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Join Date: Jun 2010
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No modifications, except as I have said, the pump does not have a fuel enrichment solenoid which was on '69s. So it looks like I have a later model pump.
The engine was rebuilt and the fuel lines look new. When the car warms up, I go rich, unless I give it some air, in which case, I go fast.

Old 07-01-2010, 04:45 AM
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I do have brand new stainless exhaust system. But, it is stock configuration.
Old 07-01-2010, 04:47 AM
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Well you answered my question on post #58. Leave the main rack adjustments alone. As for trying to adjust the idle mixture on the pump, it's not going to make any difference with the engine running at 2000 rpm, it's out of the tuning range of the pump governor springs past 1500 rpm.
Is it possible that the thermostat is sticking during the warm up and only expanding half way at times? Then after a good warm up period the thermostat then snaps open the rest of the way. Were the TB idle air screws, timing, and idle mixture adjusted at one of the times the thermostat stuck at a half way point? Car runs great, idles fine then all of a sudden out of nowhere idle goes up to 2000 rpm because the thermostat finally expanded all the way. TB's are a little worn causing a lean idle and rpm starts to go up a little, distributor also starts to advance and now your at 2000K.
We are all just trying to help and I'm no "expert" but I do know the inside of the MFI pump.
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Old 07-01-2010, 07:33 AM
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No mods, new stainless stock-type exhaust.
Fuel lines OK.
This morning I adjusted the main enrichment 6 clicks rich. It was really rich, smoke, and I could race the engine on butterflies alone, pump disconnected. So I reset it lean 3 clicks past where I started. Smoke cleared up, the engine did not race when I opened the throttles and idle went down to a relatively plesant 1400 rpm. I was pretty excited about that. I put the air cleaner on and went for a ride. The engine ran rough, misfired occasionally and I got small backfires from the fuel. So I reset it one click rich. The misfires went away, the engine ran very nicely on the road, but idle went to 1800. At least that is down 200. For the first time I feel like I have some control over what is going on. I redid the exercise with the main screw two more times going one click back and forth. I could predict the onset of rough running and the idle speed. It appears that the main screw is in control. I went to the idle screw, made a dramatic change and it didnt make any difference with the fully warmed up engine.
Old 07-01-2010, 11:30 AM
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Max Sluiter
 
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I wonder if something is crossed-up in the three centrifugal governor springs? Has the pump been reconditioned to the point where it was opened up that far?
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/
Old 07-01-2010, 11:35 AM
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No idea what, if anything, was done to the pump.
I watched the thermostat through the port this morning. It went through a complete throw. No indication of sticking. When fully extended, there was just a minimal bit of additional movement of the lever. Probably by design. I think the thermostat is working properly.
Tomorrow I think Ill lean the main screw and see how far down the idle goes. It will probably be too lean to run well on the road, but it would be interesting to get it to the point the idle screw did something.
Old 07-01-2010, 08:06 PM
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Off to Parade tomorrow.
Not driving the 911. Taking the 356C.
Old 07-01-2010, 08:08 PM
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With the main screw one notch leaner than it really wants to be, Im idling at 1200. Im getting light roughness and backfire. I will have to go one notch richer to get it to go away, but I think I will work on getting the idle down while it is actually in the idle range. My left side TBs are flowing more air that the right. Probably because it is so much harder to work on that side, I have done a better job on the right side. I owe you a number on start of distributor advance.
Off to St Charles.
Old 07-02-2010, 08:41 AM
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Correction
Exhaust system is modified. The car has a Dansa muffler.
Old 07-02-2010, 08:30 PM
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I think you mean Ansa, or maybe Dansk.

If the pump is not tuned to the muffler, incorrect fuel mixture can result through the rev range as the tuning of the muffler takes effect on volumetric efficiency through scavenging. The change in VE dynamics and stoichiometry is not equal across the rev range so the space cam should actually have a different profile to correct the fuel/air mixture. This is why MFI is hard to do when you stray far from standard Porsche setups. If the space cam is wrong, then you will only be able to tune the engine to run well at one rpm band. This sounds like how you can get the idle better with popping elsewhere or you can get it to run strong up top but idle high.
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/

Last edited by Flieger; 07-03-2010 at 12:21 AM..
Old 07-02-2010, 09:09 PM
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Flieger......so right. I have a Bursch that is specifially tuned for MFI and it has been great. Doug at Bursch told me they tuned it to eliminate the flat spot [lean spot] at 2800-3300.
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00 Saab 95 Aero wagon stick
01 Saab 95 Aero wagon auto
03 Boxster
90 Chevy PU Prerunner....1990
Old 07-02-2010, 10:13 PM
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Back from Parade. Great fun. 356C ran over 1000 miles like a sewing maching.

I took my RPM transducer to a general electronics repair guy. He checked the transistors and diodes and had no difficulty finding a broken lead to the relay. Easy fix. Call it a vibration failure, I guess. Cost $25.00 which I had to force him to take. Thanks for the insistance to fix it myself. I didnt do it myself, but learned that it is an easy board to diagnose at the component level. Fuel shut-off system working fine.

The lead to look carefully at the distributor advance curve was very helpful. Although there was nothing wrong with the distributor advance, it contributed greatly to my high idle. I have some leakage through the middle two throttle bodies. Not horrible and the outer four seem much better. I balanced them at a low flow and the idle came down to a consistent 1200 when warm. With the fuel cut-off working, I dont hang up at the 1800-2000 point. The main pump screw is unchanged, the car runs fine at speed and the idle is tolerable, actually very satisfactory.

The car starts quickly, but runs very rich initially and idles at a very low speed. It doesnt even move the tach. I can set the lever, but it is touchy, until it warms up and settles in at 1200 rpm. There is some smoke and it isnt pleasant to let it sit and idle until it warms up. This isnt perfect, but it is much better than I had.

So, I still have the dramatic change from a low to high idle, but it is at better levels. Very driveable until I learn more. I am going to get it on a CO meter when I can.
Old 07-10-2010, 06:16 AM
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Glad to see you worked most of the problems out and it runs much better. Great job.
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Old 07-10-2010, 08:18 AM
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Mine stumbles around 500 rpm on the tach when I first start up. I always need to give it throttle until it warms up. That is where the good 1500 idle is present. I am happy with the higher idle as the engine runs smoother there.
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911S
1971 chassis, 2.7RS spec MFI engine, suspension mods, lightened

Suspension by Rebel Racing, Serviced by TLG Auto, Brakes by PMB Performance
http://www.flickr.com/photos/max_911_fahrer/
Old 07-10-2010, 11:52 AM
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The rich idle could be the warm up thermostat.

Since I took mine off and replaced with an adjusting screw, it has idle so much better. I have to use the fuel squirter which I have set up with a button on the dash to start, and sometimes give it a couple of additional squirts. I live in a warm climate and this helps.

The car idles really well while warming up now.

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RGruppe #79 '73 Carrera RS spec 2.7 MFI
00 Saab 95 Aero wagon stick
01 Saab 95 Aero wagon auto
03 Boxster
90 Chevy PU Prerunner....1990
Old 07-11-2010, 05:43 PM
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