This is the continuing story of my restoration of a Morrison Olympic 600 but as this problem concerns the engine and not the cylinder drive sprocket I thought I should start a new topic.
I have finally got to the point of reconnecting the engine - a 1988 4HP Briggs and Stratton Model 112232 Type 0839-01 - to the mower. I cleaned the petrol tank and carb, fitted a new diaphragm to the pump (it's a pulsajet type carb that lifts fuel from the tank up to an internal reservoir built into the top of the tank) and generally tried to blow out all the internal orifices with carb cleaner and air. I also fitted a new gasket between carb and fuel tank. The fuel tank cap is in good nick and of the plastic variety with internal metal lugs that hold it shut. Confess I don't know how this type of cap vents but it doesn't look faulty.
There is a decent spark and I have lapped and checked the valve clearances. The valves, cylinder and piston looked in good nick - top of piston still shiney and not much carbon build up. The adjustable jet is set 1-1/4 turns out from fully seated. The carb has one long plastic tube that reaches to the bottom of the fuel tank and one short cast aluminium tube on whose side is a fixed brass jet. There is a very good description and photos of this type of pulsajet carb on the Outdoor.com website.
However, to my disappointment, the engine is reluctant to start and having started is not particularly responsive and eventually peters out. I tried adjusting the idle jet screw on the side of the carb but it seemed to have little effect. I also found that when I took the air cleaner off it started immediately but nevertheless would not continue running for more than a minute or so. Checking the spark plug (a new one) its black and I can see the top of the piston seems to be wet with fuel. I could even see liquid in the body of the carb below the air inlet. I decided to take the carb off again and clean it again just in case something was blocked. I have just reassembled it and the engine behaviour is unchanged.
Other clues? My impression is that the valves are a little noisy - when I turn the engine over with the recoil start rope on the bench I can hear them clicking closed but may be that is normal. Apart from a gentle lapping and very small adjustment to the inlet valve clearance (increasing it by less than 0.002"), I have not done anything to the valves.
My diagnosis is that something is wrong with the carb - how else would so much fuel be entering the cylinder? I am wondering if I am going to have to remove welch plugs to access some of the less visible parts of the carb but am reluctant to do this for obvious reasons.
Any thoughts or advice would be most welcome!