How to Compression Test a Polaris ATV

Low compression is a sign of serious issues within your ATV's engine. A lack of performance in an ATV can be caused by several issues, and a compression test helps diagnose problems with the engine. 

Watch the video above and follow the steps below to learn how to do a compression test on a Polaris Sportsman 850 XP.

PRO TIP: Check out our Polaris Sportsman XP 850 cylinder head parts diagram for additional reference.

Tools Needed - Polaris Sportsman ATV Compression Test


BikeMaster cylinder compression tester

Cruztools torx bit socket set


Preparing ATV for a Compression Test

Step 1. Run the engine for a few minutes to get it warm.

Step 2. Remove the seat by pulling straight up on the back of the seat so it'll pop loose. 

Polaris Sportsman 850 compression test

Step 3. Use a T27 from a Torx bit socket set to remove the Torx bolts holding the right-hand side panel in place over the engine. 

Polaris ATV compression testing

Step 4. Locate the three plastic Tuflok rivets near the bottom of the panel and remove them with a pair of pliers. 

Polaris Sportsman ATV Tuflok rivets

Step 5. Remove the outer cover on the top of the shifter head with a flat-blade screwdriver. Next, loosen and remove the Torx bolt inside to take off the head of the shifter. 

Polaris Sportsman ATV shifter head

Step 6. Lift the plastic panel up and over the shifter, exposing the plug wires and spark plugs on both the front and rear cylinder. Remove the plug wires. 

Polaris ATV spark plug wires

Step 7. Use a thin-walled 14mm socket to loosen the spark plugs on both cylinders, then use a magnet to lift them out. 

Polaris Sportsman ATV spark plug removal

Performing ATV Compression Test

Step 1. Hand tighten the compression tester adapter in place on the front cylinder, then connect the engine compression tester

ATV compression tester adapter

Step 2. Turn the engine over six or seven times with the throttle held open. 

Polaris Sportsman 850 ATV compression testing

PRO TIP: On the Polaris Sportsman, you have to hold the brake as well, so get someone to give you a hand on the other side.

Step 3. Read the gauge to check the compression. 

ATV engine compression test reading

NOTE: The ideal measurements for the test are that each cylinder should be within 10 percent of each other when measuring compression. The overall compression should be between 210 and 250 psi per cylinder.

Step 4. Remove the compression tester and adapter from the front spark plug opening, and screw the adapter in the rear spark plug opening. Connect the compression tester.

Step 5. Turn the engine over six or seven times with the throttle open with someone holding the brake to get the reading for the rear cylinder.



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