EC 300 cylinder pressure
 

Anyone know what the minimum cylinder compression pressure in psi for an ec 300 is, going to measure it next week after looking into exhaust port there is small amount black/brown build up 10mm below bottom ring, is this normal?

Thanks in advance!

My 2008 250ec runs about 190psi. Build up below the bottom ring seems strange. A little oil wouldn't be awful, but if it sticks the ring, that's bad.

It starts and runs ok, but dont know if this build up is normal :confused:

I think engine compression is something better monitored rather than just take a reading when something seems wrong. I don't know how people get these high readings, my gauge must be off or something, because I get around 125 in both my EC250 and 300. Aren't these pistons like 10:1, 14 psi atmospheric pressure, so the max it could be is 140? I never understood that.

I thought these bikes were 14:1.
Anyway, to get a consistent reading, put a couple drops of oil down the sparking plug hole.

The 300 I had before consistently read 180-185psi. My 200 before that read similarly.

Ron @ RB Designs did the squish band mod on my 250. He requested that I perform a compression test before sending him the head. He was not surprised by the reading.

Bottom line, if you get consistent readings you're probably fine, as long as it runs fine.

I am not sure what a GG is, but I looked up a Honda CR250 2t and it was at 8.5:1. I never understood these high compressions.

Compression test pressure is measured by kicking the bike fast with the throttle open until the reading tops out and stops rising. A good 250 will be around 185 - 200, 300s a bit less, more like 175 - 185. You have to kick fast enough to make normal leakage through the rings and in the case of the 300 the bleed hole not a factor. Compression ratio is the volume of the cylinder from the top of the exhaust port plus the volume of the head divided by the volume of the head. No way 14:1, usually around 7-8:1.

So if its 8:1 and atmospheric pressure is 14 psi, how do you get 185 psi? I am not saying you don't, just wonder how. Like I said, my gauge may be off, but I am getting 125 in both my bikes and 90 in my boat (which is what the shop had marked on the heads also). I know in multi cylinder motors they say if the pressures are all close you should be OK

compression
 

My 300s around 180 most of the bikes ive owned have been 170-200
If you really had 125 psi your bikes would likely not even start. ive had bikes needing rings test at 150 first symptom was hard starting.you have to make sure throttles wfo and you have a good seal on the connection toi the plug hole ,is there an oring? I've only used oil in the cylinders to check rings if you have low compression .test dry first then if readings low add some oil if it goes up alot rings are suspect.
Dont know anything about boats...............

Yes, good advice, check your gauge hose and seal, also the check valve in the gauge could be leaking bleeding the pressure off. 2stroke outboards generally have less compression.

Your doing the math basically right, but the concept and method of the test is not the same. Think of it as "dead heading" an air compressor for a peak pressure reading at equilibrium, that will determine efficiency. To make your idea into a valid test would require just one rapid cycle of the piston. Also, you would have to take the volume of the hose and gauge into consideration, both difficult to acheive and why this method not practical.