Rich in cylinders 1&2

[Stearman]

Texasdoc,
Another trick used on small aircraft engines to find an manifold/intake leak is start the engine and let it idle, using starting fluid....spray in small bursts in specific zones along the intake gaskets. A leak will be verified by a rise of RPM's.
Also, about the black inside the intake runners. I was not able to open the pic, but here is 2 more cents... Remember, you can only get rich indications or heavy black carbon deposits AFTER the fuel mixture is ignited. So, if a black fuel rich "soot" was inside a runner...that means an intake valve would be hanging open after the mixture was ignited and through the exhaust stroke. But if you did a leakdown test by using compressed air at TDC on the compression stroke, you could hear air hissing through the intake opening.
So, that leaves an oil leak as a possibility. If most of the black was on the top of the valve and in the head...suspect would point to a loose guide and/or seal. If it looked equally black on both sides of the intake runner AND the head runner...suspect would point to gasket. That would explain the head being black in the cylinder chambers too.

3rd...I may have missed it and forgive me ahead of time...but if this was a SBF block, if you had a older block, the deck height was different. 9.480" compared to a 9.503", there is specific intakes to go with each block. But that would be more of a problem than just two cylinders. Again, I may have missed that detail.

Hope this helps close the gap. The starting fluid trick nails it pretty quickly, and it's one of the best degreaser too!

[olddog]

Originally Posted by Stearman

Remember, you can only get rich indications or heavy black carbon deposits AFTER the fuel mixture is ignited. So, if a black fuel rich "soot" was inside a runner...that means an intake valve would be hanging open after the mixture was ignited and through the exhaust stroke.

On family car engines, this reasoning is good, however it depends on the cam. If there is a lot of overlap on the valve timing, the intake valve opens at the end of the exhaust stroke. Exhaust gasses do enter the intake at idle, which is why the manifold vacuum is low.

My cam is 236 deg intake and 242 deg exhaust at 0.050" lift. Exhaust carbon is all the way into the plenum on a Victor Jr. Single plane intake. I have EFI so there are injectors drilled into the ports. The fuel washes some of it away, as the carbon is heavier before the injectors. If it was a Carburetor, the fuel may wash some of it away. When I pull the throttle body, it wipes off with a rag.