MFI, idle speed slows down after coasting downhill

Helllo,
My Porsche is 1972 911S with a stock mfi.
I have a problem of that the idle speed slows down to 800rpm after coasting downhill for some time or shifting down to the lower gear before stopping the traffic signal.
I have checked the RPM transducer and the micro switch work correctly, which make the shut off solenoid on the pump also works correctly. If I leave off the pedal at more than 1500rpm, I can hear “click” and the engine speed comes down to 1000rpm immediately. The idle speed except those cases is 1000rpm in steady.

What do you think that my problem with mfi comes from? Any suggestions or comments are welcome.

Thank you
Takahiro

Sticking throttles sounds like the most likely cause. Where is your idle after blipping the throttle?

Remove the wire from the shut off solenoid and repeat test. If it idles correctly now,readjust the microswitch. Back out screw from switch entirely, turn screw until you hear switch click, go one turn further, repeat downhill test, if problem is still there check ignition timing, if it is at 5 deg. retarded try 2 deg. retarded.

Thank you for your replies.
The idle is 1000rpm after blipping.

I will try to tighten the turn screw more and to make 2 or 3 deg forward in the iginition timing this weekend.

Thank you
Takahiro

edmayo
I tried to make the ignition timing forward from 5ATDC to 2ATDC with 1000 -1100 rpm. Today I had a test drive of 200km with that ignition timing, I did not have any idle drops after the long coasting downhill. I hope the above problem had gone.

Thank you
Takahiro

edmayo.

As I wrote at the above, I set the ignition timing to 2ATDC or 1ATDC from 5ATDC, I got the good result at that time. Today I drove the Porsche for an hour, the idle speed dropped after the long downhill,though.
Then I removed the wire to the shut off solenoid, the idle speed did not down to 800 rpm, but it came down to the idle speed 1000rpm slowly. I readjusted the microswitch as edmayo suggested, I could not get the good result, though.
Any suggestions are welcome.

Thank you
Takahiro

There are many possibilities, we don't know what you're fuel /air ratio is for example, but it is possible that the distributor advance mechanism is sticking. Try removing the vacuum hose from the distributor, plug the hose and repeat test, see if it will at least be consistant in the idle speed.

edmayo,

Thank you for your reply.
I will borrow the LM-2 and measure the AFR this weekend.
If I remove the vacuum hose from the distributor, the idle speed will hunt between 1200 and 1500 rpm, as the ignition timing will be 5 BTDC. It must be very hard to start Porsche.
Is this normal?

Thank you
takahiro

Yes, the idle speed will be higher, won't hurt for the test. If that did solve the problem then you could reset timing WITHOUT the hose on to TDC and leave it that way, or get the distributor fixed. I'm just trying to isolate the problem.

Thank you for your advice.
I will try it.

Takahiro