MFI Speed switch failure?

[Halm]

Several months ago, my engine speed would hang at 2,000 rpm in a situation such as rev’ing the engine to 3000 rpm. In other words, it would settle back to 2,000 RPM not the noral 800 rpm idle. If I then held the break and used the clutch, I could easily bring the idle back down. I was also getting what I thought to be over run backfiring. To correct this, I replaced the Speed switch and everything was fine relative to engine speed and backfiring.

However, now I have difficulty modulating engine speed smoothly at approximately 2000 rpm (again!) after the engine has warmed up. For example, when driving around town in any gear at, or about 2K, and I let up on the gas and /or gently press it, the whole drive train seems to snap. This does not happen at any RPM above 2500 rpm, or when the engine is cold.

Is this a speed switch problem? Did I read somewhere that there are 2 different speed switches depending on the year and model? If so, could the wrong switch cause this behavior?

[Rot 911]

Check out these threads:
microswitch... which is terminal 30b?

MFI microswitch-- Warren?

[Halm]

Kurt,

Thanks for the links. Somehow I had missed them.

[Early_S_Man]

Hal,

There are two 911 speed switches, one for MFI, and one for carbs [with a paint dot on the case] ... but neither one would cause the symptoms you describe! The MFI 'Blue Book' actually suggests that the carb. speed switch be used on MFI engines! The actual difference in operating/release rpm points is very small ... 1350 rpm/1550 rpm for the MFI version and 1600 rpm/1700 rpm for the carb. version.

[boyo]

You can verify correct functioning of the speed switch by attaching a voltmeter to one of its outputs (see diagrams for which one) and seeing the voltage drop to zero when the throttle is released above the RPM limit. With some long leads you could take a test drive until the high idle repeats itself, and confirm it's really a factor.

But, I'm guessing it's not. There are a lot of people running without the speed switch, and who don't have this problem. (Myself included - my speed switch was completely disconnected by the PO.) A typical malfunction is the engine stopping when it shouldn't.

So I can't understand how a faulty switch would keep the RPMs high. The RPMs should drop when you let off the throttle, just because you have closed the intakes and aren't letting as much gas+air into the system. This will occur if the speed switch is functioning, or if it isn't.

I suspect there is an additional problem - possibly a mechanical linkage which is hanging up somewhere, possibly a loose linkage with too much play in the carb or MFI butterflies, or possibly something else which is causing the throttles to stay partially open when the throttle is released. May be as simple as your throttle cable sticking, or a missing spring, etc.

I'd put my bets on the sticking throttle, meaning the speed switch doesn't actually get engaged when you let off the throttle. This could be checked visually, or you could use the voltmeter on one of the switch leads to confirm.

FWIW, my 1948 Willys Jeep used to have this same problem, traced back to the butterfly sticking. No speed switch there.

-Boyo

[NYSCAR]

Check your distributor timing curve, sounds like your mechanical advance could be stuck or gooey. If it sticks advanced, it will make your car idle high and do all sorts of backfiring and wierd bucking at lower RPMS.

Brad

[Halm]

Brad, Is there a "down and dirty" way to check the advance? Also is the dizzy something that needs lubrication on a regular basis?

[Paul Thomas]

I might take a beating for this but.... Hanz at H&R fuel injection in New York, who worked for Robert Bosch in Germany, told me to disconnect my speed switch/stop solenoid.
He said that it is out- dated emissions crap, and most of the time all they do is cause the car to idle too low or cut off when the clutch is pushed in on deceleration. I experienced this more than once and have had no regrets about unhooking it.
When Hanz rebuilds a pump he suggests that the stop solenoid is removed and he makes a plate that goes over it.

Paul

[NYSCAR]

Halm,

Yup, you should have timing markings on your pulley. Just hook up your timing light and check your markings. You should see the Z1 (top dead center) mark near the top at 900 rpm idle (I think your timing should be something like 5 deg atdc, there should be a mark for that as well). You should have two more marks clockwise from Z1 that mark 30 and 35 degrees btdc. As you rev the car you should see the pulley advancing towards those marks (maxes out at about 6000 rpm, so earplugs come in handy ). If your distributor mechanical advance is gummy or broken, you probably won't see it move much, and it won't return to 5 atdc at idle. My car had the problem and the previous owner actually re-timed it to 5 atdc with a broken distributor -thus it idled fine, but i was driving a car with no distributor advance, or i could advance it so that max advance was around 30 degrees, but at idle it would only return to 20 degrees btdc and give me an idle of about 2500 rpm. Hope this helps

Brad

[Halm]

Brad, excellent. thanks.

[SFE]

Originally Posted by Early_S_Man

Hal,

There are two 911 speed switches, one for MFI, and one for carbs [with a paint dot on the case] ... but neither one would cause the symptoms you describe! The MFI 'Blue Book' actually suggests that the carb. speed switch be used on MFI engines! The actual difference in operating/release rpm points is very small ... 1350 rpm/1550 rpm for the MFI version and 1600 rpm/1700 rpm for the carb. version.

I use this quite old post to try to figure out if all the carbs speed switches should work on MFI cars ?
The part number is 901 615 113 00 (Bosch 0 336 611 006).

The Blue Book says yes for those with a green paint dot but what about those without the green dot ???

Thanks !

[SFE]

Up !

[SFE]

It works......in case somebody's looking for that info...