Small Engines (Lawn Mowers, etc.): Briggs V-Twin, internal combustion engines, internal combustion engine

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
Let's start off by first setting something straight. You have always heard a engine needs three things to run, 1 Fuel 2 spark 3 compression, this is the most misleading phrase in the field of internal combustion engines. It is what your old gray haired English teacher would maybe call, from my stand point, a incomplete sentence. For a fuel burning internal combustion engine to run it requires 3 things, 1 The correct amount of fuel, atomized to the correct ratio to burn efficiently to fire the piston back down the cylinder. 2 it needs a hot enough spark to fire the correct fuel air mixture (keep in mind the spark itself does not fire the fuel mixture, but the heat caused by the spark jumping the correct gap does). This is why a weak spark can not ignite a mixture, it is not producing enough heat. 3 The correct amount of compression is required to aid in the ignition of the fuel air mixture. Now with this in mind we have to determine what is lacing in your situation. Is the spark strong enough, is it a hot snapping blue spark? Is the carburetor supplying the right amount of fuel, and the correct mixture? Is the compression with in allowable range? These are the questions you have to answer. Has this engine been laid up for a period of time? Does the carb need to be rebuilt, due to being stored poorly, and the gas turned to gum inside it? Has the flywheel, and magneto began to rust causing a poor spark at the high tension lead to the spark plug? Just what is the compression is it with in range? Hope I helped. I tried to teach you in this answer, not just help you. Let me know.   Tom