New Engine idle

[Inline6Merc]

So this last week I put together my new engine and transmission. It’s a 200ci with CI Head, Holley 4160 600cfm carb, DUI Dizzy, and 278 solid lifter cam. Got it all in there and buttoned up. Fired the motor up and broke it in for 20min at 2000rpms. When I fired it up, it automatically was around 2500rpms and I lowered it with the idle screw to 2000rpms. So today I go out in the garage and fire it up and try to tune it but I can’t get the idle down past 2000rpms. I adjusted the floats and screwed the idle and mixture screws every which way but it won’t help. All it would do is fluctuate from 3000rpms to 2000 rpms. Any ideas fellas?

Thanks,
Curtis

 

[nitrousnick]

make sure the throtle plates are closing,cable or linkage might not be long enough and holding throtle plates open,pop the linkage off to make sure.make sure you have no vacuum leaks.does it stay running with the idle mixture screws all the way in?If so you need a new power valve in the carb.

 

[Inline6Merc]

sure does. Thats what i was coming to the conclusion off is a new power valve.

 

[nitrousnick]

before you come to that conclusion you need to make sure the throttle plates are closing because your idle mixture screws will have no effect unless the plates are into the idle transfer slot of the carb.

 

[Inline6Merc]

The linkage is dissconected so no worries on that. yah i can look down the carb and see the throttle plates are open just a hair because of the idle screw. nothing that should make it run at 2000 grand. as for leaks i have all places on the carb blocked off and everything tight so i cant say 100 percent sure there are no vacuum leaks but im pretty confident that there are none. Guess i should put a vacuum guage on it. cant hurt!

 

[Anonymous]

Did you block off the vacuum line on the rear baseplate of the carb?

 

[JackFish]

What is the timing set at?

 

[addo]

I'm wondering about cam timing.

 

[MustangSix]

It takes very little throttle to make an engine run 2000. With the idle adjustment screw fully backed out, the engine should not run at all. If it is, you have air coming in from somewhere.

The first thing I'd look at the linkages to ensure the primaries are closing. Look at the throttle plates. Make sure none are bent and preventing them from closing fully. Sometimes (rarely) the secondaries are open slightly. There is a secondary stop screw that you can get to by removing the carb and looking at the bottom.

If that checks out, next I'd look for a vacuum leak. Make sure the bottom of the carb matches up to the manifold so that all the vacuum ports are sealed by the gasket.

Make sure all other vacuum lines are plugged and connected. The trans modulator line, if left disconnected, can allow a fast idle like that. Same with brake boosters, distributor advance canisters, PCV valves.

 

[Bort62]

You have a vacuum leak by definition.

 

[Inline6Merc]

So it turns out Doug was right on the open vacuum port under the bowl. I plugged it up and set the timming and got it down to about 1200rpms in park. Havent been able to stick it in drive to see how much it drops because there is no transmission linkage hooked up. Thanks for all the help. Does 1200rpms sound to high with a 278 solid lifter cam with a 110lobe center and 10 to 1 compression?

Thanks,

Curtis

 

[MustangSix]

1200 is a little high if you still have a stock converter. If so, you need to try to drop the idle a little more, maybe to 800-1000 rpm.

 

[Inline6Merc]

I guess i should have mentioned it has about a 2500rpm stall converter.

 

[MustangSix]

Should be ok, then. Make sure you have a good trans cooler installed to handle that converter.