holy smokes......rings or valve guides?

DougXS2

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my 72 just started smoking like a 2 stroke! It has about 20k miles on it and mainly smokes from the right cylinder (and of course uses oil).

I've done a compression check and get 146 to 148 on each cylinder.... and nearly identically across both.

Is there a kind of rule-of-thumb on how to determine if its rings or guides? my logic was, that if it was rings, the compression would be down and likely unbalanced?

and that leaky valve guide would not affect the compression?
 
Have you checked oil level and made sure the oil doesn't smell of fuel? High oil level and fuel contamination can occur with float or float valve failure and can cause what you're experiencing. It's also possible for an internal seep to develop at the head gasket that allows oil into the combustion chamber without significant loss of compression.

Re. rings and guides, oil rings often fail before compression rings. The only way to be sure is to break down the engine, measure valve guide/valve stem clearance, replace guides if out of spec, hone the cylinder, and replace the rings.

But since the problem emerged suddenly, the best guess would be that valve stem seals need replacement; rubber only lasts so long. It's possible to replace valve stem seals without teardown, so that's the first thing I'd try; look in the Tech section for procedures.
 
my 72 just started smoking like a 2 stroke! It has about 20k miles on it and mainly smokes from the right cylinder (and of course uses oil).

I've done a compression check and get 146 to 148 on each cylinder.... and nearly identically across both.

Is there a kind of rule-of-thumb on how to determine if its rings or guides? my logic was, that if it was rings, the compression would be down and likely unbalanced?

and that leaky valve guide would not affect the compression?

What does it matter what the cause is? At 20K miles, its due for a top end overhaul............rings,valve seals, valve lapping, camchain, front camchain guide, gaskets, oil seals, etc. Once the engine is opened up, you can measure the pistons and cylinders, to determine if they are still OK or need an oversize. You will also measure the valve guides for wear using a dial indicator.

You may also have a passing head gasket. The compression readings don't really mean much. Oil drawn into the cylinders over time causes a heavy carbon build up on the piston tops and on the head. That carbon build up can make the compression seem higher than what really exists.
 
i just went trough this same problem. It turn out to be a bad intake vavle seal. you can take the carb off and look at the vavle to see if it has oil all over it. thats what i found wrong with mine. I ended up ripping the motor apart but i notice there are forums that you can do it with the motor together.
 
so you guys are actually talking about the rubber stem seals, and not the valve guides?
 
so you guys are actually talking about the rubber stem seals, and not the valve guides?

The valve stem seals are rubber of some type, and apparently they can be changed without removing the engine or head from the bike. You might get lucky and stop the smoking, but chances are high that the rest of the top end is well worn and needs replacement/machining.

The valve guides are an interference fit that fit into the head. They require removal of the head/engine to the work bench.
 
If it smokes while the throttle is applied, or all the time, it will be rings. If it smokes mostly under deceleration, it will be valve guides/seals.
 
DLD1, mine is definately under throttle.

I've heard people mention that scenerio - accell vs. deceleration. whats the logic behind it? is deceleration sucking up oil and acceleration forcing it in?
 
I've been told that with a car if it smokes only on start up but not when running it's seals. When it smokes while running it could be either.

The reason is that after the motor has been sitting more oil will run down the stem to burn than if it was running, when it would get burned off quicker.
 
yeah. I've always heard that and it makes sense.

I just wondered about the how's and why's of the other ways
 
Under deceleration the vacuum created in the intake is very high, and will literally suck the oil by the seals if they are bad. This situation is minimized under acceleration. What XJWMX said about startup is also true about the seals.

If the rings are bad it will allow oil by the rings and smoke while accelerating. Pressure in the crankcase can cause this too, if it's not vented well.

DLD1
 
Did it start smoking from both sides suddenly, only more from the right side? In that case if it's not a coincidence, it's conceivable oil is getting past the head gasket. If that's the case, might check that the head is on tight.
 
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