73 MFI Problem

[gunnar]

Car: '73 911S - stock

Getting my restoration project back on the road finally!!!

Engine starts ok when cold, and once warmed up, runs
very nicely without any mid-range surge. Idle is good,
and CO is at 3.5% at idle and 3-4% at 2500-3500 rpm,
so the rack seams to be adjusted correctly. Ignition
timing is also on correct.

When the car is warmed up, the engine will start if
you try immidiatly, but if you let it sit for more than
a minute, it will not re-start. The cold-start
injectors are not squirting (as they shouldn't) at
this point. I don't really know if I'm getting
too much or too little fuel at this point.
The car will re-start fine in about 2-3 hours.

I removed the injectors to soak them in Ventil Sauber
overnight, and when disconnecting the fuel lines from
the injectors, it seamed like the lines were dry (at
this point the car had sat for 4 hours). Is there some
sort of check valve in the MFI pump to keep the lines
filled, or is the pump expected to re-fill the lines
once you crank?

I'm not quite sure where to go from here, so if any one
had a similar problem, any thoughts or advice??

Thanks,

Gunnar

[Halm]

Last week, P-Thomas got my MFI right including using the Gunston analyser. We got it running very smooth with no load running in the driveway with about the same values. However when we took it for a test drive, we had to fatten up the enrichment. Then it ran like it should.

We also found that the primary rod length that is to be set at 114 mm, needed to be slightly longer for the rack to balance. Thus, our theory is that as these engines and MFI systems age, the precise factory settings for a new car may or may not work.

[earlycar]

Gunnar, the injector lines will fill again when the pump starts turning. this will take a few cranks. have you tried pressing your trottle to the floor when starting warm- if this works i would assume that you are too lean on start up, but doesn't mean you should go adjusting your pump. on my car the start circuit is disconected and a wire runs to a switch on the dash where i choose how to prime the engine. hope this helps, chris

[Paul Thomas]

I put a button on my dash to operate the cold start too. For hot starts i will usually just tap it for a little extra juice. the CO numbers are fairly low for it to start without a little help (my opinion). Like you said, it sits for a while, cools off and when you restart, thet cold start comes back on.

You could test this by running a hot wire off of the the 3-fuse panel by the CD and when hot starting have someone touch it bfriefly to shoot a little gas in there.

Also, the injector lines still have fuel in them, if you unhook both ends, it will come out. since it didn't come spurting out at least you know you have good pressure in the pump.

Paul

[power]

That was going to be next weekends project. To put a pushbutton momentary contact switch in the dash, for the start enrichment circuit. I have had trouble in the past with this circuit not working correctly. Glad to hear it works!
P.S. always be cautious with live wires/sparks and fuel lines.........

[gunnar]

Thanks for the replies. I have already tried to hook
a wire up to the cold-start solenoid, and given it
a couple of squirts while cranking hot. No start......,
not even a hint of fireing.

Gunnar

[Early_S_Man]

Gunnar,

It sounds like an electrical problem ... not a fuel/mixture problem, though your CO sounds awful low for an MFI engine!

One thing you may want to check is the distributor point plate ground wire ... as detailed in the following thread:

small copper wire in '73 bosch distributor