compression test results

[SIKEWARD]

what kind of test results should I see from a compression test. A reliable friend says 170 psi per cyl. The service manual says 199 to 227. Mine is right around 125 psi.

 

[jp111]

Are you holding the throttle open while doing the test? I know that can create a big variable. If you have access to a leak down tester that is more accurate than a compression gauge.
I have seen on small 4 cylinder car motors burn the head gasket between the cylinders and it reduce the overall reading as it was blowing the compresion in the other cylinders.

 

[Beenba]

Make sure all the plugs are out when testing each cylinder, and yes, WOT.

 

[cannondale27]

Due to the decomp even a small valve clearance variance can cause alot different compression reading.Leakdown is a better test just be sure motor is past point decomp is activated against exhaust valve.

 

[SIKEWARD]

I don't know a thing about doing a leak down test or what tools are needed. As for how I performed a compression test I did it step by step as the service manual stated. It didn't say anything about wot. It states its results are at 400 rpms. Which I am assuming is the starters rpms. Why would you do this @ wot? You do a compression test with the motor off correct? Sorry to sound dumb on this topic.

 

[kilometers]

You do it at wot so the piston can draw the full amount of unrestricted air. If the piston struggles to draw enough air your readings will be off. theirs potential to loose some due to vacuum lines, and the vacuum between the butterfly/throttle valve etc.

 

[Supa Dexta]

And yes, thats engine off, throttle pulled wide open..

 

[grizztracks]

From the 08 Nytro manual:

 

[SIKEWARD]

guess i missed that step. Thanks grizz. What kind of numbers should i expect to see from this?

 

[sorenson1610]

Why are u doing this? Having problems?

 

[grizztracks]

guess i missed that step. Thanks grizz. What kind of numbers should i expect to see from this?

You can redo the compression test as described by Yamaha but if you get numbers that are out of spec you may want to do a leak down test. I prefer the leak down test over the compression test also. It requires compressed air and a leak down tool (I made my own). I'm not sure if Autozone has the tool to lend but you could give them a call. Because you overheated the engine there's a good possibility the cylinders are scored and the rings are damaged.

 

[jp111]

If you attempt to do a leak down just besure to keep the motor from rolling over when you supply it with air.
I am not to familiar with doing it on over head cam stuff as in my line of work we only have the vrod and have to be just pass the decompression valve on the cam. We also check the motor at 3 positions top, middle, and bottom. This helps tell us how out of round a cylinder is.
Grizz has a good idea check with autozone or napa as they should have one on a loaner deal.

 

[SIKEWARD]

grizz good idea on the autozone loaner. I will look into that today. Can you verify what kind of pressure I should be getting on the compression test?

 

[yox185]

Can you verify what kind of pressure I should be getting on the compression test?

It's right on the instructions page he posted.

 

[srx901]

what kind of test results should I see from a compression test. A reliable friend says 170 psi per cyl. The service manual says 199 to 227. Mine is right around 125 psi.

how did you make out with your nytro I am having low idle when warm issues