800 cfi 85-90 psi???

[robstar1337]

Hello,

My assault 2009 just got 85-90 psi on both cylinders, o-rings in head? they did not seemed to be 100% good, it got upgraded pistons and head, so I guess around 140psi is the right pressure?

Do you think o-rings in the head can be my problem? I have not pulled of the cylinders, but what I can see its no damage to those..

thanks for answers, and excuse my english

 

[Goinboardin]

Has it been run yet? If the rings haven't seated, the motor is not warm, or if there is no oil it would read low. Are they even? Compression tests can vary alot based on altitude and the gauge itself. I had a piston port 500cc read 85psi with my crappy gauge, there was nothing wrong with the motor, I was just at high altitude and had a cheap gauge.

 

[robstar1337]

I did test on my m800 and it showed me 130-140 psi, just 1 min after I tested the assault to verify the gauge..

EDIT: Yes they are even..

 

[Snowbird11]

that sounds pretty low, even with the updates. will the engine run? how hard do you have to pull the pull start on the M8 compared to the polaris to turn over the pistons?

if your values between the two cylinders are less than 5% difference then you may not have a problem. however, given the track record of that motor, i would be concerned enough to pull it down and have a look. while you're in there you can put the fix kit in or something that will keep it from grenading in the future.

 

[diamonddave]

Ok. Is this a motor you have just rebuilt? If so, the readings can be lower than normal as like previously stated, the rings may not have had a chance to seal yet. Compression tests of rebuilt engines is not reliable data.

Elevation has a drastic effect on compression readings. As you go higher in the elevation, the compression becomes less (goes down)

Now, a 2008-2010 CFI-4 800 with the 2010 updated head which can be identified by the extra boss of material just below the spark plug hole will only get a maximum of 120 lbs. of compression at sea-level on a motor that has been broken in. I typically will see 115 lbs. at 300 feet above sea-level.

 

[robstar1337]

thanks for answers, its almost at sea level and its not rebuild, it have about 3000km. you think it should fire up with 85-90 psi? it won't start...

have been hard to start and bad idling and now it wont start at all..

 

[rocket]

If the motor is "intact" internally (no broken parts), I believe it should at least run with that much compression. However, I agree completely that 85-90psi is too low to be correct. I will assume you have verified spark is present, as well as fuel when you're trying to start it. As far as "What might be wrong?", the o-rings in the head basically keep coolant out of the combustion chamber. If your motor isn't flooded, then the o-rings aren't the problem. I'd suspect the pistons have been "smeared" and the rings are partially or completely stuck, thus the low compression.

 

[PowderMiner]

My stock pistons/rings are only lasting me about 1800 miles. no cylinder wear. lots of people rebuild their topend every season. cost of doing business

 

[Snowbird11]

thanks for answers, its almost at sea level and its not rebuild, it have about 3000km. you think it should fire up with 85-90 psi? it won't start...

have been hard to start and bad idling and now it wont start at all..

you might benefit from a new top end and a tune up. use new gaskets while you're in there.

have you changed the spark plugs with new ones yet? how about checking the the Throttle Position Sensor (TPS)? it certainly can't hurt to tune it up.

 

[donbrown]

thanks for answers, its almost at sea level and its not rebuild, it have about 3000km. you think it should fire up with 85-90 psi? it won't start...

have been hard to start and bad idling and now it wont start at all..

It won't start? Did it run yesterday? Last month? April?

It should fire up with 85-90 psi.

The big tell tell sign is the pressure difference between cylinders.

You have spark?

Put some oil/gas mix in there and see if ity fires up.

You english is great but what do I know I'm in Los Angeles

 

[Kraven]

thanks for answers, its almost at sea level and its not rebuild, it have about 3000km. you think it should fire up with 85-90 psi? it won't start...

have been hard to start and bad idling and now it wont start at all..

Agreed on the compression being too low, but as for the failure to start issue;

How old is the fuel?

If it's questionable, pump out ALL of the old fuel and put into your truck.

Use fresh fuel into the sled and see if it starts

 

[BB454]

Squirt some oil in on top of the piston through the spark plug hole and recheck compression. I think it is too low to run. If it comes up then it's rings. Time for pistons and rings.
Check the reeds too.
Ever changed the fuel filter?