Small Engines (Lawn Mowers, etc.): Two-cycle rpm variation

Question
Hi Eric,

I have an old Jacobsen 420  single stage snowblower with a two cycle engine and a Walbro carburetor.  It has no throttle control so it runs at the working speed at start up.  At start up the rpm seems normal, but sometimes during use, the rpm climbs way too high when not under a loaded condition.  If I immediately put the blower to work again when the rpm rises sharply, it will settle right back down.  At those times when the engine starts to "scream", as I'm backing it up, I have found that doing controlled choking brings the rpm down and if then sitting idle a bit it will calm down and run at normal rpm. When used at a later date, the engine may just operate normally from beginning to end.  The carb has only one adjustment screw and the manual says to adjust it when the engine is off until it seats lightly and then turn it one half turn counterclockwise.  I have done that.  The carb was supposed to have been recently rebuilt, but I did not perform the work.  Do you have any ideas as to what causes the sharp rise in rpm?  What can be done to limit that extreme speed?  Thank you for your help.  I appreciate it very much.

Duane

Answer
There could be an air leak causing the engine to lean out but this in not too common.

I found the parts diagram and your machine should have a Walbro carb.  The parts list part number for the carb says the parts are obsolete...it is wrong.

The carb you have is a Walbro SDC series (I believe) but you would have to check for sure.  Those carbs have an internal governor.  It could be malfunctioning but I have never seen one malfunction.  

I would check the carb butterfly shaft to make sure it is not wore and sticking...I have seen some wear cause the problem you described as well as gunk inside the shaft holes that caused the butterfly to stick.  

Are there any linkages connected to the carb other than the fuel and primer hose?

Let me know.
Eric