Bad Fuel Distributor? w/ pressures for 73.5 CIS

[rockreid]

My 73.5 CIS 911 Targa (US) which ran yesterday for the first time since 1990 is not running now. I did a complete engine rebuild myself (2.4 CIS stock except for 0.25mm main bore and 0.25 oversize bearings.) Anyway the engine ran great for about 5 minutes then died. It cranks as it did before but no start. It occasionally catches but does not run for long. Today I checked the fuel pump which works, however the pressures I'm getting seem low. My fuel distributor evidently is not the stock fuel distributor as was determined on this forum a few months ago. I think it was from a 74 911.

Following the instructions on this forum my fuel pressure tester kit connected from the Fuel Dist to the WUR gave me the following:

ambient temp : 65 deg F
WUR resistance : 20.2 ohm

System Pressure : 42psi (3 Bar)

Cold Control : 24psi (1.7 Bar)

Warm Pressure : 42psi (3 Bar)

Time Delta for Warm Pressure : 3 minutes to 3 Bar

Residual Pressure:
5 min : 20psi (1.4 Bar)
10 min : 10psi (0.6 Bar)
15 min : 8 psi (0.5 Bar)
30 Min : 0 psi/ Obar

Also what I found at first is that the WUR was not working and was not getting 12v from a problem in the harness. I wired the WUR directly from its fuse in the engine bay and it seems to operate now according the the pressure tests. I haven't been able to find the specs for a 73.5 but these pressures seem low compared to other years I've read about in this forum. I suspect I'm bout to send my FD off for rebuild but I want to make sure this is the problem before I do.

[jason2guy]

i think system pressure should be around 5.2 bar. there is a pressure regulator built in to the side of the FD, there is an oring in there that can fail. i would check that

[timmy2]

System pressure is low, could be FD or the pump.


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[RarlyL8]

We have many rebuilt components in stock, a -004 fuel distributor on the shelf and a 2 wire conversion kit for your early WUR as well if interested.
What are the part numbers of your current components?

[E Sully]

The fuel pump flow rate can be check by disconnecting the fuel return line and using a container to measure volume. It should flow 850 ml in 30 seonds.
You can remove the cap nut and check the o ring. Adding shims under the spring will raise fuel pressure if the fuel pump is working properly.
1973.5 System pressure should be 4.5 to 5.2 Bar
Cold control pressure should be 1.6 @ 10C
Warm is 2.7 @ 35C

[rockreid]

so these shims will raise system pressure without affecting control pressure? .

One other rule related thing.. I know the Throttle lever next to the handbrake is supposed to affect starting , but what component is the Throttle cable supposed to be attached to in the engine bay? I can't seem to find this Throttle cable assembly on any diagrams I can find.

And I want to now try and run my fuel system dry and clean out my fuel tank. I bet old crud from the lines has made its way into my freshly restored tank. I don't think its enough to plug up any lines and maybe my new fuel filter is already done.

[E Sully]

I don't recall which components your car has, so there may be some differences depending on your fuel distributor and WUR.
Shimming the spring will change system pressure, but I am not sure of it's effect on control pressure.
Here is a link to Jim's CIS primer, which is indispensable for understanding CIS.
911 CIS Primer - Description Basic
One of the unique things with the '73.5, 74 and '75 is the regulator on the throttle body. It affects control pressure depending on the throttle position. It is designed to slightly lean the mixture in the mid range, and richen it up at full throttle. While checking control pressure, moving the throttle actuated valve should be done to make sure it is slightly changing control pressure. If it fails, it can't be repaired and needs to be replaced. Later CIS systems used vacuum controlled wur's for this effect.
The hand throttle between the seats connects to the throttle rod in the tunnel. Check out this thread, it goes into the details with pictures.
Hand throttle lever
It basically works the same as lightly stepping on the gas pedal. Having the throttle partially open also affects the micro switch on the throttle body for actuating the cold start valve.
Running the tank down will give you a good opportunity to check the filter screen that screws in the bottom of the tank on top of the fuel filter in the engine bay, Also make sure the return line is clear, any cloggs on the return will affect the control pressure.

[rockreid]

ok, here's the latest.

Volume at fuel return line past 4-way junction was 1.5 liters- more than spec. I shimmed the fuel distributor spring 2-3mm but made really no difference. System pressure still at 44 psi, way below spec. The o-ring at the tip was not torn and looked to be in good shape. I'm thinking the fuel distributor is leaking internally or is somehow plugged.

My engine is not ready to run anyway because I checked the running voltage at the battery (while the engine was running for 5 its brief working minutes) at 17.75 volts so I have a new voltage regulator coming this week. The new alternator is a brand new Bosch.

