79sf compression

[donebysunday]

is very high on the 2 SFs I have, Horse with no name is 180 on all, Old time rock n roll I just checked 1/180, 2/170, 3/160, 4/190 this is crazy yes/no ?

Also the tank cap was crusty and could hardly push air through, took it all apart and everything was clear. Cleaned up the exterior surface with brass tooth brush and air flows freely ?

Also when I got it home it was full of E10, wanted to change that out for non ethanol, the tank was empty, did not notice drippng, don't see signs of ever dripping ?

Thanks for any comments ! Done

[Radioguylogs]

Wierd

Seems wrong to have higher-than-spec compression.

Only reason I can think of is excess fluid (gas/) in the cylinders, but it would seem so unlikely that all 8 cylinders could have that. Besides, the not-under-test cylinders would blow out any fluid if the plugs were out.

[rdmcguy]

High compression numbers

I think those numbers are very high as well..as mentioned excess fluid in cylinders or HUGE carbon deposits?

[donebysunday]

Indeed, Horse (180 all) showed black sooty plugs, Old Time (160-190) the plugs were dark brown and a bit glossy.
Explanation it most likely will be simple.
I'll check the two 81s next the LH has been tested before at 150 all, SH has not been checked.
I've run Horse about 20 mi. since I checked the compression, I'll check it again.

[natemoen]

No, they really are not high, they are good! Lots of people have that as compression, especially if you check when the carbs are off. 190 seems to be about the top for compression, never heard anyone hit 200.

[donebysunday]

No, they really are not high, they are good! Lots of people have that as compression, especially if you check when the carbs are off. 190 seems to be about the top for compression, never heard anyone hit 200.

Thanks Nate !
I had thought 150 was perfect ? There would be a deference in the performance from a bike at 150 compared to 180/190, YES ?
Carbs were on but throttle held wide open. Don'e

[crazy steve]

No, they really are not high, they are good! Lots of people have that as compression, especially if you check when the carbs are off. 190 seems to be about the top for compression, never heard anyone hit 200.

Yep, I've gotten those numbers on pretty much every motor when tested with the plugs out and the carbs off. Yamaha's rather low 'factory' spec was probably to give them plenty of 'wiggle room' for warranty purposes...

And I have seen 200; one I checked came in at between 190 and 200 across the four cylinders and ran great... still on regular fuel.

[TADracer]

I have consistently had those kinds of numbers for years. Some of it may have to do with how much the head gasket compresses if you have ever changed it to an aftermarket gasket. Spark plug tip/insulator configuration can have a slight affect also. Then there is always that set of pistons/rings that are right in tolerance and let virtually no blow by through. If it runs well I wouldn't worry about it.