Thirty year old 4hp Briggs problem on shredder
I am trying to resurrect an early 1980's Kemp shredder with a 4hp Briggs engine. The shredder came to me with a blown head gasket and a tank full of old gas (ten years maybe?), and upon dissassembly I saw that one of the head bolts was deformed. I replaced the head gasket and bad bolt, torqued everything, drained the tank, and after fiddling around with it for an hour finally got it to start. It ran great for one hour, and I was impressed by the machine, and then died as if out of gas even though there was a third of a tank left.
Figuring that I had a carb problem, I dissassembled the carb, cleaned the orifices (didn't replace the diaphragm, yet), made sure the pick-up tubes and screens were clean, inspected the tank (no rust), and reassembled. Again, it took me about an hour to get it started, and then it ran for an hour and again died like it was out of gas.
This engine has a very simple control arrangement: the only speed control is the governor, which exerts force against a spring to close the throttle if it over revs. I put a mini-tach on it just to verify engine speed and it was right around 3400 rpm which is where it is supposed to be according to the specs on the engine and the shredder.
I re-built the carb completely, replacing the diaphragm and needle valve. I double checked the fuel pick-up tubes to make sure there was proper seal at their bases, since the long fuel pick-up tube was loose when I dissassembled it, replaced the gaskets, reassembled...and NOTHING. I have good spark. The fuel cup in the tank is full of gas. I tried spraying starting fluid into the carb, and it doesn't even catch. I tried pouring a thimble full of gas in the carb and again, no luck.
I am beginning to think I have a compression problem. Will an engine run fine with bad compression but not start? The head looked very clean when I took it off, as if someone had previously cleaned it, and the valves "looked" good but I didn't have a way to really check them then. I have a leak-down tester and am going to try it to see if I can hear a leak in the cylinder.
If it is a compression problem, is it worth fixing? I have less than $40 into the machine between head gasket, carb kit, new plug, oil, and carb cleaner. I could buy a new 6.5 hp engine from Harbor Freight on sale for about $115, but I hate to throw this one away if it is serviceable. How difficult is it to replace a valve on one of these?
Thanks for the advice.