cant get her to idle under 1000 rpm

[Stang_1973]

so ive been resetting my curb idle speed every 3 days now. ive backed out the curb idle speed screw to the point that it has no effect on the throttle at all. however im still idling at 1000 rpm. if i push on the throttle down with my hand i can get it to about 800 but as soon as you press on the gas it goes back up. and after about 2 or three days is pregrssivle begins to go higher.

how do i fix this. what is the problem.

 

[htwheelz67]

sounds like a vacuum leak

 

[Stang_1973]

idk. last time i used a vacuumm gauge when tuning idle mixture and i was able to get a steady max 14 reading. idk if that helps any.

 

[69gmachine]

The curb idle screw needs to be set so that the throttle blades are in the transition slot. Take the carb off, turn it upside down and adjust the throttle stop so that there is .040 of the transition slot showing. Don't make any more adjustments here. Next, with the engine off, turn the idle bleeds in all the way, then carefully back them out 1.5 turns. This should be very close to where they should be once you find and fix the real problem. Be careful turning in the idle bleeds too far, you can lean the engine out and damage something. Disconnect the vacuum line to the distributor. Are you connected to ported vacuum or manifold vacuum? If this solves the problem, make sure you reconnect to ported vacuum. If you have power brakes, disconnect the line to the booster and plug it. If that solves the problem, replace the booster. Next, use a propane bottle (unlit of course) to check for vacuum leaks around the base of the carb and any fittings or hoses that go into the manifold. Is the Holley new? If not, you may need to rebuild it. There could be internal leaks, both fuel and air. The fuel bowls could be adjusted too high allowing fuel to overflow into the carb.

Good luck and let us know how it goes.

 

[72 fastback]

You might need a stiffer return spring. Before you do that take the throttle cable and disconnect it and then see if you can set idle. If your running the stock throttle bracket you might have to bent it forward just a bit. If that works check for wide open throttle after.

 

[hotcobra03]

high idle

have you checked the screw for the choke?maybe the cam gear is stuck..look close on the back side of choke

 

[stonecoldtx]

Assuming this is a 4-barrel, have you checked to make sure that the secondary throttle blades are not stuck open a bit? I've experienced this issue personally before . . . check it out and let us know what you find.

 

[ForceFed70]

I had the same problem with my old holley 750cfm carb. Never did figure out what the problem was. A stiffer return spring did help a little.

In the end, I gave up and decided that the carb was just too big for the engine. I'm pretty confident it wasn't a vacum leak. When I swapped to TBI this problem went away, so unless it was the carb itself that was causing the vacum leak....

 

[2nd Mustang]

New or old carb?

I could never get my first carb to idle under 1200 rpms. It turned out the vacuum leak was because of worn throttle body butterfly bushings. The vacuum was a steady 12 inches (mild hydraulic cam) even with new carb.

IMO, carbs aren't that accurate so idle speed fluctuates between 900-1200 depending on the temperature of my engine. Anything less than 900 at idle on my engine and the car shakes too much I'm assuming due to the cam.

 

[65ShelbyClone]

I had a problem with an idle on my car that wouldn't stay where I set it, but would return if I pulled up on the throttle pedal. I found out the throttle was loose on the primary shaft. Welding it fixed that problem, but now I have the 1000rpm idle problem with my 650DP. I got it used, put a renew kit in it, and got it working 95% right, but never could clear up an off-idle stumble. After messing with it last weekend, it won't idle below 1000(EFI smog cam capable of 600rpm idle). I have the secondaries closed completely with ~.020" of the primary transfer showing and it still won't idle below 1000 with 12° advance and no detectable vacuum leaks.

 

[D.Hearne]

With a Typhoon intake, the first thing that comes to mind is a huge vaccum leak due to the poor quality of these POS intakes. The cam isn't that wild, You didn't say where the initial timing's set at either.