Surging is a sign of a lean condition. There are many reasons your engine could have a lean condition. The first of which could be a leak at the diaphragm you just installed. Next it could be as simple as the new diaphragm needs a little more pressure on it to keep is closed longer. Hence making more fuel run through the engine rather than bypassing the fuel to the front of the pump.

I'm not going to go into a detail explanation on how the pump works here but you could just need to dial the pump screw IN (CW) until the surging stops. Or it could be more of a problem like discussed above. You might be surprised what 1/8th to 1/4 turn on the pump Richer (IN or CW) will do to a surging condition.

I have never seen the little valves and springs inside the pump fail. I have seen debris get in there and stop them from closing but this produces a full rich condition not a surging lean one. Not that it can't happen I just have never seen it. The pumps when they fail start to leak in the main body or the shaft of the pump. The piston that is driven by the pushrods or the pump body gets worn either at the top where the tappets are being moved by the pushrods, or the little piston inside the pump body becomes excessively worn.

And yes these problems can cause lean conditions. They can also cause rich conditions too just depends on what happened.

My first step would be to dial the pump in a bit and see if you can get it to stop surging at the midrange...this should also increase your rpms as its providing more fuel....No promises on the rpm increase but I have seen it happen.


Troy Newman
Team YS