911 73T CIS issues

[tkrefeld]

I am having a cold start issue with my 73T CIS. When warm it runs/idles fine but when cold I have to start it about 10 times bafore it will stay running. And, giving it fuel sometimes the tach jumps about 3K when cold which worries me. There may be a crack in the air box and the PO mentioned the throttle cable may be broken. Is this enough to cause this problem?

[Jim Sims]

There is not a throttle cable on a 73.5 CIS. Instead there is a linkage of rods and bellcranks with springs to return it to a closed position. The linkage runs from a bellcrank at front pedal cluster though the tunnel to a bell crank on the front driver's side of the tranmission, then up through the engine sheet metal to a third bell crank on the top of the engine. From there it goes to a throttle body on the driver's side on the top of the engine. Within the tunnel just below the emergency brake, heater and throttle handles there is a plastic arm attached to the throttle handle. When the throttle handle is pulled back for starting the throttle linkage is pulled open by this plastic arm; one should see the gas pedal depress when the throttle handle is pulled up. This action should cause a microswitch mounted on the throttle body to close. The closing of this switch allows the cold enrichment valve (or cold start valve or CSV) to operate when and only when the starter is turning. The cold enrichment valve is like a seventh fuel injector to enrich the mixture for easier starting. Several things could be wrong: the thottle handle could not be moving the throttle linkage due to the plastic arm being broken, the microswitch could be out of adjustment or broken or disconnected and the CSV could be bad or disconnected. Are you starting the car by pushing on the gas pedal? Doing this and a dragging linkage due to worn bushings on the bell cranks and on the horizontal rod in the tunnel along with a broken return spring (there are two on the top of the engine) could be causing to momentary jump to to 3K rpm which isn't too bad in itself. The real problem with using the gas pedal is "blowing the air box" due to an improper mixture distribution; does the car have a "pop-off valve" (pressure relief valve) in the air box? Look under the filter element for the pop-off valve. And yes, a cracked airbox can also cause the starting problems you are having. Good luck, Jim

[azasadny]

Jim Sims,

Your description is 100% accurate (at least for my '74). My WUR was bad and was replaced. My hand throttle lever was broken and was replaced. My throttle microswitch was not working and was bypassed and the entire CIS system was checked and I still have a cold start problem. Weird thing is, tonight when I came home from work, I tried to start the car and it started right up! Tomorrow i will test the cold start valve and thermotime switch to make sure they are functioning correctly.
Thanks for your description of the throttle, I understand the system alot better now! Now if I just knew "what wires went where" on the rear electrical relay box on the driver's side of the engine compartment. The wiring is a rat's nest but I have a difficult time reading the factory wiring diagrams.

[Jim Sims]

In a 1974 with a manual transmission when the hand throttle switch (this is the microswitch on the side of the throttle body) is closed, power (voltage) is sent (while the starter is turning and only then) through a yellow wire to the thermal time switch (or thermo-switch) located on the cover of the driver's side chain case. There should be two terminals on the thermal time switch; they look like little nail heads and the wire lugs or connectors slide sideways onto the heads. There should be a grey and white wire with the yellow wire (they share a connector) and they attach to the same thermal time switch terminal. The grey and white wire will go back through the engine wiring harness and connect to a plug on connector that attaches to the cold start valve. There is also a red and black wire that is attached to this same cold start valve plug on connector and this wire goes back the other way through the engine wiring harness to the other terminal of the thermal time switch. The power going to the thermal time switch will also travel through the grey and white wire to the cold start valve (and cause it to inject additional fuel during starting while the engine is cold). The ground for this circuit is back through the red and black wire to the thermal time switch. The thermal time switch begins to heat as soon as it gets power (voltage) and once it heats to a certain point it opens the ground connection to the red and black wire. This takes different amounts of time depending on how hot the switch is; hence the name "thermal time switch". Note that a portion of the thermal time switch body is exposed to the heat of the engine through the chain case cover and the oil splashing around inside the chain case. Check this circuit with the engine cold. Remove the connector with the yellow and grey and white wire. Check for voltage at this connector while someone cranks the starter with the throttle handle pulled up. Take care that one remains clear of the fan and fan belt as the engine will be turning over and may start. Note there is not a fuel pump relay in 1974 models that one can pull to stop the fuel pump. If there is no voltage at this connector then there is a problem between the connector and the hand throttle microswitch or maybe the hand throttle microswitch isn't getting power. A yellow wire from terminal 50 of the starter solenoid provides the the hand throttle microswitch with power during starting and only during starting. If there is power to the yellow and grey and white wires then disconnect the the other connector on the thermal time switch and check the red and black wire for voltage while starting (do not reconnect the other connector to the thermal time switch while doing this). If there is not voltage at the red and black wire then there is a problem with the CSV or it's plug or the wiring to it. If there is voltage at the red and black wire then check to see if the terminal on the thermal time switch that held the red and black wire is providing a ground (check for continuity to ground). If there is no ground and the engine is cold then the thermal time switch is likely bad. If there is voltage through out the circuit and continuity to ground then then the CSV itself may be bad (burned out or jammed or plugged or fuel hose missing). Clear as mud? Cheers, Jim

[Sunroof]

Great description Jim.....
I have had similar cold start problems and I did replace the plastic brittle claw for the hand throttle which was easier then I expected, then went on to a new cold start valve and further to a new warm up regulator. I know I could have tested each component but it gave me an excuse anyways to freshen up the system as part of the restoration process. Interestingly I got several different diagnosis from two separate mechanics (cracked air box and bad WUR).
What my problem actually was, was that a mechanic somewhere in the thirty years of this cars life applied a good amount of locktite to the mixture adjustment screw. When I thought I was making adjustments, I was just cranking down on the locktite and never really got a bite of the hex adjuster! Talk about bizarre! I finally cleaned out the screw area with a dental pick and re-adjusted. Everything form then on has been perfect. Its amazing how improper mixture can raise havoc in the CIS. Over the last year I installed a new air box, intake runners, fuel pump, accumulator and pop-off valve. I am fortunate everything is running well............thus far.

Regards
Bob
73.5T

[azasadny]

Jim Sims,

Another great explanation! Thanks again!!!

[azasadny]

Many of you have followed my intermittent "cold starting" problem that eventually became a constant starting problem that caused my to have to crank my car 20-30 times, with several loud backfires, prior to the car starting. Once the car started, it ran great but the starting problem was driving me nuts. Needless to say, I replaced alot of components (fuel filter and accumulator, cold start valve and thermotime switch, throttle microswitch, plugs, CD unit, rotor, coil, Pertronix unit, WUR, etc...


Each replaced item improved the situation temporarily, but the problem still persisted. The cold and warm pressures were within acceptable limits and I had good spark at plug #1, but the car just cranked and refused to start. To make a long story short, this is what the mechanic found:

- The plugs were improperly gapped (too wide) even though I followed the instructions on the PermaTune CD unit for spark plug gapping specs.

- The plug wires and their "boots" are bad and I will replace them

- The "new" throttle microswitch that Jordi Riera sent to me worked perfectly and the old one was bad

- One of the wires coming off the Pertronix unit had rubbed until the insulation was off and bare wire was showing.

My mechanic "fixed" these things and the car starts on the first crank, idles at 950rpm and runs great. I REALLY appreciate all of the assistance so many of you have provided. I have learned alot from you kind folks. I'll post this update on the other posts that I have been on so that anyone with a similar problem may be helped. I had checked the spark at plug #1 and had a nice spark, but my mechanic showed me that others were weak. I hope this helps others out there....