Help need to interprete compression test results

XJ 550

Dry test

Cyl 1 165
Cyl 2 125
Cyl 3 160
Cyl 4 170

Wet Test

Cyl 1 300
Cyl 2 230
Cyl 3 260
Cyl 4 260

Do I read that as worn seals on Cyl 2?

Is there a good thread on next steps? ( I am guessing engine rebuild is going to be painful for a novice.......)

 

Was the dry test done on a warm engine? The dry readings are within spec IIRC.
#2 could just be a stuck ring or a valve out of spec.

 

Yes, on a warm engine......

Doesn't look great does it?

 

It actually looks good. Yamaha calls for 90psi as a minimum. Are your valves in spec?

 

Are valves within spec?

 

I thought the cylinder compression had to be within about 10 % of each other too?

I re-shimmed and everything was in spec but I might just check it all again to be sure to be sure........

 

Having cylinders within 10% of each other is part of the spec, but #2 could be low due to an out of spec valve or a stuck ring (I'm wagering on a ring). I don't see evidence of a rebuild being needed just yet. More diagnosis in needed.

 

ok, great. I have heard about leaving penetrating oil in the cylinder to assist with stuck rings. Does that sound like a reasonable next step?

 

The minimum for a 550 is 100psi IIRC, I'll have to check the book when I get home.

Meanwhile; ALL of the other ones are high, beyond "max" for a 550 even dry. These numbers look like a "wet" test and a "wetter" test; did you already dump some oil in it?

 

No, no oil but.......I failed to get the bike to run dry of fuel..... even with the vacuum tube removed, the bike carried on running when the petcock was in the central position. It did get spluttery but never seemed to completely run out of fuel! Maybe that was the wetness? I can go back to it and take the fuel pip off all together but was trying to avoid spilling fuel on the hot engine.

 

Dark here now so one for tomorrow........... (I need a workshop!)

 

The clymers manual says
Standard 121 psi
Minimum 100 psi
Max 135 psi
For the 550's

Have you tried a different compression tester yet.

 

they do look high to me. I think the max for the 550 is 135. Look into the spark plug holes and see if you possibly have a lot of carbon build up.

 

That begs the question: did a PO "breathe" on the engine?

 

I went to see another bike yesterday (also a 550) and compression tested it. I was getting about 110 on each cylinder using the same compression tester so I think I can rule out a problem with the that.

I didn't understand you k-moe... what is a 'PO'?

Peeing down out there but I'll try to get a light into the hole when it dry's up tabaka45......

 

PO: Previous Owner

aka, pillock that you bought it from
aka, horse's arse that you bought it from
aka, the bloke that had the bike before you that didn't have a clue what he was doing and cocked it all up

 

Oh right, thanks! Just as likely to be me unfortunately.... trying to do things right but making mistakes along the way. Would be lost at sea without this forum!

One thing I have been learning the hard way is to do it right the first time, don't take short cuts or make assumptions. The carb cleaning was a great lesson.

Everything I did before cleaning the carbs, I want to check again and that included valve shims and valve tensioner. Maybe I have made a mistake.

Can I do any harm putting in some cleaning agent down the plugs (think marvel mystery oil or redex) and leaving for 24 hours too see if that affects the compression? Good move?

 

It would be great if others would read this quote and take it to heart! It's GREAT self-analysis that we should all keep in mind.

Dave

 

Fuel shouldn't be the issue. There would be fuel in the bowls anyway, even proper functioning petcock. By all means, take the tank off for your next test. And no matter what, get that petcock rebuilt to avoid future problems with fuel in the crankcase (if any of your float valves ever get stuck open).

Don't know why you are getting such high readings, never have heard anything like it.

But if bike runs reasonably well, you can put a couple of hundred miles on it and see if that will loosen the rings on cylinder 2. I put seafoam in mine and ran it in there for 30 miles as well. Putting kroil in the cylinder for a few days helps as well. Kriol and Marvel mystery oil in the cylinder will not hurt anything at all.

 

It's giving high readings because it's "wet" somehow; there is either gas or oil in the cylinders in enough quantity to raise the test results.