Cadillac 500 unable to idle

[tschmitt]

Recently rebuilt Cadillac 500, just broke the cam in yesterday. Runs great above 1200-1300 RPMs but cannot get to run at anything less than approx 1100 RPMs before it sputters and dies. Starts up first shot no problem, hot or cold/next day/same day. Vacuum advance is not hooked up. Engine is on test stand, only thing attached is the PCV, all other vacuum ports are capped. Vacuum at 1200 RPMs is 14-15. Cannot get a good timing reading due to being unable to get even close to a decent idle. Noticed both sides have significant nozzle drip throughout operation that doesn't change with idle screw adjustment. Rebuilt this carb a couple of weeks ago with kit from Cliff as well as put in new bushings for the primaries and it went smoothly, well plugs installed too. Fuel pressure at 5 psi via a pressure regulator. I can adjust the idle mixture screws and it doesn't seem to change anything unless I bottom them out and then the motor will die. Below are the stats for the carburetor and cam card. I can cover the secondary air flaps with a towel and no change. Any advice at what to look into next would be greatly appreciated.

Cadillac 500 8.5:1 Compression
LSA 112+4
Overlap 66
Valve Lift Intake .475/.493
Duration at 0.050 214/214

Carburetor # 17059205
This is what I have done to the carburetor following Cliff's book so far.
APT 3 turns out
Idle mixture screws .090
Idle screws 4 turns (I have varied this several times but hasn't seemed to help)
Main accelerator wells .040
Secondary emulsion tubes .036
Secondary air flap open distance 1.330
Secondary air flap spring 3/4 turns
Idle tubes .036
DCR .046
Upper IAB .070
Lower IAB .070
Idle Bypass Air .056
No holes in plates
Main air bleed .080
Hanger: O
Secondary Rod: AX
Primary Rod: .036-.049
Primary Jets: 75
Accelerator pump exit holes .028
Fuel inlet seat .135
Float level 1/4

[Cliff Ruggles]

Next time it's up and running tip-in or gently hold the choke flap partially closed and see if it will idle down without stalling.

You also need to make sure that the distributor is dropping the timing out when you try to go to idle speed. 

Other things to check are the secondary throttle plates to make sure they are FULLY seated in the bores when closed and I'd check to make sure your screw in plugs aren't too high and holding the main casting away from the baseplate and not sealing the gasket.

You can't move forward with tuning the idle system until you make sure that there are no fundamental issues with the carb and/or distributor. 

It could be as simple as needing more initial timing with the larger cam, although I consider that a "tiny" cam in a 500cid build at 8.5 to 1 compression.  It should be making a LOT more vacuum and nice smooth idle without much more than 8-10 degrees initial timing in that build.......

[tschmitt]

Thank you very much Cliff, I will check into all of those things today and see what happens. I appreciate your help.

[tschmitt]

Just performed the tip in procedure. Set the RPMs at 2000, vacuum was at 10 at the manifold port. RPMs increased with tip in by about 100-150, vacuum increased to 12. Idle mixture screws were at 3. I've sprayed carb cleaner on the gasket surfaces of the carb and the intake but not seeing much change. I'll dissemble the carb and check into the base plate to main casting. Does the base plate and main casting need to sit completely flush without the gasket in place? I'll check the secondary and primary blades as well to make sure they're sitting correctly in the bores and that the secondaries are fully closed.

[lightning boy]

Does it have a hot air choke? Seams like it would need more by-pass air.

[tschmitt]

No, electric choke that I disable when on the test stand so choke is open.

[tschmitt]

The incredibly low manifold vacuum is definitely a issue that I'm not sure extra bypass air will resolve. Checked compression on all cylinders and they are all 150# or above. May need to take the intake back off and put on another set of gaskets reseat. Gotta be a major leak somewhere unless the cam is wrong.

[Cliff Ruggles]

Agreed.  At 2000rpms that engine combo should be easily making over 20" of vacuum.  10" would be really low at 1000rpms assuming you had adequate timing in it.

At this point I would check for a huge vacuum leak someplace, or ignition timing WAY off the mark. 

Hopefully it's something easy and something worse like the camshaft out of phase, etc....

[tschmitt]

Pulled the intake off and applied a new gasket. Intake gasket did have one area that looked like it may have been leaking internally. Also tore the carb down again to make sure everything was in place, rechecked the base plate to main casting surface and indexed the throttle blades again. Rechecked the fuel inlet set with a vacuum and it failed this time. Reseated it 10+ times with a check ball and it finally held. Will let the new intake gasket and RTV sit 24 hours and then fire it up again tomorrow. Fingers crossed.