Sunday, March 4, 2012

Polaris 120 Snowmobile Rebuild

I have an older Polaris 120 snowmobile that I am rebuilding.  The engine had been neglected, so I decided to tear it down and inspect everything. While I had it apart I also removed the governor.  This is the first step to making these machines run faster.

With no governor to control engine speed the motor will rev up until the valves float.  To increase the RPM that this happens at I decided to install a set of "high performance" springs from Recreational Motorsports.  These springs are little bit longer and stiffer than the stock springs.  After the governor elimination this is the simplest way to make more power. The spring on the right is the new one, the original spring is to the left.

Whenever you are assembling internal engine components be sure to coat all the moving surfaces with some kind of assembly lube.  I normally use Lubriplate.

On these tiny motors I use my simple homemade valve spring tool.  You can see this tool and a manufactured one in these old posts here and here.

These small motors motors have a splash lube oil system.  There is no oil pump to distribute the oil around the engine.  The bottom of the connecting rod cap has a long thin "dipper" that sticks down into the oil reservoir.  As the crank spins this dipper splashes oil up onto the cam shaft and around the crankcase. 

Make sure the timing marks are lined up when the cam shaft is installed.  The small punch marks on the edge of the gears are the timing marks.

This photo shows the main parts of the governor assembly.  The governor can be bypassed by messing around with the springs on the throttle linkage, but this puts a higher load on this plastic gear that will eventually lead to it breaking and making a big mess.  If the governor is not being used it is best to remove this gear.  This photo also shows the governor control rod.  I cut it in half and remove the part that goes inside the crankcase.

When installing the spark coil it must be mounted as close as possible to the flywheel.  I have found that the easiest way to do this is place a piece of paper between the coil and the flywheel when tightening the bolts.  When you turn the flywheel you are left with a paper thin space between the parts.

This covers some of the high lights of small engine rebuilding.  When I get the rest of the machine together I will have more photos of chassis work.

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