Compression Check

[frankfast]

Checking the compression of my friends 250 Mark 3 the tester measured 80lbs. We're assuming it's the stock piston and don't know how many miles are on the bike. Does that sound satisfactory or is that too low? Can't find any information.

[Jon Pegler]

Sounds low to me.
Good would probably be 140-160lb
Are you sure you're turning the motor over fast enough?

[frankfast]

We kick it about six or seven times until the needle on the tester stops going higher.

Bike hasn't run in at least six years.

[ranton_rambler]

There’s definitely a technique to measuring it right. I found I got about 90psi initially , but if you take it just past TDC and then give it a good go, the piston speed is higher and should give a better reading.

[George]

Don't forget to hold throttle wide open

[frankfast]

Good point. We forgot.

[themoudie]

Aye frankfast,

I would suggest an egg cup of ordinary mineral oil or Redex upper cylinder lubricant (UCL) down through the inlet and the spark plug hole to lubricate and help seal the rings in the bore and their lands. After letting it sit a wee while and then turning the engine over at least 10 to 20 times, use a rag to catch the surplus oil and then set the compression tester up.

Even after sitting for a couple of months, let alone "at least six years"! All standing engines loose compression, as the oil that was on the walls of the cylinder and the ring lands drains away. What was usually a 2 or 3 kick starting Honda XBR or Yamaha SRX single can take at least 25 to 50 kicks to get the compression up sufficiently for the engine to start and run, if you don't put a wee drop of UCL down the plug hole first.

Once lubricated, bring the piston up to TDC and give it a swinging kick with a full follow through and either the throttle wide open or remove the carb altogether (It will need cleaning anyway!). I would agree with Jon that you would hope to see between 140 and 160 psi. Anything above 120psi should run.

If in doubt, measure your piston ring end gaps, with the individual rings, one at a time, just below the top of the cylinder bore, below the wear ridge at the top of the liner and square in the bore. You may get away with either needing nothing further, a light hone with 800 grit wet and dry paper, with plenty of paraffin. Alternatively, a machine hone and new set of rings, or a rebore maybe required. All of these processes will require the cylinder be thoroughly washed in brake cleaner and then boiling water, with Fairy Liquid, followed by a thorough sluicing with cold water, a quick dry and then a coat of mineral engine oil or ACF50. An initial bit of 'flash' rust (Brembo cast iron discs) won't do any harm, providing the oil is applied sooner rather than later.

Good health, Bill