Folks, I'm still having some issues getting this little 850 engine, SU Carb adjusted correctly. She wants to stall out at idle after a long run. The mixture flows well to cylinders #1 & 2 but really does not want to direct the flow back towards 3 & 4 evenly as it should. I've had to find a middle point where the front 2 were running a bit rich and the rear 2 were running a little leaner. I've had to turn the idle up just a hair where it's running at 1,000 rpm or a little higher at idle once fully warmed up to keep it running after a full run and down to idle, wants to stall out. Choke it a bit when it does. Should I richen up the carb just a bit too?
I've came to the conclusion the issue has to do with the design of the intake manifold, but I've never read that this has been an issue. Am I missing something?
If I understand you correctly, you are saying two cylinders at one end of the engine are running weaker or richer than the two cylinders at the other end of the engine. I presume you identify rich or lean running from the spark plug condition? If you have the standard one-into-four inlet manifold, there is no way over the dimensions of this part that the gas flow will be measurably different from one end to the other. It will all be pretty much turbulent flow and that will ensure even distribution throughout (unless you've got some special gas-flowed manifold ??). So I think these symptoms are more likely to indicate an air-leak - and the fact that it gets worse when the engine is hot, expanded and any cracks, splits or gaps will be larger is consistent with this. Hopefully you've just got a gap somewhere in the inlet manifold, in the region of the weak-running cylinders. Worst case you've got a crack in the cylinder head. The gap in the manifold might be as simple as the gasket leaking. Or it might be a fracture in the casting. Obvious first and easiest step is to remove the manifold and check it out. Reseat it with a new gasket, and you might want to think about a film of Hylomar Blue just to be on the safe side. If this doesn't cure things, then the next step is to pressure test each cylinder, which will show if the head is faulty. If it is, then the most practical cure is a replacement head, unless again there is any sign of gasket leakage allowing air to be sucked straight into the cylinders. In this case a skim and new gasket should cure it.