95 sportsman 400l compression - Polaris ATV Forum
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post #1 of 38 (permalink) Old 09-17-2019, 10:28 PM Thread Starter
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95 sportsman 400l compression

Finally got some time to really dive into an atv I bought a few years back. The atv seems to have ran pretty good. I have not drove it in about a year. I had to replace the center tightener, and tie rod ends. While I had the plastics off, I decided I would check everything out. I did a compression test on the engine, and I am getting 105 psi. Test was performed with the throttle wide open, and the engine cold. (51 degrees outside tonight). I have the gas tank and battery removed so i cant warm it up to test compression. I put about 2 tsp of oil in the spark hole and retested. Reading 107-108.

On a cold engine, how are these numbers looking? When the engine warms up does the psi increase 5, 10 or more psi?

Like I stated, it seemed to run pretty good, but I just felt like I could never get the carb tuned right the engine seemed to always give me the impression the carb mixture was not right, perhaps this could be why.
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post #2 of 38 (permalink) Old 09-18-2019, 06:48 AM
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Compression is low if you have faith in your compression gauge - if you checked other similar engines what kind of reading do you get?

I have 3 compression gauges and occasionally I will check an engine with all three to verify the readings. They never give the exact same reading on the same engine, but they are within 5 psi so I consider all three to be accurate.

Compression on a warm engine is typically lower than a cold engine, but sometimes unknown problems within the engine can result in a higher reading when warm. If you are performance oriented, you check the compression both cold and warm and record the readings to establish a record to compare subsequent readings with.

I like to see the compression about 120 to 150 on a cold two stroke engine

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post #3 of 38 (permalink) Old 09-18-2019, 07:13 AM
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How strong was the battery while testing? Cranking speed and the accuracy of your gauge make a difference too.
Sounds a little low though. IIRC, about 120-130 is standard for that engine when fresh. Compression will show a little higher warm but not a lot. While you already have it apart it would be a good time to freshen it up if you're going to though. I'd probably set the tank back on and get it warmed up to test and compare the results and go from there. If it comes up 10-12 psi you're good for a while.
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post #4 of 38 (permalink) Old 09-18-2019, 08:31 AM Thread Starter
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I only have the 1 compression tester. It has not been used too much, and always stored in it's hard case. From what I recall it was a pretty expensive unit. When I go home for lunch, i'll get the name of the tester.

Anyways, the battery is not even installed on the ATV. All plastics, racks, tanks have all been taken off. I hooked up an everstart portable jumper to temp test the compression. when I go home for lunch, i'll hook the battery back up for a test and post the results of that test. I don't mind replacing the piston while the ATV is apart, the part that concerns me is if I need to get the cylinder bored. I live in a small town in WY, and highly doubt there is anyone local that does that type of work. I can use my flex honer to hone the cylinder, but I really hope it does not need to be bored out.
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post #5 of 38 (permalink) Old 09-18-2019, 11:10 AM
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I'll defer to latebird's assessment OP. Although it appears we pretty much agree for the most part, he has a lot more years of experience and knowledge than I do.
If you're wanting to put the machine back good and strong it will need freshened up.

Good luck and keep us updated.
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post #6 of 38 (permalink) Old 09-18-2019, 11:55 AM
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If it needs bored and you can't find a local shop to send it to, I can bore it - if you are unsure about if it needs bored and can't find a shop nearby to verify the condition of the cylinder, I can do that too - down side is I'm in Illinois, but I have bored a couple of cylinder for others on this forum. Let us know what you determine - you may only need to replace the rings to get the compression back and better compression will make it start easier and it will have more power too.

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post #7 of 38 (permalink) Old 09-18-2019, 01:02 PM
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There ya go Op.
You got any tools to measure the jug with or a machine shop nearby that could measure it for you?
If its not bad a new set of rings and gaskets and you'll be good to go.
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post #8 of 38 (permalink) Old 09-18-2019, 01:35 PM Thread Starter
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There is an auto repair shop, so when I remove the head, I'll run it down to them and see if they can measure the jug and see if it needs to be bored out. I am not sure if they have the machines to do a bore. This area is heavy on farming, so I imagine there is someone that these people take their tractors and stuff to when it needs to be bored. I have only lived here just over a year, so I am not sure what place does that stuff, or if they charge crazy prices to do it.

I'll pull it tonight, and run it down to the shop tomorrow and let y'all know what they say.
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post #9 of 38 (permalink) Old 09-18-2019, 08:46 PM Thread Starter
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Alright.. I took the top end off the motor to get an idea of what it looks like in there. I quickly realized... it looks like a piston and a cylinder...

The top of the piston is black. Not really "oily " more like carbon, or black 120 grit sandpaper. The cylinder walls when running my finger nail along the walls, feels like glass. I see some "scratches" in the pictures, but I could not feel any on my fingernail.

When i removed the jug, the gasket between the jug and the bottom end tore away1/4th. Not sure if it was weak and ripped, or weather it was torn already. The top gasket looks fine. Feels like it is made out of black plastic or a thin tin material. I am not sure if this is the oem piston or not.

What do you think. How do the walls look? Do you think a bore is needed, or just a hone and new rings?
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post #10 of 38 (permalink) Old 09-18-2019, 08:48 PM Thread Starter
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