Zenith TIN - Intake Air Leaks Solved by PMO Manifolds

[VFR750]

Summary: My observation is PMO manifolds, with the CIS-specific flange design, seals the CIS intake ports better than the old Porsche manifolds do.

History:
Over the past 2 years I have been trying to figure out the best jetting for my Zenith TIN carburetor installed on a 3.0l SC engine. Things were going pretty well for the first season, Aug-Nov 2010.

In 2011 I learned a lot more, and started to really get into tweaking the fundamentals. But after I added the replacable Idle Air Correction Jets, I started to be more aware of the idle popping, and occasional popping when coming to a halt. At first I related the problem to cool weather (35-55F) and my attempts to lean out the engine. However, by the end of 2011, the engine would shake at idle, as if it was missing a cylinder. Even going back to my baseline good jetting did not fix the problem. Ultimately, by the end of 2011 I was very fustrated. I had recently removed and reinstalled the Zenith manifold, new gaskets, etc... So an idle air leak seemed improbable.

2012 started out no better. New gaskets started out the new year, and within a month, the shaking and poping had returned. This was getting very fustrating, and frankly my enthusiasm for the carberators was waning.

Obviously I had an air leak. So off came the exhaust. Of course I found one leaking gasket at the #1 cylinder head. Unfortunately replacing all the exhaust gaskets did not solve the problem. It must be in the intake, but why? All new gaskets, manifold properly tightened, etc...

A chat with Richard Parr at PMO pretty much confirmed what I though my problem was: I needed manifolds that would properly clamp the gaskets to the CIS heads.

Problem:
The CIS head has a large cutout for the injectors. The Zenith (Porsche) manifolds fit a non-CIS head, but do not have enough clamping surface to properly clamp and support the gasket between the head and the manifold/heat isolator.

If you look at the picture, you can see the outline of the manifold/heat isolator in the gasket. This does not match the CIS head surface at all. You can also see how the gasket is breaking down, "fluttering" itself to pieces where it is unsupported. That leave almost no gasket between the edge of the CIS injector cutout and isolator. This gasket was on the engine for ~5 weeks! Previously removed gaskets looked worse.



Ultimately, the original manifold is going to have a hard time sealing a CIS head. In the picture above, the gaskets used were the CIS compatible ones sold by PMO. These are subtantially larger than the standard 911S gasket for the Early carbureted/MFI engines. But even this added material is not enough to prevent the eventual air leak.

Solution:
Here is the installed CIS-specific manifold. It clearly had a wider flange, that clamps the gasket the way it needs to be. Since then, I have had no issues. And have restarted my tuning effort, knowing changes I make are not influenced by intake air leaks.


Isolator Shrinkage:
One thing to watch out for is the shrinkage of the heat isolators. They do shrink in thickness over time. This is especially true in the middle portion, where it gets thinner than the outer edges of the isolator. The easy fix is to carefully sand the isolators and bring them back into a level surface. If you look at the first picture, you can see the isolators have been sanded. You will need a caliper/micrometer when sanding, to insure the isolator are sanded evenly, and to the same thickness.