70 911 T - Exhaust "Popping" and Other

[pozee]

1970 911T 2.2 w/Zenith's

Grady Clay was gracious enough to respond to me off line. He suggested I share the issues with others.....Hope this helps.

Congratulations on your 911T and welcome to the Porsche world. You will have a lot of fun.

You will find Pelican as a hotbed of DIY modifications. The primary reason is we have all learned so much since Porsche built your car in ’69-’70. Not only did Porsche fix many issues that somehow passed engineering review, but much has been learned from racing and simply dealing with our now almost 40-year old cars. The good news is they can be better than new. Nothing like 20-20 hindsight.

Question - 1 - The gas tank has surface rust inside. I know you
don't like tanks to be treated with chemicals so would
you recommend just cleaning it with a pressure washer?
Since the surface rust has started, is it best to buy
a repro tank from
Pelican? I think the tiny rust
particles are clogging up my mesh filter.”

Answer - Internal coating the fuel tank is much less of an issue with a ’73 and earlier tank than the ’73.5 and later CIS tanks. The reason is the early tanks don’t have internal baffles or a “swirl pot” that can be restricted by the coating.

For your purpose, it is probably
sufficient to clean the tank and the screen filter.

Question - 2 - The car has a return line from the pump to the tank. If I used a
Holly pump later...could I just plug up the return hole in the tank
(whether I end up using my existing tank or the new
one...)”

Answer - Yes you can.

There is a better solution than running the original “dead head” fuel supply to the back. It is reasonable to use a circulating fuel circuit that keeps cool fuel at the carburetor needle & seat inlets. Check this thread: Weber fuel recirculation- Questions

I think your ‘70T has the unused fuel return pipe already in the tunnel. You have the very desirable front mounted fuel pump (that Porsche mistakenly moved to the rear in ’71-’76.) You can plumb your fuel circuit with an in-line fuel pump (lower pressure version of a CIS pump) and CIS fuel filter at the front suspension cross member and then the bypass type pressure regulator
in the return circuit.

The reason for this effort is to help prevent too hot fuel at the carburetor. After heat soak, the hot fuel can flash to vapor at the needle & seat causing poor running on hot start. This in turn can lead to backfire in the intake and carburetor fires and worse.

An additional “fix” to reduce this issue is to install phenolic spacers between the intake manifolds and the cylinder heads as Porsche did
with the ‘71 911T (currently available from PMO.) It takes slightly longer studs that attach the manifolds to the heads. I use two spacers at each head (2 spacers, 3 gaskets) for even better insulation and more extreme measures on my carbureted race engines.


Question - 3. The car "pops" out of the exhaust. Based on the
forum...this is a "lean" issue. Could the fact
that
the fuel flow is being restricted by the rust
particles be causing the lean state?”

Answer - Yes, this can be from a lean running condition. If so the first place to look is a restricted idle gas jet from a piece of dirt.

Another likely cause is a pin-hole leak in the exhaust system. This usually is at a head-to-heat exchanger gasket or heat exchanger-to-muffler gasket. It can also be in the muffler (very common with aftermarket mufflers).


Some other issues you should be aware of are:
A) You may have hard rubber chain ramps (guides) in the engine. Those should be replaced with the current plastic ramps. If a piece of the old ramp comes off and goes through the cam timing chain, the cam jumps timing and bad things happen. This is a DIY procedure and can be accompanied with a chain tensioner and idler sprocket support up-date. There is mild controversy as to which later chain tensioner to use. My choice is the “Turbo” version from the early ‘80s with collapse protection. Some others are proponents of the more recent “Carrera” version.

B) Your transmission has what Porsche calls a “simplified” differential. There was an up-date in ’71 that prevents total failure. You should research this. The up-date is relatively easy DIY.

C) The attachment method between the front strut and the ball joint was up-dated in ’72 and used since on all 911s. It is prudent to inspect this for any looseness or corrosion. If/when the opportunity arises you should apply these later parts. These are front struts, ball joints and wedge pins that replaces the pinch bolts.

D) While this is more appropriately long-term maintenance, it is wise to replace the brake master cylinder and all four flex hoses with new original parts. Just for safety.

Enjoy.

Best,
Grady