High Idle

[Blue69Gen1]

Just installed a new Demon 750 vac secondary with electric choke on a new ZZ383 crate motor. I can't get the idle down below 1500 RPM's. Timing is set a 10° with vac advance disconnected. Throttle linkage is disconnected. The 4 idle mixture screws are all 1 turn out. I can turn them all in and it makes no difference on the idle speed. When I turn them out, the engine becomes noticably rich. Front and rear fuel bowls levels are set at the lowest mark on the sight glass. New Holley mechanical fuel pump that says it delivers 6-8 psi with no regulator required. The engine idles at 2000 rpm when cold and then after it get up to temp, it drops down to 1500. When I shut the engine off, it wants to diesel and spin backwards.

Thanks,

[MikeDVC]

I had a high idle problem with my zz4 / 650 demon carb. This may sound odd, but the primary butterflies were not closing all of the way because they were rubbing the inside of the carb; and as a result the throttle was not returning all of the way forward. I didn't notice this until I put my foot on the gas pedal (after the car sat for a while) and the gas pedal was difficult to press, almost like is was binding. As the butterflies settled into place from the pressure of the return spring, they were sort of stuck in place.

[Y-TRY]

This may seem silly, but you didn't mention anything about setting your idle-speed screw. Does it idle like that with the screw all the way out?

[Blue69Gen1]

Sorry about that, yes the idle speed screw is backed all the way out.

[Norwoodx55]

Is the choke adjusted properly? If it is too far out of adjustment, I think it can hold the primaries open. I may be wrong on that... it's been years since I have run a choke.
Brett

[streetk14]

Whenever I've had that problem it was always timing related. Just curious, is this the 1st time you've run the engine, or did it idle fine before with a different carb? Are you sure your timing marks are correct? What is the timing like at the 1500 rpm idle? To me it sounds like either the throttle is open more than it should be (stuck or choke related) or the timing is way advanced. Or maybe you have a big air leak. Did you make sure to plug any vacuum ports that weren't needed? Keep us posted.
-Andy

[Y-TRY]

Cool about the idle screw.

Streetk14 is also on the spot with vac. leak and timing. I second those two dianoses

[Blue69Gen1]

No luck over the weekend. Pulled the carb and could not find anything sticky with the choke mechanism. Put the heavy springs in the distributor - hopeing to kill any mechanical advance with the higher rpm's. No vac leaks. Plugged the PCV line just to make sure - no changes. Was able to get the idle down to 1000 rpm with advance at 8°. Below that the engine really starts to stumble. The butterflies are completely closed, with none of the transfer slots showing. Idle mixture screws still make no difference.

[Norwoodx55]

Are the secondary throttle blades fully closed? I think there is a screw adjustment for them also.

[Blue69Gen1]

Yes, fully closed.

[Y-TRY]

I know this doesn't help, but "that is weird". I feel for you. With no air/fuel going to the engine it shouldn't run at all.

As a thought, just as a stretch, here's a couple suggestions:

1) Use a straight edge to confirm that your intake is square at the carb flange. If there is a gap or gouge, or it's uneven, it could allow unregulated air through, even with a good gasket.

2) Pull your carb off, put the nuts down on the studs and set the carb back in place on the nuts. Turn on your fuel pump (obviously not engine) and let it run while looking under the carb (space now created by setting the carb on the nuts). The idea is to see if any fuel is leaking from the carb. I mention this because of the mixture screws being inaffective. You shoudl be able to lean it enough to kill it. Fuel has to be coming from somewhere.

There is still the chance your distributor isn't phased right, but that would only explain a portion of your symptoms.

Good luck, and keep us posted.

[harshman]

I had the same issue when I installed my cam in my 396 BBC. The cam was large enough that the idle never went below 1200 – on matter what I did. Timing, vac., block screws – I tried them all and turns out after talking with many a seasoned hot rodder that the carb needed to be “adjusted” - a bunch. Meaning I sent it in and had the metering plates opened up a tad as well as some other performance mods.

Now, I'm not saying that you cam is large enough to have issues but I would say that you might want to look there as well. PM me and I'll try to find the carb guys info that worked on mine.

[Neil B]

My car had the exact same symptoms when I bought it. It wouldn't take more than 8 degrees initial without the idle going to 1,500rpm. The cause was an open vacuum port under the rear carb bowl. My car had an aluminum heat shield between the intake and carb and the vacuum port could not be seen with the carb installed.

IMO, I don't think the ZZ383 cam is radical enough to need major carb tweaking.

[Norwoodx55]

I know this doesn't help, but "that is weird". I feel for you. With no air/fuel going to the engine it shouldn't run at all.

As a thought, just as a stretch, here's a couple suggestions:

1) Use a straight edge to confirm that your intake is square at the carb flange. If there is a gap or gouge, or it's uneven, it could allow unregulated air through, even with a good gasket.

2) Pull your carb off, put the nuts down on the studs and set the carb back in place on the nuts. Turn on your fuel pump (obviously not engine) and let it run while looking under the carb (space now created by setting the carb on the nuts). The idea is to see if any fuel is leaking from the carb. I mention this because of the mixture screws being inaffective. You shoudl be able to lean it enough to kill it. Fuel has to be coming from somewhere.

There is still the chance your distributor isn't phased right, but that would only explain a portion of your symptoms.

Good luck, and keep us posted.

Mechanical fuel pump.

[Y-TRY]

Oh, well nevermind that part. You'll just have to pull the pump off and pump it back and forth REAL fast..

[harshman]

My car had the exact same symptoms when I bought it. It wouldn't take more than 8 degrees initial without the idle going to 1,500rpm. The cause was an open vacuum port under the rear carb bowl. My car had an aluminum heat shield between the intake and carb and the vacuum port could not be seen with the carb installed.

IMO, I don't think the ZZ383 cam is radical enough to need major carb tweaking.

agreed on the cam thing... i was going to say the vacuum port under the carb too. daymn, great minds um... well... um....

[streetk14]

Did you ever solve the problem?

[GMachineDartGT]

VACUUM LEAK! Have you tried putting your hands over the top of the carb? Does it stall?

[gmachinz]

Get a Holley instead. LOL, J/K! Question-if your electric choke is wired up, does the primary butterfly open up slightly? Your curb idle is almost impossible to set until engine has warmed up. With the electric choke wired, you should only have to depress your gas pedal once to set the throttle shaft on high idle and the choke will prevent the shaft from moving to idle speed until the choke has warmed up enough. Maybe you have a warped shaft? Or, do you have a strong enough throttle return spring? A worn spring works well on a worn shaft but a nice, new firm shaft needs a pretty rigid spring...oh boy is this about to go in another direction here...lol. Does your accelerator cable have any binds to it? Is your throttle cable bracket adjustable like the Holley version? I bet it's in the cable/return spring adjustment. -Jabin

[Blue69Gen1]

Pushed the car to the side and have not touched it. Went to Vegas to visit the in-laws for a week. Now I am thrashing on my late model stock car. We have a winter festival coming up with a road race downtown. They close down several blocks and create a 1 mile coarse. Lots of fun. Tech is this Saturday and the race in on Feb 18-19th.

The high idle is roughly the same when the engine is cool or fully heated. Choke connected vs disconnected is no difference - hot or cold. Throttle linkage is disconnected. No obvious vac leaks. New dual return springs. Transfer slot is covered by butterflies. Idle screws make no change. Base of carb and top of intake checked ok with a straight edge.

Did not think of covering the carb with my hands to see what happens.
After the race, I will pull the Holley off the late model and see how it runs.