Small Engines (Lawn Mowers, etc.): Surging Tecumseh, throttle shaft, head gasket

Question
Just bought a minibike with a 3.5 HP Tecumseh engine(1970). The engine surges and recedes without the throttle being applied. Seems to do this in a "cycle".  Any ideas? Thanks.

Answer
This is a common problem on older engines but unfortunately it is very difficult to troubleshoot.

Surging is almost always caused by the carburetor.   The problem with older engines is that many small parts on the carb wear adding to the problem.  

To start with, I would check the intake gasket and head gasket for leaks.  The next place to check is the carb; dis-assemble and check for worn parts, especially the throttle shaft.  If the throttle shaft isn't wore too bad and the surging continues then I would check the engine RPM with a single cylinder tachometer.  Adjust the engine speed as necessary.  If the surging continues the final part I would replace is the governor spring.

If you don't have all the special tools to check the parts you may want to start by replacing the governor spring.  governor springs are fairly cheap and you can rule out a defective spring and concentrate on other areas like the carb.

Let me know if this helps.
Eric