ST1100 low compression in one cylinder

Capital

Capital

Joined
Aug 30, 2020
Messages
114
Location
LTU
Bike
2002 ST1300A
1991 ST1100 with 150k kms. Kinda poorly maintained when I bought it half a year ago.

Went out for a ride ant returned on 3 cylinders... Played with exchanging ignition coils/wires trying to understand why removing front right ignition wire barely affects RPM. Whilst finally measured compression yesterday.

Three cylinders hit ~140-150 easily, as the front right one hits 60-ish. Adding some oil to cylinder bumps it up a bit (maybe to 90), but nothing significant.

Suspicion is on the blown head gasket (and I really hope that IS the problem). That would also kind of explain the cooling problems I experienced, when coolant pushes through the radiator cap prematurely - guess is due to exhaust gas contaminating the coolant and building up the pressure. So far it is only a theory.

Will start head disassembly sometime later in the summer when I regain the spirit for it. The thing is that I need to take of radiator and timing belt in order to remove the OH cam (at least that is what the manual says) - that itself is a labor.

Wish me luck.

Also - all comments/experiences are more than welcome.
 
My first reaction is, bugger.
But the second was, this is your opportunity to make sure it's right going forward. If you're contemplating one head gasket why not make it two. It would be blummin annoying to go that far in, only to have to do it all again. Obviously the cam belts might as well be done too. New inlet rubbers and hoses are a must.
Good luck and I know those that have been this far before (there won't be many) will help along the way.
Consider yourself a reluctant trailblazer, I think beer and whisky will help.
Upt'North.
 
Thanks for support! In fact, I've been that far during winter, changing the belt, cleaning carbs, changing boots, new hoses & radiator. That is one of the reasons I am so reluctant now to do it all over again :eek:

But I do like your idea to change both gaskets while at it, I wouldn't have enough strength for 3rd time, that's for sure.
 
Upt' North said:
My first reaction is, bugger.
But the second was, this is your opportunity to make sure it's right going forward. If you're contemplating one head gasket why not make it two. It would be blummin annoying to go that far in, only to have to do it all again. Obviously the cam belts might as well be done too. New inlet rubbers and hoses are a must.
Good luck and I know those that have been this far before (there won't be many) will help along the way.
Consider yourself a reluctant trailblazer, I think beer and whisky will help.
Upt'North.
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Ill hold the beer and whiskey for you!
 
My '93 suffered a burnt exhaust valve on cylinder #4 (right rear). Try feeding air slowly into the spark plug hole with that cylinder at TDC. Then listen for the air at the intake, exhaust & oil filler cap. In my case I heard the air escaping through the exhaust so knew there was an issue with one or more of the exhaust valves in that cylinder. Also remove the cooling system cap and watch for any bubbles. You should be able to diagnose the issue before disassembling anything.
 
+1 what @wjbertrand said. When you add oil to the cylinder, it will seal the leaking rings, but not a blown head gasket or valve. My money would be on a valve since a leaky gasket often burns coolant that can be smelled in the exhaust or pressurizes the cooling system. The latter will get you coolant blown out of the overflow tube, which you have not mentioned. You will have to make up a fitting that allows you to pressurize the cylinder with a compressor, then you can pull the radiator cap and see if you are getting bubbles in the radiator.
 
Yes I may do a test with compressor. And I do have an issue with mid-warm engine already pushing coolant through cap (2 new caps tested, 1 of them OEM).

thanks for tips!
 
Find a borescope to borrow or buy one of the cheap ones that attach to a smartphone. If the cylinder is all scored, no point in even going into it, just get a used engine.

RT
 
check valve clearance. too tight or broken valve spring. If you think its a head gasket, fill coolant to top of radiator, leave cap off and crank engine over. If it blows out while cranking you definitely have a leaking head gasket.
 
Good point. Checking valve clearances is also a diagnostic step that you can do prior to any disassembly. Forgot to mention that one.
 
Al st1100 said:
check valve clearance. too tight or broken valve spring. If you think its a head gasket, fill coolant to top of radiator, leave cap off and crank engine over. If it blows out while cranking you definitely have a leaking head gasket.
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it slowly starts going up after 1-2 minutes if i leave the cap off. At least was like that before the cylinder went out of order completely. Will check it out again with cap off in the garage.
 
Capital said:
it slowly starts going up after 1-2 minutes if i leave the cap off. At least was like that before the cylinder went out of order completely. Will check it out again with cap off in the garage.
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That's probably normal. The coolant will expand obviously as it warms. Normally that will just push over to the catch reservoir and return to the radiator upon cooling, assuming the over flow hose is intact (but that's another issue) and the cooling system cap is in place. I'd be concerned about seeing bubbles not the level rising.
 
Capital, just curious what did you end up doing to your bike so far? I bet it was a learning curve. Having the repair manual is a must, but a little practice gets you lots of confidence later on. I hope the bike is running good now.
 
Dream12 said:
Capital, just curious what did you end up doing to your bike so far? I bet it was a learning curve. Having the repair manual is a must, but a little practice gets you lots of confidence later on. I hope the bike is running good now.
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I haven’t gotten to it yet, but will start this week. Will prepare everything for head dissasembly first, then take it off. Was busy riding friend’s VTX for a couple weekends… And I do have the manual, would be tough without it :)

PhotoDoctor said:
Having more than one good running bike makes this type of work less painful.
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You’re absolutely right!
 
Went to the garage. Fired it up without rad cap. Video - are these enough bubbles to conclude? Dissasembly will begin now.. :)

 
Can you smell anything that smells like exhaust at the radiator neck? I'm stuck deciding if the bubbles are due to entrainment from the swirling when the engine is revved or exhaust bubbles. Looking closely, I could not clearly see any bubbles coming up from below the surface in the video. An exhaust sniffer would be ideal. I haven't heard of an ST head gasket failure either. There have been a few high mileage units with cracked cylinder heads, but I think this just allowed coolant to leak to the exterior and did not affect compression.
 
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