Demon too rich at idle?

[bigblockmark]

383 hot engine used for street and strip. In a S 10. Has headers and Edelbrock single plane intake. This truck ran 7.3 in 1/8 mile a few years ago and is just being brought back out after a period of inactivity.

Engine struggles to start when cold. Better after warmed up. Very low idle vacuum. 25 degree initial timing. Sounds good above idle.Power valve is 5.5. Carb has new gaskets and float level is at bottom of sight windows.

7" vacuum at idle. Idle A/F screws give highest vacuum at less than 1/2 turn from full in. idle air bleeds are 70. I am going to try larger idle air bleeds. Correct? How much bigger? What else to try?

i am going to perform cranking compression and cylinder leakage tests to see if engine is in good mechanical condition.

[Right hand drive]

What Demon?

Most Demons have .037” - .038” IFR up high.

[bigblockmark]

I find no ID on the carb. Throttle plates are stamped 1.69. Where else should the carb ID be?

I changed from .070" to .076" idle air bleeds, not much if any difference. Try .080"?

i tried moving the timing up and down, but it wants 25* plus initial just to have 7" vacuum. If i take it down to 20* it only has 5" and struggles to keep running. I went a head and disassembled the base plate of the main body and was thinking about the drill the plates with holes deal. but i found the center of the mounting plate has what looks like an air bypass built into it with a spring loaded needle valve that has 4 exits to the throttle plates and would feed air from the air filter stud hole. It has a stud in the hole because it has an air cleaner, so I don't know if the cancels the bypass or not.

The only other things I thing might be going on is the hydraulic lifters might have too much preload. I can back them off and see. Also this engine was built normally aspirated with high compression. Later it was to be converted to Procharger. Supposedly the pistons were changed to lower the compression in preparation for the Procharger. But that never was completed. It has a pretty grump camshaft. Maybe the lower compression and the wild cam don't like each other?? No cam spec available.

I did find the MSD distributor has a spring broken on the return weights and replace both and new cap and rotor. No difference except now does not vary as much RPM at idle.

Still going to do compression and leakage test after carb is back on.

Once this thing is moving down the road, you would swear nothing is wrong with it. Brakes the tires loose in all 3 gears.

[Right hand drive]

Identifier of older Demons before putting a laser etch p/n on the air cleaner ring was a casting on the main body. With those throttle plates you have a 750 or 650. On top of the main body next to one of the vent tubes will be a two digit number with the second number half eliminated by a drill dimple. No dimple it’s a race Demon. A 750 will have a 40 to denote 1.400” venturi and a 650 will have 28 for 1.28” venturi. Bottom pic is 650. The owner was advertising it as an 850.

Hydraulic lifters should adjust to their preload. They will self adjust to keep the valve closed, they will not self adjust and overcome valve spring pressure. Low comp and late closing intake valve timing (read lumpy cam) are a recipe for low idle vacuum so that is something inherent in your engine.

The ‘Idle-Eze’ in the center of the base plate was to eliminate drilling holes in throttle plates in order to keep correct transition slot exposure. I have found they don’t work as well as they should but give it a try. I think a flat blade down the air cleaner stud thread turned anti clockwise is supposed to let more air in.

[bigblockmark]

It has the "40" on top so a 750. Thanks for all your knowledge.

No matter what I do, is the proper amount of air bypassing and the transition slots only barely exposed, really going to do anything to help increase the low 7" vacuum at idle? Are the proper size idle air bleeds going to do it? Just stuck with low vacuum with the low compression pistons with the high compression cam after the carb is correct? Any reason to think another carb might be helpful?

Does having a stud in the Idle ease port defeat that feature or does it get air from somewhere other than the vertical hole the stud mounts in?

[Right hand drive]

[GTO Geoff]

With 7" of vac, probably wants a LOT more idle timing. Maybe 40+*. Turn dist with engine running to see what it likes. If it likes 40 ish, use vac adv connected to manifold vacuum to give you the reqd timing. The Crane adj VA will work down to about 5~6"of vac & can add a total of 30*. Adjust Allen Key fully CW. If you do not have a dist that accepts VA, get one. Idle timing needs to be sorted before playing with carb.

[Right hand drive]

[bigblockmark]

Thanks for the tips. I have rebuilt the carb and it is ready for assembly. I will raise the float level up in the glass. I see how the Idle Eze works now as you described. It has 73/81 jets, have not changed that. 6/5 power valve. Nothing else stand out ad anything to effect the vacuum.

As far as lowering the IFR, I did drill, tap and have 6-32 X 1/8" long brass set screws. Set screws are in a proper height. But, my technique i have used to make 10-32 air bleeds is not good enough for a .032" hole being drilled. The drill bits I have will not get the job done, break. I am using a mini chuck in my drill press. squared and centered in a fixture I have for holding the set screws. Tried different RPM and lube or no lube. So it is going back together for now without them. Good thing I did not remove the upper restrictors. Would really like to see the effect of the lower position on this carb and engine.

As far as timing, it was set at 3*5 when it came in. I lowered it to 25* before it started to drop from 7" to 5". Did not try to raise it above 35*. Will do that.Once the engine is warmed up, it is a lot better than when cold.

Any chance going even leaner on the idle air bleeds will help make the A/F screw want to be out further, 1 1/2 turn ish. next step would be from .076" to .080". not responsive now. Maybe i can find a setting on the Idle Eze to let me close the throttle blades more and help it.

I fear that this is all mute and the real issue is internal to the engine, like big cam on low compression.

[Right hand drive]