a timing light on it and see what rpm it starts advancing at, set your idle around 900rpm hot you want the advance to start at around 1050-1150rpm, if its moving before that rpm when raising rpm from idle then tighten the advance springs by running a shorter or slightly heavier spring until it does not move until around those rpms

[wilson1970]

Distributor is MSD Billet 85551. Curve has all timing in at 3000 at 34 degrees final. Not sure what springs are in it either. This gives me an idea on what direction to go. Thanks

[Tuner]

Can't promise this because production tolerance makes it so no two MSD advance mechanisms are exactly alike, but ....

..... the black bushing with two blue springs usually does not begin to advance until 1200 or so and totals between 2500 and 3000 and should be close to 18 crank degrees.

It is best to use matched springs in Delco type advance mechanisms, MSD is a copy of Delco.

When the springs are not identical in both rate and installed tension the strong spring pulls the mechanism off-center of the shaft and the grease eventually all moves to the slack side, the tight side drys out and sticks.

Even when the grease is fresh, uneven springs cause enough added friction in the advance mechanism it will be slow to retard when RPM drops and there may be 500 RPM (or more) lag where it hangs advanced even though the engine slowed down.

Measure across pins where the springs install and note the two sides may not be identical. Use needle nose pliers to adjust the free length inside the loops of the springs so they will have identical preload.

[F-BIRD'88]

Too much advance travel coming in (and going out too) way too fast. (Light tension adv springs.)
Recurve for a limited 10 deg mech advance curve.
24 initial base +10 34 max at 3200 ish rpm.
A slower more stable adv curve.

Combine with PORTED vacuum advance.
Now the timing will not surge with rpm and vacuum +/-
Better idle quality. Better throttle responce.
A online company sells the correct 10 deg diz adv bushing..

None of the msd bushings are correct.
Get the correct bushing here
http://4secondsflat.com/MSD Distributor