Why is it…..

[viclioce]

That you measure an existing piston, rings & cylinder and they all measure within specs, but you put a compression tester on and you only get 85 psi???? Victor

[ossa95d]

Vic,
Among other things it could be a glazed cylinder wall. It might need a light honing to restore the hashmarks and allow the rings to work with the oil provide a seal. Good luck.

[Bullfrog]

A bit of oil in the cylinder AND hold the throttle wide open while kicking thru . . . (?)

[Bullfrog]

PS: Got the correct head and correct piston . . . ?

[taber hodaka]

Is it a L ring, has the cylinder been ported, did you lap the head? has it ran at all yet, I like my ring end gap on the tight side. I have one tester that is bad - does not read correctly. --- Clarence

[Joe Ormonde]

The first thing to do is make sure your Compression Tester is reading correctly. Check the Ring or Rings in the Cylinder by holding the Cylinder up to a light . If you see any light in between the ring and Cylinder either the cylinder is out of round or the Ring is defective. I recently had this problem with a fresh bored Cylinder. It is a fully Ported +.080 with a Webco piston. The rings were out of round. BUT I had a set of Yamaha rings that were Perfect! The Rings that fit the Webco 52mm Piston are: Yamaha 335-11611-00 (52mm STD). Joe

[viclioce]

Thanks folks for your answers. The following replies are offered…

Ed. Yes correct piston & head combination. It’s an Ace 100 with an Ace head and an Art piston with NPR rings.

Testing was done with numerous kicks with throttle wide open. Multiple tries never went above 85-90 psi.

So I’ll be pulling the cylinder and piston and inspecting with a light from underneath to look for irregularities. These were brand new rings. Piston was not brand new but did have a very tight .004 clearance with the cylinder and no rings. Clearance was measured with the rings just above the exhaust port and not at the very top of the cylinder. Victor

[Bullfrog]

I'd check compression on a different engine as a way to check the compression gauge, before tearing things down.

[viclioce]

OK! Good idea, Ed! Victor

[dirty_rat]

Have you tried to start it? Even with low compression you can often start one (might have to push start it). Then, once it has run for a while (giving the rings time to seat in), try a compression test (on cold engine).

[viclioce]

It will not start. Numerous attempts. Probably more than 50 kicks, minimum. Victor

[Bullfrog]

Didja ever find out what was going on . . . ?
Ed

[viclioce]

Not yet. Still working on raffle bike.