CIS Fuel Pressure

[Clark Griswald]

I have a 77 2.7 with CIS.

Just bought a fuel pressure tester from JC Whitney. Unfortunely, the instructions don't tell me what the fuel pressure should be for my CIS.

There are two pressures that are needed, if memory serves terms are Supply Pressure and Control Pressure (or something close to that).

Anyone know what the pressure should be?

Thanks
Chuck

[emcon5]

Chuck, If you don't have an answer by the time I get home (in about an hour), I will dig out my Bosch book.

Tom

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82 911SC Coupe

[Clark Griswald]

Thanks Tom.

Hey, did I see you pulling into a Mcdonalds off Winchester in San Jose a couple days ago?

Looked like you and your car but with a carrera tail, and I seem to remember you picked up a tail.

[emcon5]

No, it wasn't me at McDonalds. I do have a Carerra tail, but it isn't on the car yet. At the moment it is at a friend's house near Seattle. I bought it from a guy on Rennlist who lives up there, and I had him go check it out for me. He was supposed to ship it this week, but I haven't talked to him.

How much did the tester run you?

According to the "How to Tune and Modify Bosch Fuel Injection" by Ben Watson, the numbers for your 1977 911 are:

Cold Control pressurem (All numbers ±3):
20psi @ 50ºF, 32psi @ 75ºF, and 49psi @ 100ºF.

System Pressure: 65-75psi
Warm Control Pressure: 39-45
Rest pressure: 16psi
Return flow 850cc/30 sec

Hope this helps.

Tom




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82 911SC Coupe

[john walker's workshop]

system pressure 4.8-5.2bar
control pressure 3.4-3.8bar
1 bar= 14.5lb roughly.

[emcon5]

How did it go Chuck?

Tom

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82 911SC Coupe

[Clark Griswald]

Finally got around to testing pressure today.

Problem is, I got three responses to my question asking what the correct pressure readings are. And all three responses where different.

Everyone agreed about the system pressure - 70 psi. But the recommendations for control pressure ranged from about 40 psi to 55 psi.

My actual reading was 55 psi control pressure and 70 psi system pressure.

I did find a vacume leak (before fixing the leak Control pressure was 35 psi).

Car is still missing a bit. I think the problem is mixture related.

I tried tweaking the WUR to adjust control pressure, to no avial ("knocking" the plug)

Is there some other adjustment to manipulate control pressure?

[dwil]

on my 77 2.7 the wur is hooked up across
the throttle valve compensator and is effectively out of the circuit once it is warmed up. so i believe knocking the plug
(i would recommend putting a screw and nut in
the plug so it could be backed out) is done when the engine is cold and wur 12 volt is
unplugged. the specs i had were system pressure 4.5 bar, cold control pressure 1.5
bar(room temp). my wur was full of crud but i
completely dismantled it and cleaned it out when i made it into an adjustable wur

[Clark Griswald]

I did modify the WUR with a screw for adjustment. I'm wondering what other adjustments are available to tweak the control pressure.

[john walker's workshop]

the plug adjustment on the WUR is only for warmup period, so the engine is responsive, as opposed to popping and farting when you rev it up. the adjustment does not change the hot running pressure. the only way to do that, and then only slightly, is to change the system pressure, which is controlled by the amount of shims behind the check valve in the fuel distributor.

[Clark Griswald]

JW,

So if my control pressure was off, how would I correct that situation? Seems my system pressure is perfect.

Btw, of the three responses I received on pressure recommendation, yours was the only one that says my contol pressure is in range (even then at the very top end of the range). I do respect your knowledge, but I would have felt better if the other feedback I received lined up.

[john walker's workshop]

the pressures i posted are CIS system pressures from day one. if you are within spec, why do you think your pressure is incorrect? the control pressure is a byproduct of the system pressure. so if you want more or less you adjust the shim pack in the fuel distributor. this adjustment will only affect the control press by a small amount and you must stay within the system press specs when doing it.

[This message has been edited by john walker's workshop (edited 10-02-2001).]