MFI High Idle

[JB]

I have a 1971 911E and after the car warms up, it idles at approx. 2000 rpm. I can turn it off, and then start it and it will idle normally for about 10-15 seconds and then gradually rise to 2000 rpm. I have tried disconnecting the throttle linkage so there is no pressure on the rods and no change. If I try adjusting the idle control at the pump it drops the rpm on initial start up but it always increases to 2000 rpm after 10 -15 seconds.

Any ideas? I just bought the car so don't know the past history. On the road it runs great and revs to redline in any gear. I just can't figure out how to lower the idle.

Thanks,
JB

[jluetjen]

You might want to read through some of these previous threads on the subject. If you still have questions, post them here and I'm sure lots of folks will chime in.

[Jess]

Hey Jim,

Have you checked out the distributor at all? Could it be advancing too early and causing the idle to increase?

It's just one thing that could be sending you in the wrong direction.

Jess

[304065]

JB, you and I have the same problem. There is "no substitute" for reading all the threads on MFI, printing the ones you find useful and putting them in a 3-ring notebook for reference. Also, there is a publication called "check measure adjust" which you can find here if you search, it is the BIBLE for the MFI owner and must be followed exactly to make things work.

Anyway, there are only a handful of factory-trained MFI experts left in the country, so what do you do? YOU become the expert. Like learning to fiddle with carburetors, it takes time and the right techinical information. Half of that you can get right here.

Anyway, without short-circuiting the need to search for the answer, I offer the following:

1. Do NOT mess with the idle mixture adjustment until you have followed CMA exactly. Your problem is most likely not fuel, it's air.

2. You are getting too much air because the throttle plates are either not all the way closed, improperly adjusted, idle bypass screws letting too much air by, or the throttle bodies are worn out.

3. To truly check the idle, you must remove all the linkage rods. Be careful that you don't mix up the rods. A piece of masking tape on the center of the rod to label them will keep them from going back in the wrong place. The lengths of each individual rod are absolutely critical.

4. On the throttle shaft of a MFI car there's a microswitch which signals the speed switch that the throttles are closed. Often the screw that actuates the microswitch is improperly adjusted to be too long and it holds the throttles open at idle. So, loosen the lock nut and use a screwdriver to back the screw all the way out. Test the idle again.

5. If that doesn't fix the problem, you must obtain an STE synchrometer from our host here, and go about the process of Check Measure Adjust. You want to make sure the rest of the engine is OK before you begin adjusting correlation.

6. The MFI system is equipped with a thermostat that richens the mixture until the engine temp reaches approximately 180 degrees. Are the paper hoses to the thermostat intact? A malfunctioning thermostat could be contributing to the problem, but it's not the core problem-- an excessively rich mixture will make the engine barely run, which is what you want until it warms up.

Anyway, there could be other causes such as ignition advance but these checks are subsumed in CMA and you can cross that bridge when you get there.

I have been fighting a similar problem with my 71e for a couple race seasons now. . . search under my name for one man's twisted saga. . . and remember. . .

Good luck and keep the MFI faith. There is no better induction system.*

*Just kidding you EFI guys.

[Flying Toaster]

I used to have the same problem. I read the CMA, bought the synchrometer & adjusted my idle bypass screws. They were quite a bit out of wack & the car idle came right down to 900. Nice & steady. LMK if you need a synchrometer. I can send you mine if you pay shipping. When your done, send it back. -John