Compression low one cylinder

Ian Robinson

Joined
Jan 22, 2013
Messages
62
Location
England
Bike
ST1100 2000
Thanks for previous advice.
3 cylinders tested with throttle open and all 180psi cold.
1 cylinder 130 cold and 180 wet cold (squirt of oil)
Valve clearance at lower end of tolernace for inlets, bang in middle for exhausts.
Bike runs fine 4000 miles this Summer with no problems, should I leave alone or start worrying?
(Previous owner had both exhaust valves replaced on that cylinder 3 years ago - due to "lack of power symptoms")
 
From my limited mech skills, I remember if you add oil to the cylinder and the pressure goes up its usu bad rings....

As to leaving alone , I would guess it's a push/pull between the cost of an engine job vs the value of the bike... or possible to get a second engine cheap and swap out

Since you said it ran well... If repairs are on the high end compared to value, I'd just ride till I had to fix.
 
I would not necessarily worry just yet. Does the bike burn oil? If not, I suspect that the low cylinder had some fuel from the carbs either drip into the cylinder, or if the engine was started or turned over for just a second with the choke on and simply washed out the lubricant from this cylinder, this will cause a low compression reading. You should never do a compression test on a cold engine unless there is no other way. I'll bet if you run the engine for a bit and then do another test the compression will be fine.

Dan
 
Hi thanks for the advice. The bike does burn a bit of oil but only needs topping up evey 1000 miles or so. Occassional puffs of oil and not when under load i.e. going up hill etc.
 
How many miles on the bike, and what is the condition of the rest of the bike? As an auto tech myself (cage) and owner of a variety of value/condition vehicles, I would do as mentioned above...gauge the condition/value of the bike vs. expense of fixing it. Is it worth investing a bunch of money in it? Does the rest of the engine have too many miles on it to make it worth fixing the engine vs. looking for a low mileage one? If the whole bike has high miles and has seen better days, just run it and enjoy it the way it is. If the bike has a lot of miles but has been well taken care of, I would look for a replacement engine. If its still relatively low miles, I would dig further into your problem and fix it right, or else find a low mileage engine if you find expensive problems.

The next step I would take would be a leakdown test. That will give you a more accurate measurement of the condition of that cylinder, as well as let you know WHERE the air is going to if there is leakage.
 
Tks... he got around to it where I thought he should have mentioned earlier... Cyn on TDC so valves are closed and then fiq out where air is going.
 
I have done leak down tests on 2 strokes - only way to see if crankcase sealed really.
On 4 strokes I have put cylinder ar TDC then injected air in through spark plug hole (need a seal here) .... then listen where it comes out.. carbs= inlet valves leak, exh= exh valves leak, crank(harder to hear) piston by pass.
 
Its a crude test on 4 strokes really but can show up a problem - expect some cylinder by pass if cold of course, usually hear air coming out of crank breather. Never done on a ST1100, as you may realise only just got one.
ian
 
It should be done on a warm engine. Also when putting pressure to the cylinder it is a good idea to move the crank back and forth a bit to make sure the rings are seated against the cylinder walls. I am not an advocate of doing really low pressure leak down tests say at 10 psi. Piston rings during the compression cycle are pushed out against the cylinder walls to provide a good seal, 10 psi simply is not enough pressure to insure the rings are fully seated. When I do a leak down test I use at least 20 psi or more. I have seen tests done at 10 psi that show a problem and when the psi is increased the rings seat and everything looks fine.

Dan
 
You can read a pretty good overall explanation at the link below. One thing they didn't touch on, is if you have a bad head gasket. Besides listening at the oil fill cap, intake, and exhaust...you can also remove the radiator cap and look for signs of bubbles while the cylinder is pressurized. Obviously do this while the engine is warm but not hot. The safest way is to loosen the cap 1 notch and warm the engine, turn it off, then remove the cap carefully. There should be little to no pressure at that time.

http://www.carcraft.com/techarticles/116_0406_cylinder_leakdown_tester/viewall.html

While a professional quality tester will run close to $100 or more, you can find fair quality ones for much less, especially ebay.
 
Spray a water soap mixture around the gasket if you suspect a leaking gasket, either with test above or with engine running.
 
Soapy water will rarely help with a head gasket- the leak usually goes one of 3 places- Crankcase, Cooling system, or the adjoining cylinder.
 
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