[E Sully]

It is making me wonder that the system pressure matches the control pressure. How are you hooked up to check system pressure? Is there a chance you are not reading the system pressure properly? The gage off the fuel distributor needs to be dead ended to read properly.
This thread gives information on testing the CIS, including how to check the throttle position valve and gage hook up.
Engine Backfires + CIS airbox Repair

[T77911S]

i am with sully. when something like the system pressure equals the control pressure, you need to go back and see what you could be doing wrong. first thing is connecting gauges up correctly. the shut off valve needs to be on the line or side that connects to the WUR.

[Hil911]

I was wondering how much difference those little shims would make! I think that the spring strength would be much more critical.

It's nothing to do with the low pressure, but control pressure equals system pressure when the inlet to the WUR is blocked. There is a fine grill that plugs up easily. I cleaned mine with solvent and mechanically, then blew a bit of compressed air in the other direction.

[T77911S]

thats what the shims correct for. spring "strength". the spring should not change.

there is a screen on the input to the WUR. if it gets clogged or partly clogged it will raise the CP. the system pressure (or pressure reading) is not effected by the WUR input screen.

you can try putting a thinker shim in there to see what happens, but i really dont see the spring changing enough to drop the pressure that much unless it just broke.

i hate to put bad thoughts in your head because i know i have all kinds of ideas for whats wrong with my 930, but i have 2 sets of gauges, both bought brand new at the asme time.
one gauge reads .5bar different on the high readings but they both read the same on low readings, like CP.
system pressure was 6.2 on one set and 5.7 on the other.

[Hil911]

Springs do weaken with age, although I suspect that the life of that spring would be pretty long given its small range of motion.

I don't think that leaking within the fuel distributor could reduce the system pressure. There are, I think, only five routes for fuel to exit: to the injectors, past the pressure valve, to the start valve, to the WUR or down into the air-box. The only real leak would be down into the air-box (which could happen with a very worn plunger or failed O rings) but the results would be hard to miss! I suppose that the start valve could flood the engine, but again that would be pretty obvious.

It seems to me that the pressure regulator must be sticking or the spring is weak if the 'O' ring is good. You shouldn't be running the fuel at 1.5 litres if it hasn't reached pressure: the valve should stay closed. Incidentally, at least on the '76, the hole feeding the control pressure line is tiny - I'm not sure that you could pump 1.5 litres through it to by-pass the pressure regulator, even if the WUR provided no back pressure. You could cap the WUR line, just to check?

[E Sully]

If you check this diagram for early CIS, System pressure is regulated by fuel passing by the spring loaded needle and o ring of the pressure regulator and the primary restriction in the fuel distributor. As long as the air metering plate is in the fully lower position, no fuel will flow to the injectors due to the piston blocking off the orifices.

To properly measure System Pressure, the gage must be attached so no fuel is allowed to pass by the gage to flow through the WUR or Throttle Actuated Valve. All fuel will flow by the Pressure Regulator Valve into its return line when testing System Pressure. The line to the Throttle Actuated Valve is connected directly to the fuel distributor output and must be disconnected to properly read System pressure. Adding shims behind the spring for the pressure regulator needle valve will raise system pressure. Thinner shims will lower system pressure. If the needle and o ring are sticking, pressure would be too high. If the needle and o ring are worn, pressure would be reduced. System Pressure must be set before you can check Control Pressure.

Control pressure is measured with the fuel flowing past the gage through the WUR and Throttle Actuated Valve. Control Pressure is regulated by fuel flowing through restriction at the WUR and Throttle Actuated Valve. Later CIS systems eliminated the Throttle Actuated Valve and used vacuum controlled WUR's to adjust the Control pressure at different throttle openings.

To test if the Throttle Actuated Valve is working properly, the Control pressure readings need to be taken with the throttle body closed in the idle position, halfway open for cruising mixture, and wide open for full throttle, with the WUR fully warmed up. The readings should be Idle 2.8-3 bar, Part throttle 3.4-3.8 bar, and Full throttle 2.6-3 bar. The Throttle Actuated Valve is not able to be adjusted or repaired, so if it is not working properly, it must be replaced. I have not tried, but it should be possible to eliminate it and adapt a later 2.7 vacuum WUR to eliminate it.

Total volume of flow is measured after the junction of the 3 lines with the majority of volume from the Pressure Regulator spring plunger in the fuel distributor.

As mentioned previously, any blockage of the fuel lines to the WUR or Return line to the fuel tank will affect pressure readings.