So i have a 2008 polaris sportsman 400 ho. I did a compression test and it came up with 70. Is that low?

 

Yes, that is low. Most likely cause is that the valve lash has never been adjusted. 2nd: bad air filtration.

 

Actually, that isn't considered low on that engine. Spec is 50 - 90 PSI.

There is an automatic compression release in the cam so it shows low compressing readings during cranking.

 

Thanks for that. I didn't know if the 400 H.O. had that too. Other than displacement is it basically the same engine as the 500 H.O.? They had one engine that I think was the same as the 500 except for the bore, but I can't keep track of them all.

 

Yeah, if that Polaris has that feature on the exhaust cam, you will indeed test low. My error.

 

They sure look identical but so did the 425. Not sure how exact they are but I'd have to say they are pretty damn close!!! I never specd them out that close but Id say you are right.


No worries. Everyone is here trying to help. All is good!

With all this being said, a cylinder leakage test is the best way to check a 4 stroke. It tells you a lot more info. Problem with that is a lot of people dont own a compression gauge let alone a cylinder leakage tester but it is the better test.

 

True. I have a cylinder leakdown tester, but I rarely use it. If one does not want to buy one, he/she can make an
adapter to pressurize the cylinder with compressor air and listen for leaks at the intake, exhaust, and crankcase. Of
course the cyl has to be at TDC on the compression stroke, and constrained from moving. I don't know if the Polaris
auto-DV holds the Exh valve open when engine is NOT turning.

I have 4 compression gauges, and they all give different readings.

Related to this auto-decompression feature on newer quads: most, if not all, of the current crop of small Honda
and Honda-clone OHV small engines have this feature. As the valve lash goes out-of-spec, the DV stops working,
and they get super-hard to pull-start. Moral: check and set valve clearances!

 

I agree to that but I also think that's not the best way to do it. Reason is, is my cylinder leakage tester applies like 5 psi to the motor. it's a specific motorcycle/atv leakage tester. On Polaris I usually put a wrench or socket on the drive clutch bolt to hold the motor at TDC. Well I bought an automobile leakage tester that put out like 25 psi. When I tried using it on a Polaris it turned the motor enough I thought the clutch bolt may actually break . The motor turned over a considerable amount and tightened the bolt. I didn't like it. So that being said, if someone rigs up a hose to try this test to shop compressor air pressue(my thoughts 125 psi minimum) how quick could issues take place due to the high psi. Just my thoughts, never tried it personally that way.

 

94HARLEY: you can use any pressure from the air compressor that you want, tho leaky rings or valves will be more
apparent with higher pressure, say at least 20psi. If you hold the engine at TDC, the torque on the crankshaft is minimal.

I do not use my leak-down test rig for single cylinder engines. It is made by Moroso, and I think it calls for 100psi from the
compressor, IIRC. I have used it mostly with 4-valve V-8s where cam overlap was significant. That meant that you could get decent
compression #s with a compression tester, and still fail the leak-down test.

I realize that many ATVs have centrifugal clutches and it may be difficult to keep the engine from turning. This is when you
want a manual trans and someone to hold the brakes.