QuestionQUESTION: Mike, I need someone to check my diagnosis of the following problem. My
high mileage (53,000 mi) engine suddenly developed a case of no
compression on a single cylinder. I checked the clearance on the cam to see if
the valve/valve seat had recessed in the head and the clearance was within
what's specified in the usual Haynes/Clymer manuals. There is no popping as
there might be from a burned valve and no signs of smoke from the exhaust
on that side of the engine which would tell me there's aproblem with the
piston. The bike will run on the three remaining cylinders with the fourth
simply pushing air back out the carb (on that cylinder).
I've been a dedicated shade tree mechanic for years and I've never
encountered this particular combination of symptoms, symptoms that really
don't seem to add up.
I think I need to pull the head and cylinders to go any further unless someone
(perhaps yourself) can convince me otherwise. If I do need to proceed to
disassemble to top end of the engine, do you have any techniques you might
share about keeping the timing chain tight against the driving spocket on the
crank? I'd like not to have to pull the crank cover in order to reseat the chain
if I pull the top end apart. I have a complete gasket set in case it comes to
that but I'd rather not if I can avoid it.
Thanks in Advance for any observations and advice, Deane Bristow
ANSWER: Deane, the symptoms initially sounded to me like a holed piston, but as you've said there is no smoke or oil coming out the exhaust it's hard to say for certain. Especially since you also said the air gets forced back out through the carb, which is usually indicative of stuck valves.
I'm assuming you removed the spark plug and did a compression test with a cylinder pressure gauge? Is it possible, with the gas tank removed, to shine a light down the spark plug hole and get a look at the piston surface when it is at TDC? With the rocker cover removed, do both valves fully open (particularly the exhaust valve) when they are on the cam? It is remotely possible that the valve clearance - which is checked on the base of the cam - is within spec but the cam lobe has worn out and is not opening the valve(s) anymore.
A bizarre, but not unheard of, condition is to have the end gaps of all the piston rings in one cylinder rotate around until they're all lined up, in which case the compression will blow right through the gap into the crankcase... but that doesn't explain the blowback through the carb.
So, I'm going to go out on a limb here and say it's a worn cam lobe on the exhaust valve. remove the rocker cover and crank the engine over while you watch the valves on the problem cylinder. Let me know what you find and we'll take it from there.
---------- FOLLOW-UP ----------
QUESTION: Mike, I did take a compression check, wet and dry and was unable to raise
any sign of compression on the #1 cylinder, one in question. As to a stuck
valve, spinning the engine over with the starter showed that the valves for
that cylinder were opening and closing appropriately as the high spot on the
cam came 'round. Since this situation appeared after I had a professional
mechanic set my valves since I didn't want to invest in a shim kit, I thought
he'd over shimmed that cylinder but, as I said, the clearance was right around
.002 so I couldn't go off on his work. When he did the work, he said only the
exhaust valves were out of spec. I talked to him and explained the situation
and he was as baffled as I am. If I had a valve seat that came unstuck, I'd
expect to hear a rattle. If I had a bent valve, I'd expect to have it hang up in
the guide. If the cam was worn enough to not open the valve(s) on that
cylinder, what causes the air coming back out the carb?
I hadn't considered the ring gap alignment option but then I don't have a
pressurized crankcase either.
BTW, thanks for the time helping me puzzle on this.
AnswerIf the lobe on the exhaust cylinder was worn, the intake and compression strokes would operate as normal, but the missed exhaust stroke would keep the compression in the chamber. When the intake valve opens again, the compression charge would escape back out through the carb.
I'm just as baffled as you, but if you are certain the piston isn't holed and the ring end gaps aren't aligned, then the problem must be in the valves... and I suspect a worn exhaust valve cam lobe, because that is the only way that the compression charge could come back through the carb, via the intake valve.
