Right, my engine has had low compression (AGAIN). Took it apart expecting to see another scored cylinder, but the cylinder is perfectly fine. I'm very confused at this point. So I decided to have a close look at the valves, visually they seem to be ok, but I've noticed on the valve seat where it meets the valve is covered in soot and general muck, shouldn't the mating surface be clean?

 

I'm utterly stumped, the bore looks good, the valves look good, no signs of compression leaking past the head gasket. I'm utterly lost.

turn the cyl head upside down and pour petrol over the valves. If they are leaking, the fuel will seep through.

 

Are the rings within spec and can you see the hone marks in the cylinder wall?

3 minutes ago, drewpy said:

turn the cyl head upside down and pour petrol over the valves. If they are leaking, the fuel will seep through.

 

Are the rings within spec and can you see the hone marks in the cylinder wall?

Poured electrical contcate cleanmer into the head (a generic solvent) and it didn't leak past the valves, unless the cam was bumped up aganst it.

I can clearly see the hone marks, but those are only 5K only, I had a compression issuie a while back and decided to hone the cylinder and get a new piston and rings (standard size), solved the issue for about 1.5-2k then it came back.

Obviously if the valves don't leak it MUST be the cylinder, but I'm kind of amazed that the piston, rings and cylinder can all look poractically new and still be so worn. It's my own fauilt for not getting an oversize piston in the first place, I'll just get it bored out tomorrow and fit a .50 oversize piston and hope it fixes the issue. Not the valves, must be the cylinder, I suppose it can look new after the spruce up but still be out of spec. All I really did was put lipstick on a pig in hinde sight.

4 minutes ago, Variomatic said:

. All I really did was put lipstick on a pig in hind sight.

ha ha, oink oink

Just now, drewpy said:

ha ha, oink oink

I've taught myself a lesson with this one, do it wrong, do it twice, along with a load of other shit.

You cant polish a turd , but I think I read on here about someone rolling 'em in glitter ! That's a tough one mate , you'll sort it .

Yes, those surfaces should be clean!! Are the valve clearances in spec when the motor is assembled?  Your description of soot on the mating surfaces of the valves would suggest they are not closing properly when the engine is running.

If the valve is seating correclty, I wouldnt expect to see crap on it. Stems not bent is it? Guide ok?

On 27/09/2016 at 5:41 AM, anyoldiron said:

Yes, those surfaces should be clean!! Are the valve clearances in spec when the motor is assembled?  Your description of soot on the mating surfaces of the valves would suggest they are not closing properly when the engine is running.

 

Yep, the valve clerances are always within spec, they're adjusted every 2k or so, but for a while the exhaust valve did have a little preload on it meaning it didn't close (as in 10-15 miles of riding before it was fixed)

 

On 27/09/2016 at 1:19 PM, neversaydie said:

If the valve is seating correclty, I wouldnt expect to see crap on it. Stems not bent is it? Guide ok?

Well without taking the valves out of the head I can't tell for sure, but from a close visual inspection they seem to be opening and closing correctly, and every other comonent in the top end seems good so I can only presume the guides are ok.

Like I said I poured solvent into the head and it didn't leak past the valves so I presume they're sealing, but I would have thought that the mating surfaces would have been clean. I might have to try and get my hands on a valve spring compressor and take a closer look, check runout/clerances ect ect as well as lap them in.