ST1100 low compression in one cylinder

Well I have my compression low, and rpm unaffected upon removing the spark wire. Additionally - suffering from premature coolant overflow to the tank.

For the smell… hard to tell what I smell there. I may be already too paranoid to stay objective ;)
 
wjbertrand said:
Unless it is simply a tight valve clearance.
Click to expand...

I'm making the assumption that the OP has checked that as it was mentioned several times in the responses. If not, he certainly can do so on his way into the engine. If he pulls the valve covers and finds tight clearances, he can stop right there and address it.

RT
 
Valve clearances are in spec. IN on the higher end, EX on the lower - but still in spec. Both right cylinders are the same (one with good compression, other not).

Prepared it all for removing the head. Need to do some reading and get to work.
 
Beheaded my ST today :shrug2:

Right front (yellow/bronze in the pics) is the cylinder with low compression. What I've read somewhere, is a piston gets clean when it gets washed by coolant.

IMG_2304.jpg

A couple close-up pics to compare both cylinders (don't mind the water, it got there upon head removal, as I haven't drained the drain bolts - noted for future):
IMG_2306.jpg IMG_2305.jpg

How the head looks from below with the old gasket on (top is the "ill" cylinder):

IMG_2307.jpg

Gasket off:

IMG_2309.jpg

And finally - the gasket split into two when I barely touched it to take it off. I assume gasket breaking apart is a candidate for a root cause of my problems:

IMG_2308.jpg

What to do next? Well, first of all order new head gasket, valve cover gaskets (and small "gaskets" under valve cover bolts). May also do a check to see if valves seal correctly. Then probably disassemble the head, seat the valves and adjust them upon assembly.

A couple dilemmas still left:
a) Are the rings on cylinder good? I assume they are, as there are no oil traces in that cylinder. On the opposite, it looks too fresh.
b) Should I go into the left head? I don't want to, but I may regret it sometime sooner than later...

Any other insights are more than welcome - I like the critical thinking from members here ;)
 
Capital said:
Beheaded my ST today :shrug2:

Right front (yellow/bronze in the pics) is the cylinder with low compression. What I've read somewhere, is a piston gets clean when it gets washed by coolant.

IMG_2304.jpg

A couple close-up pics to compare both cylinders (don't mind the water, it got there upon head removal, as I haven't drained the drain bolts - noted for future):

How the head looks from below with the old gasket on (top is the "ill" cylinder):



Gasket off:

And finally - the gasket split into two when I barely touched it to take it off. I assume gasket breaking apart is a candidate for a root cause :
a) Are the rings on cylinder good? I assume they are, as there are no oil traces in that cylinder. On the opposite, it looks too fresh.
b) Should I go into the left head? I don't want to, but I may regret it sometime sooner than later...

Any other insights are more than welcome - I like the critical thinking from members here ;)
Click to expand...
I have never had rings go bad from a head gasket. If rings are worn or weak the edge around the other piston will be clean. From what I see carbon all the way around the piston is good. The clean piston got steam cleaned from the anti-freeze. Your pretty far into the job there is not much more to do the other side
at this point. just clean and carefully check for cracks I doubt it but.
 
I can’t seem to find cylinder head bolt torque sequence in Honda’s service manual. Torque values are there of course, but not the bolt order upon assembly. Anyone happens to know it? Maybe Haynes / Clymer has it?

Thanks!
 
I'm curious to hear what others suggest, but my guess would be to tighten the two in the center first, then do an X pattern on the outer ones last.

Since its not specified, Honda may consider it a non-issue anyway.
 
Below is a pretty standard torque sequence when a particular pattern is not specified.
I would also suggest tightening the bolts in stages to more evenly compress the gasket.

1627233275090.png
 
Thanks! Yes, final torque is 56 Nm (if I recall correctly), and reaching that in couple stages is a smart way to do it.
 
These are normally excellent engines. I don't think I'd take apart the other side if there's nothing suggesting issues. One thing you might want to do is a leak-down test. If you can't borrow one, they are easy to make from hardware store parts. You'd just need a hose from a compression tester for the spark plug hole. This is the one I made.
tester.jpg
 
You've done a nice job cleaning them up. What is the condition of the cylinder wall on the bad cylinder? Compared to its neighbor? Any ridge at the top? Grooves, scratches or scoring? If a ring was cracked, or damaged, there should be visible or tactile damage. I assume you have run fingers over the bores, fingernails looking for scratches, etc? Have you checked the valves and seats? A bad valve is usually obvious, as is a case where its not seating correctly.

You going into the other side as well?

RT
 
rwthomas1 said:
You've done a nice job cleaning them up. What is the condition of the cylinder wall on the bad cylinder? Compared to its neighbor? Any ridge at the top? Grooves, scratches or scoring? If a ring was cracked, or damaged, there should be visible or tactile damage. I assume you have run fingers over the bores, fingernails looking for scratches, etc? Have you checked the valves and seats? A bad valve is usually obvious, as is a case where its not seating correctly.

You going into the other side as well?

RT
Click to expand...
From all the inspection I’ve managed to do - everything else seems without damage. The only thing is previous owner running tap water in cooling system, and a lot of green residues all around.

For the left side - I decided I won’t.There’s a limit how much I am willing to invest into this particular bike. It is an old one, and you can change everything you touch. I’d rather ride it if it runs after me going in, and then decide if it needs to get well deserved rest, or sell it & buy something newer.

:old1:
 
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