Idle Mixture Screw Not Doing Anything

Turning the idle mixture screw all the way in or out does absolutely nothing. What's going on here?

Maybe relevant: When I turned the car off, I got like an explosion of gas back out of the carb (detonation?).

1965 Mustang 200 with C4 auto, Pertronix 2 in factory distributor, new Pertronix coil, new Autolite 1100 carb.

Timing and idle speed have been set to factor spec. Timing was set with gun, and idle with tach.

Well it sounds to me like the carb is way Rich, or there is heat soak, or maybe even flooding. Have you checked the fuel level setting in the carb? Have you checked the fuel pump pressure? Have you checked the distributors condistion ie shaft wear during the Pertronix install? Who did the Autolite carb rebuild? Best of luck.

Hi, in general, if the idle mixture screws have no effect the idle may be too high (you do not say what the idle RPM is), or there is a vacuum leak or carb issue like Bubba said. First I would try to lower the idle as low as possible with the adjustment screw on the throttle shaft. This will close the throttle plate more. Also make sure the return springs are pulling the throttle completely closed. Good luck

This might not help. But this subject made me think of my old website. I found my html pages on an old hard drive. See Below:

Tips On The Autolite 1100 (The Misunderstood Carburetor)


CFM Rating = 185
Stamping Numbers = C5OF-E, C5OF-H, C5OF-R, C5OF-S, C5OF-N
Parts Nubmers = C5OZ-9510-B, C5OZ-9510-G, C5OZ-9510-C, C5OZ-9510-D, C5OZ-9510-E
The numbers listed above are for a 1965 200 CID Mustang. The Autolite 1100 was also used on the 144 and 170 engines. Plus on the Falcon, Failane, Ford Taxicabs and Police Specials.

For automatic choke carbs clean out the groove on the carb spacer/heater plate or the choke will not operate properly. (This is a vacuum passage)
groove


Use the proper gasket from the spacer to the carb. This is a 4-hole gasket. It lets the groove from the spacer plate line up with the hole on the bottom of your carb.
gasket

Always use a new fuel filter with the installation of your carb and use teflon tape on the threads of the filter. Two to three layers or wraps are fine, but Do Not put the tape over the opening of the filter. Pieces may shred off and get stuck in your carb. Instead, start the tape at least 1 to 2 threads back from the opening of the filter.

There are really only two on car adjustments that can be made to your carb.
1. Adjust the curb idle speed with a hot or warmed up engine, to 650-700 rpms in neutral or 550 rpms in drive with a automatic transmission. The screw is located on the rear of the carb, drivers side.
2. Adjust the idle mixture screw. Turning it in or clockwise, shuts down the fuel or leans it out. Backing it out, adds fuel or richens the mixture. To adjust, turn the screw in until the engine slows down. Then back it out until the engine slows down again. Finally, turn it back in once more until the engine runs the fastest. You are also looking for maximum vacuum at about 19 pounds. The screw is located on front passengers side of the carb.

Autolite 1100
Autolite 1100 Exploded View


© Copyright 2001. All rights reserved. Contact: 65coupei6 Powered by Inline Performance

I'd go back 1 step.
Many of us rebuild the "rebuilt carbs" to assure just one more aspect of a full, entire system is what it needs 2B.
@ the very least checking the 'wet fuel level' of float in bowl. Many of these ppl don't leave the bench w/the carb'n drive FI vehicles...
(I can just C all the crv & rav 4 in the prkin lot)
:rolflmao:

Ended up rebuilding the original carb and ditched the new one, and the idle mixture screw issue has been resolved. Car seems to be running okay now.