Small Engines (Lawn Mowers, etc.): Briggs V-Twin, engine runs poorly

Question
Hiya,
Well, I just purchased a used 35hp Briggs & Stratton V-Twin offline. I have a hobby of making go-karts, and this is the biggest "small" engine I've ever seen. I believe that it is the biggest made by briggs...and I have already made a frame for it. I'm afraid that I can't supply you with a model number, as most of the cowling and the OHV valve covers had been removed when I got it. I asked the original owner for them, and he looked for them without any luck. I had to buy some second-hand, and they are made for a 24-hp engine, so they might not be much help. The engine is electric-start, but even after carefully cleaning all the fuel lines and the carb, and replacing the head crankcase gaskets, AND replacing the oil sensor and damaged cam and rods, this engine just won't start! It has spark, fuel and plenty of compression, but for some reason, it won't run. It tries, and will run rough for a few minutes, but then promptly shut down. I payed $500 for this engine, and will not give it up. There appears to be no mechanical damage anymore after the replacement parts, and the engine is well-balanced and when it does attempt an idle I can tell both cylinders are firing. Do you have any guesses? It's almost Christmas, and I was hoping to give this go-kart to my 14-year-old son, Andrew. He loves engineering and anything to do with engines and cars, and this would be his first motorized vehicle of his own. Please help me!
-Scott Johnson

Answer
Hi Scott,

It sounds like you could classify this as 'Engine Runs Poorly' since a 'no-start' condition would have the engine not even turning over.

I would be checking to see if the fuel mixture is running too rich, or too lean?
Does the carburetor need to be adjusted?  Is the air filter plugged up?

Is there any vacuum leaks anywhere?  Check for a leaky gasket.  IS THE FUEL  TANK VENT  & FUEL TANK SCREEN PLUGGED UP?  

Do you need to check the combustion chamber for excessive amounts of carbon?  Possibly is the flywheel loose?  Is the Spark plug fouled?  Is the plug possibly not gapped properly?  Could you find any valve springs weak?  Is the valve clearance properly set?

These are the questions I'd be asking, and looking for the answers in these areas.

I hope this helps you out.
Good luck,

Howard B.