1973.5 CIS Fuel problem

[ahklein]

My 1973.5 911 T with CIS has developed a fuel starvation problem. At first, when the fuel tank was low, on reserve, it would die. Thought it was out of gas. Filled up the tank but it would not take the full amount (1-2 gallons shy). Would not start at the gas station. Fuel pump running, turned over but no start. Waited for a tow, it eventually started, it ran ok, a little rough over 5000 RPMs. Next day ran fine. Ran well until that tank went low. Same problem: No start at gas station. Waited for a while, it eventually started and is running ok, a little rough. It was not particularly hot on either day.

I am thinking that there must be some sludge in the bottom of the tank that is causing issues when low. Any thoughts on how best to proceed? I need to replace the fuel sender as it is not accurate below half tank (reserve light works fine). Might I be able to see inside with the sender removed? Is there a screen that could be clogged? Do I have to remove the tank?

Thank you for help. Alan

[jess p]

On my 73.5 2.7 with CIS when I ran out of fuel and upon filling the tank I had to remove the cover and prime the fuel distributor even though the fuel pump was running. Looking in that tank would be difficult. I would start with the fuel filter and if there is sludge it would appear at least on the filter.

[HarryD]

I had a similar problem. At a HPDE my car would not rev over 5000 rpm. Suspecting fuel starvation, I replaced the fuel filter and sock (no plugging in old sock) in tank. Did not help. I replaced my fuel pump and problem solved.

I suspect my fuel pump had been failing.

[jess p]

One more thing, most electric fuel pumps often die a slow death.

[ahklein]

Thank you all. I did replace the fuel pump last year with the original style. I will check the filter.

[E Sully]

You may need to empty the tank and replace the fuel tank strainer. It is in the bottom of the tank. It needs a fairly large allen wrench to remove. Maybe flush the tank a bit if there is debris inside.
https://www.pelicanparts.com/More_Info/91120114100.htm?pn=911-201-141-00-OEM&bc=c&SVSVSI=4256
https://forums.pelicanparts.com/porsche-911-technical-forum/768320-cis-gas-tank.html