CIS - bad cold control pressure symptoms

prebordao · 07-29-2009, 02:19 AM

My car has been giving me a hard time when starting from cold. It starts but takes some trying. Usually I turn the key for a few seconds and it seems to be starting, but when I release the key, it dies. After some more tries (and turning the key for longer) it starts roughly, unevenly. A few seconds later, it "clears its throat" and the idle stabilizes. Afterwards it's smooth sailing and it responds well across the rev range.

I begun to suspect the cold start valve and assorted circuitry, but it doesn't seem to be the culprit... Now I'm thinking about a possibly wrong cold control pressure, but what would be the symptoms for it ? Something similar to what I described ?

914efi · 07-29-2009, 05:28 AM

It sounds like it could be either too rich or too lean. Best move is to measure control pressure, hopefully you can buy a gauge locally.

If you disconnect the cold start valve, does it get better or worse? Reason for asking is that if it is too rich (low control pressure) it might improve with less fuel from the cold start valve. The problem is that is is easy to chase things like this, the only way to really know what is going on is to measure fuel pressures.

If it is too rich you could also try leaving the key on for a bit before you try to start, this would allow the heater in the control pressure regulator to act and lean it out a bit.

If it is too lean, there are not many ways to game the system to test for it, maybe open the airbox and lift the air sensor plate (key on) to give an extra squirt of fuel before starting. The problem with this is the time you spend doing it will also allow the heater to lean the mixture!

Before you go too crazy, do you have a pop-off valve in your airbox? Good safety measure if not.

prebordao · 07-29-2009, 05:52 AM

Well, unplugging the cold start valve is an easy test... I'll do it first

Paulporsche · 07-29-2009, 06:00 AM

As Art said, it's best to know if your fuel and control pressures are correct. You'll need a fuel pressure gauge to check them.

Poster "Ricks911S" has a thread on how to conduct the tests.

You also need to verify that your mixture is correct.

Another thing you can try for now is to slightly richen your mixture before a cold start. 1/16 turn of the screw on the mixture control unit should be enough. If the engine starts fine, then your mixture was too lean, most likely either from an incorrect setting, a too high cold cp, or a vacuum leak. You can always return the mixture setting to its original position if this doesn't help.

I'm assuming your '74 doesn't have the hand throttle between the seats.

You should always make sure your ignition components are functioning correctly and you have no vacuum leaks.

prebordao · 07-29-2009, 06:03 AM

My 74' does have the hand throttle... Does it change anything written so far ??

Paulporsche · 07-29-2009, 07:08 AM

Just to make sure that you are operating it and it is adjusted correctly. I'm not completely certain, but I think that handle should be all the way up for cold starts. This performs the same function as the AAR on later CIS cars to keep the revs up. I'm not sure if it also functions the same as the AAV to provide a gulp of air to aid in starting.

Are you sure it is functioning proprly and you are using it correctly?

prebordao · 07-29-2009, 07:12 AM

Yes, I'm using it. Although, I think the only thing it does is to "simulate" your foot on the throttle.
It acts on the throttle cable and allows it to stay slightly open when you remove your foot.

ossiblue · 07:31 AM

The hand throttle on your car performs two functions: First, it closes a microswitch on the throttle body that activates the cold start valve. Second, it opens the throttle plate a bit (similar to putting your foot down on the accelerator) to allow more air into the intake.

As you've done, you should cold start your car with the throttle handle pulled all the way up. The function of the handle was replaced in 76 by the thermo time switch and AAR, as Paul has said.

Follow the advice give so far--system pressure checks sould be the first.

prebordao · 07-30-2009, 12:09 AM

I tried disconnecting the cold start valve by removing one of the wires at the microswitch.
I turned the key and it start immediately... Wow !
The car had been sitting in my underground garage for 5 hours so I guess it was reasonably cold.
This morning, after a full night of "coldness", it took a little longer to start, but still less than before...
So it seems the mixture was too rich. I suppose it could be the mixture adjustment or the cold start pressure being wrong, and the only way to be sure it to measure the pressures.
I should buy a gauge...