Early 911 Warm Start Problem

[Danimal16]

I know that this topic has probably been brought up many times. I have reviewed the Pelican FAQs and there is a discussion regarding trouble shooting warm starting problems for a CIS 911.

When I go to restart my 1971T with stock carbs, it is rather hesitant at times and once in awhile will not start at all. Can someone share with me there experiences with similar problems and possible solutions?

Thanks in advance.

Danimal

[Grady Clay]

Danimal,

Yes, there is an “art” to starting a carbureted 911. There are three distinctly different situations. I am assuming proper operation of everything.

First, cold start where the engine is totally at ambient temperature. For instance it has set outside overnight. In this case you need to turn the ignition to “on” and let the fuel pump fill the carburetor float chambers. “Pump the gas” appropriately, every engine is slightly different due to accelerator pump settings and more. With the throttle closed, crank the engine as you open the throttle. As it fires keep the throttle close to closed and, if necessary, slightly pump the throttle. Once the engine is running, give a couple of light revs to clear any excess fuel.
Start driving immediately, don’t let it sit and idle like your Honda.

Second, and most difficult, is when the car was driven and is hot and has been parked for, say, 15 minutes. This is the worst case scenario. Do NOT turn the key to “on” to refill the float chambers. With the throttle closed, crank the engine and then open the throttle to WOT while cranking. It should fire after about 3-7 seconds. When it fires immediately let it rev to redline. Rev it high several times and immediately drive it. Drive high revs and full throttle as much as possible in the first few minutes.

Third is when you turn off a hot engine and immediately restart. Again, start cranking the engine with the throttle closed. While cranking, slightly open the throttles. When it fires, rap it a couple of times and immediately drive.

The big issue here is to prevent a “backfire” past the intake valve and cause a fire in the carburetor.


While driving, the latent heat of vaporization of the fuel and the dramatic change in air pressure across the throttle plate and venturi causes the carburetor to run very cold.

When you park your carbureted 911 hot, the head heat rises and can boil the fuel in the carburetor. This can lead to potentially disastrous results. With little fuel in the float chamber, those cylinders run lean and tend to backfire back through the intake. In extreme cases Webber carburetors can have the brass float expand and stick in the down position.

In about ’71 Porsche added a phenolic spacer between the head and the intake manifold. That was designed to be an insulator to reduce the heat transfer from the head to the carburetor. Every carbureted 911 should have these spacers two or three deep. They can also serve as matching the ports between the head and manifold.

I had a 914-6 SCCA CP/GT-2 that between sessions we would ice down the intake manifolds to prevent a problem. The problem was at a USRRC driver’s school at Riverside, I had a float stick down on the 46IDA. I came through the Carrousel with huge 10' flames coming out of the grill after the fan belt burned through. We raced the next day.

Lessons learned.

Best,
Grady

[Danimal16]

Grady,

Thanks, the information has helped a bunch.

[75 911s]

Bump. Just solid info for someone new to carbs. Rip Grady.