poor hot starting, cis injection 1973

[1973CIS911]

Hi,
I have been having a hard time starting a 1973 911 when hot. Always goes but can take a while. Appears to be standard on early cis injection cars. I have done a bit of investigating and it appears that I lose fuel pressure when engine stopped. I further tested by clamping the warm up regulator return fuel line as soon as the fuel pump was switched off, this stopped the fuel pressure at 30psi. It seems that the fuel is bleeding back to tank via the WUR. I see there might be a later modification to stop this happening and I think its called a transfer valve. I think it fits in the metering body in place of the system pressure valve. It meters system pressure during running and then stops the fuel bleed when engine stopped. Does this sound like the upgrade I need to sort the bad hot starting? Cold start is no problem. Thanks for any help. J

[prebordao]

Had the same problem. After many attempts, cured it by installing a solenoid controlled fuel shutoff valve in the return line to the tank (after the WUR).

You can find details here:

Quicker starts for CIS - hot or cold

[tirwin]

Does your '73 have a fuel accumulator? That is one component responsible for maintaining residual pressure in later CIS cars. Also in later CIS cars I believe there is a check valve on the return line to the tank. Those are the most likely culprits from what I know of the later cars.

[prebordao]

Earlier cars (73, 74) miss one additional valve in the FD.
My shutoff valve compensates for that.
You may replace the accumulator and pump check valve as I did, but it probably wont solve the issue.

[Discseven]

I had same experience with my '74. As I recall it turned out to be the warm up regulator.