Question for MFI Guru about transducer !

[SFE]

Time to time, particularly when the engine is not warmed up and after a decell (foot off the pedal), rpm drops very rapidly when engaging the clutch and engine is stalling.

I checked that with 2 transducer (speed switch), one rebuilded and one NOS.
The classic transducer test is ok on both (set 2500 rpm, press the microswitch, then rpm is cycling between 900/1900).

It's like the shut off solenoid is not rapid enough to authorize the fuel to come back at 1300 rpm or so.

When I disconnect the transducer, no problem.

Could my shut off solenoid be kind of lazy and I may have to change it ?
Could it be the MFI pump too lean ?
Anything else ?

Thanks !

[mysocal911]

Originally Posted by SFE

Time to time, particularly when the engine is not warmed up and after a decell (foot off the pedal), rpm drops very rapidly when engaging the clutch and engine is stalling.

I checked that with 2 transducer (speed switch), one rebuilded and one NOS.
The classic transducer test is ok on both (set 2500 rpm, press the microswitch, then rpm is cycling between 900/1900).

It's like the shut off solenoid is not rapid enough to authorize the fuel to come back at 1300 rpm or so.

When I disconnect the transducer, no problem.

Could my shut off solenoid be kind of lazy and I may have to change it ?
Could it be the MFI pump too lean ?
Anything else ?

Thanks !

Test - Just press throttle switch and raise the idle to about 1500. Oscillations will occur.

[SFE]

Yes they occur !

[SFE]

I did my tests with 2 "known to be good" transducers.

The original relay for 911E/S : 0 336 611 001 (ON at 1520rpm and OFF at 1350rpm).



With this one I still have some engine stall as described even with the engine fully warmed up.

The substitute relay : 0 336 611 006 (ON at 1610rpm and OFF at 1500rpm).



With this one I only have engine stall as described when engine is cold ; with the engine warmed up I still see the rpm dropping very low but it's coming back to idle at 900rpm right away.

So it's seems the substitute transducer improves the behavior because it authorizes the fuel to come back sooner (at 1500 instead of 1350).

I would therefore tend to blame the solenoid, unless an MFI pump too lean could induce this issue (but I doubt it) ?

[john walker's workshop]

What are the CO readings at idle and at 3000? Both settings like to be on the rich side. Factory specs from back in the day rarely work well with today's fuels.

[SFE]

Originally Posted by john walker's workshop

What are the CO readings at idle and at 3000? Both settings like to be on the rich side. Factory specs from back in the day rarely work well with today's fuels.

Here is last datas at idle (950 rpm) :

CO 2.91
CO2 13.3
PPM 205
O2 0.35
AfR 13.63

[SFE]