120 Track Driver

I have several 120 cc snowmobiles that we use for racing and playing.  These are small childrens machines powered with "lawn mower" type engines.  All of the major manufacturers sell these machines and they are all put together with very similar components.  I have a few older posts about them here, here and here.

These machines use a 12" wide track that is normally driven with involute drivers that engage the track on the rubber "lugs" on the inside of the track.  After a lot of hard use the track and drivers on our Skidoo Mini Z wore out.  In this photo you can see the lugs sticking up on the right side of the track, just outboard of the clips.  The left side of the track is supposed to have lugs also, but they have all been ripped off.  This track is interesting because there is no windows in the track.  The clips are attached to the track with no open holes.

This photo shows a Polaris track with nice tall lugs next to the clips.  You can also see that there is a "window" between each clip.

The track in the Skidoo was obviously junk and the drivers on the drive shaft were pretty worn out.  In the above photo you can see the old drive shaft on the right side (I cut it in half to make it easier to get out of the old bearings.)  As you can see the teeth on the black plastic drivers are rounded off.

The drive shaft on the left side is a Polaris shaft with new "anti ratchet" type drivers.  These drivers are designed to drive the track be engaging the windows on the track and pushing on the clips.  This set has 5 teeth rather than the original 4, this will change our gear ratio quite a bit, making the machine faster on the top end but given up some low end torque.

I ran out of photos, but the new Polaris track and drive shaft fit right into the Skidoo with no problems.

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.

Polaris 120 Carb Cleaning

In my last post I talked about drive chains being the number one problem with 120 or mini snowmobiles.  A less common problem is carburetor troubles.  But if you put enough water and dirt in the gas tank it will eventually give you trouble.

The carburetor on this Polaris machine is located on the back side of the engine.  The carb is slides over two studs that are threaded into the engine.  To remove it you need to pop the cover off the plastic airbox and remove two nuts.  The orange colored arrows show the location of the studs. 

Here is the carburetor on the work bench. 

When I pulled the float bowel off the bottom it was full of water, rust, and dirt.

Here are all the important parts.  The blue arrow points to the pin that holds the float in.  It should slid right out to remove the float, sometimes you may have to tap on it

The Green arrow points to the float needle.  On most carbs this is a separate part, but here it is connected to the float.  This needle controls the flow of gas into the carb.  If you have a machine that lets the gas run through the carb and flood the engine your problem is right here.  Occasionally something will get stuck by the tip of the needle and prevent the needle from shutting off the flow of gas.  On my old Polaris 440 I had that problem and when I took the carb apart there was a 12" long piece of musk ox hair stuck there!

The red arrow points to the main jet.  If you have a machine that seems like it does not have enough gas/ only runs with the choke on then the main jet may be clogged.  The jet is made from soft brass, make sure you have a screw driver that fits well if you are going to take it out.

This is what it looks like all apart..  Once the needle/float and the jets are removed I simply scrub it out with a little carb cleaner and blast it clean with compressed air.  Be careful putting everything back together, it is all soft brass and aluminum.

Mini Snowmobile Chain Repair

I recently had a Polaris 120 in the shop.  This is a mini snowmobile designed for kids.  It uses a 120 cc "industrial" engine.  This is a "Briggs and Stratton" type engine like you would see on a lawn mower or other outdoor power equipment.  Arctic Cat and Skidoo also make 120 cc sleds and they are all put together the same way.  There is a simple centrifugal clutch and a chain drive to the track.  The chain drive is the biggest source of trouble on these machines, it needs to be lubricated and tensioned regularly. 

When installing the chain a master link is often used.  They come in a few different styles, but they all work the same.  There is a chain link with a removable side plate and some type of clip to hold it together.  These are quick and easy to install, but are also prone to failing.  Over the years I have had several of them come off on different vehicles.   

A more secure way to connect the chain is with a chain rivet tool.  This tool is used to press the pins in and out of a regular link in the chain link.  It takes a little longer to use than a master link, but will produce a more secure chain.