300 compression

[stay_upright]

What compression should I have after a rebuild?

Previously my bike has always been at 150psi which seems very low to me though perhaps the gassers have very low compression?

 

[Eric K]

My '06 EC300 had 162 psi compression 4 hours of riding after brand new.
After the RB Designs head mod and rebuild it had 185 psi compression before running the engine. After 12 hours of riding the compression measured 205 psi.

As a note, my '06 EC300 has a 0.14 inch diameter compression release hole in the cylinder wall above the exhaust port. It is much easier to kick start than my '01 XC300 which does not have a compression release hole in the cylinder.

 

[stay_upright]

Thanks for the info - I may aim for 185 via the basegaskets when rebuilding.

I wonder if my 05 has that compression release? How does it work, what stops the air leaking out during normal running? maybe this affected my compression tests...

How did you measure compression if it has a compression release?

 

[Eric K]

The compression release has neglible affect on normal operation. It is primarily to ease starting. You can tell if you have one by looking at the cylinder wall above the exhaust port - when the head is off.

Do a compression test as you normally would - hold the throttle open and kick repeatedly until the pressure gage stops rising.

 

[stay_upright]

After 12 hours of riding the compression measured 205 psi.

.

Why woud the compression go up after riding? Do you think it was new rings adjusting to a worn cylinder?

 

[Eric K]

Presumably the rings seating in.

 

[stay_upright]

Rebuilt my 05 300 - exhaust and powervalve covers are not on yet but tried 0.3mm worth of gaskets - compression only comes out at 150psi with new piston and 0.4 to 0.5mm less gaskets not sure why was expecting compression to be much higher - maybe the compression release is bleeding it off but there again Erik you had much higher values with an 06 which sound like it had teh compression release.

PS any idea of the level of compression you would get from oen good kick e.g. cold start at a race? I get 75 psi... not sure if that's good or not?

 

[Skidad]

Button everything up and run the bike a bit for some heat cycles and then do the test with the bike warm/hot.

Compression test makes no sense doing it cold as that's not how the bike will be running and using the motor. It may give you an idea of a # but get it hot then do the test. Heck, do it both ways just to see the difference if it makes you feel better.

 

[stay_upright]

Yeah thanks I think I will have to try that next...

 

[iancp5]

I've also heard that the reading will vary according to your technique for kicking and how accurate your guage is. That the important thing is to take a baseline with a good piston/rings/ barrel.

 

[stay_upright]

Replaced the reeds with new Boyesen C/F ones (expensive I thought at £60) and compression was 160 so a slight increase so i'll rebuild with the 0.3mm base gaskets based on these readings.

 

[jefgil123]

What is the "squish" (piston crown to combustion chamber measurerment?)

(See tech articals for how to do this.)

Mine had 2.1mm / .080" so I sent it to RB Designs and they milled it down and now it has 1.3 mm / .051". Has higher compression and more grunt.

(my compression reading are relatively low because I'm at 6,000 ft.)

 

[stay_upright]

I think it was around 1.5 to 1.6 with the 0.3mm base gaskets

 

[jefgil123]

That's not too bad. Probably a neglidgible increase if you milled it a bit....