Rich on one side?

After the first race in my D-13, I pulled the plugs to see what they look like. Cylinders 1 & 2 were just fine, but 3 & 4 showed carbon sooting. Is it common to have this, or is there a distribution problem in the intake manifold? Plugs all seemed to be gapped at .033 to .035. Any ideas? Thanks!

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Originally Posted by Bardahl77

After the first race in my D-13, I pulled the plugs to see what they look like. Cylinders 1 & 2 were just fine, but 3 & 4 showed carbon sooting. Is it common to have this, or is there a distribution problem in the intake manifold? Plugs all seemed to be gapped at .033 to .035. Any ideas? Thanks!

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My first thought would be leaks (making one side lean) and/or significant dents in your intake manifold.

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Originally Posted by Stumpthumper

My first thought would be leaks (making one side lean) and/or significant dents in your intake manifold.

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I was also thinking intake manifold not perfectly seated over #1 & #2.
Start her up & spray carb cleaner around the port (where the manifold & head seat together). If white smoke appears at the tail pipe...voila...you have a leak.
The manifold should be kept perfectly centered on the head port as you're tightening it down.
And always use new manifold gaskets... re-using them isn't advisable.

Glenn

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Originally Posted by pacratt

I was also thinking intake manifold not perfectly seated over #1 & #2.
Start her up & spray carb cleaner around the port (where the manifold & head seat together). If white smoke appears at the tail pipe...voila...you have a leak.
The manifold should be kept perfectly centered on the head port as you're tightening it down.
Glenn

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You shouldn't have to 'wait' for smoke at the exhaust. If there is ANY leak at the manifold, the engine at idle will stumble if you spray it with most anything liquid. WATER is a lot cheaper than carb cleaner or brake clean :-). Also.. if you have a leak (or other carb problem), the engine generally doesn't want to idle correctly. If you can't get it to idle nicely around 1000 or 1100 rpm.. ck all those things. Many seem to PREFER a high idle and that's OK .. but it still SHOULD be able to idle at 'around' 1000 RPM even if you don't leave it that way.

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Originally Posted by pacratt

And always use new manifold gaskets... re-using them isn't advisable.
Glenn

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Correct .. IF you are using the original type crushable VW gaskets. I use solid copper gaskets (only takes ONE set.. per engine, anyway) with a 'smear' of hi temp sealer. (clean and re'smear each use) And ALWAYS spray test after assembly.

As for plug reading - no, there is no 'common distribution problem' in manifolds. One issue that could cause this (other than the leak) is intake valve(s) too TIGHT on that side. Any leakage back into the intake mannie will cause sooting of the plugs as well as in the intake port .. even back up into the manifold.

Adding some nuance to the subject....

Cyl 1 and 3 run lean compared to 2 and 4. This has to do with the wet flow dynamics of the siamese ports in operation and the fact that 2 and 4 fire before 1 and 3 in the firing order. The mass of the fuel in the air/fuel mixture has a hard time changing flow paths/direction compared to the air.

Also, for some reason that I have yet to discover after 20 years of investigation, Cyl 3 is always a little leaner than 1. This has nothing to do with the ignition timing to the individual cylinders. I have experimented with different cylinder timing to no avail.

All of this is clearly visible with using individual cyl EGTs. A little harder to spot on the plugs.

A good manifold will have balanced flow side to side. Even if a manifold has unequal flow, the AFR will stay correct within each sides flow. Flow changes do not effect the AFR in the flow.

Brian