Small Engines (Lawn Mowers, etc.): 12 hp briggs timing, hp briggs, carbarator

Question
here my engine briggs 12 hp model#283707 type 1139-e1 ser#000405zd  ny son took this motor to his small engine class now he brought it home and it backfires through carbarator and kicks back is this a problem with timtng or did he cut valves to short they ground valves and teacher said they needed to cut valves for more clearance any advice thank you

Answer
ken
Do you know what the valves are cut at? Seems to me the teacher should know what he is doing,more clearance ??? for what this engine works fine on those valves. Sounds like they were trying to get more HP which is not worth the energy as the block cam and connecting rod won't handle it. The timing may be off if they took the whole engine apart and put it back incorrectly.Sounds like this teacher needs to go back to school and learn the correct disassembly of a small engine. He should not have let it out of the shop unless it was running correctly. How can you teach someone if you let things go out wrong.My Advise id to bring it back to the class and have the teacher fix it with the students so they know what is wrong and how to repair it right.