Where does it come from ?

[guy53]

Yesterday I took my B model out to make the final prep for today's  friendly competition. I could not crank it, jerk it front and back it did not seem to be stuck, so I tought that the crank was the problem. After all it did start great about 3 weeks ago when I changed the transmission oil. After removing the side cover, everything looked ok, and I could not find the cause of my problem, took the clutch out, all the separate gear where free appart from the one on the end of the crankshaft.  Than with the sparkplug out I tried to turn the motor: horror it WAS stuck. So head and cylinder off ( it was not stuck too hard ) I found water in the base, enough that the bottom of the cylinder is pitted. There was no water in the oil of the transmission ( I was not surprise by that ). It had good compression, no leak between the base and cylinder and the gas that I use is always fresh and clean. So where does that water came from, the only way it could get in the base is by the exhaust witch is in my opinion improbable. The exhaust would have to be full of water. Any idea ?

 

Guy

[lineaway]

Did you wash it without starting it afterwards?

[guy53]

Nope, always clean it dry ( with a rag.

 

Guy

[dadof2]

This is very unlikely but it is possible. Look up "latent heat of vapourization". What can happen if you leave your petrol on or your fuel tap leaks is that petrol slowly runs into the carb and evaporates. The evaporating fuel lowers the air temperature causing condensation. The condensed water than trickles into the crankcase.

Given the right atmospheric conditions / humidity this process can produce a drip of water every few seconds. I have seen this demonstrated on a 4T engine with the engine stopped so both valves are open and a draft of air can pass through. The warm moist air enters via the airbox, it is then cooled and becomes more dense / heavier and flows out of the exhaust. When this is happening the carb feels really cold on the outside and condensation droplets appear.

[feetupfun]

Thermal cycling is another possibility. This is how fuel tanks and magneto casings can collect water too.

If the bike is stored somewhere there is day-to-night temperature variation, every enclosed (and vented) space will breathe in and out with each thermal cycle. In spaces like fuel tanks and crankcases and magneto casings and gearboxes, the air cools inside the space at night. Because it cools, the air contracts, which draws in some external (evening) air. If the metal gets cool enough overnight to go below the dewpoint temperature of the air inside, liquid water will form on the surfaces and collect at the bottom. The next day when the air warms up, the metal and the air inside will warm up and the space will expel some air. By then though the amount of water vapour in the air in the space will be less than it was when it entered (because the rest turned to liquid and ran to the bottom). Because of this, the air expelled will take out less water vapour than it brought in.

Each thermal cycle that sees the temperature going below the dewpoint with the bike stored will add to the amount of water in the bottom of those vented spaces.

While you can block the fuel tank vent and the gearbox vent and the magneto vent during storage, it is pretty hard to effectively seal up the exhaust and inlet of a two-stroke. It's a good thing to run each of your bikes at least every month to clear out the water from the motor and re-coat the insides with oil. I use semi-synthetic premix oil in my fuel because it gives added protection from corrosion damage between rides, compared with full synthetic premix oil.

Another thing to consider is that if you run petrol (pump gas) that contains alcohol, there will be more water condense inside the crankcases each time after you run the motor and let it cool, compared with running petrol that does not contain alcohol. That's why racing engines that run on methanol are then run on petrol at the end of each competition.

[guy53]

Now that my blood pressure is back to normal, I'm going to have a good look this evening, from what I can see with the cylinder off, only the part below the exhaust port is pitted.

 

Guy

[guy53]

Disaster: Just for fun I took the piston from the connecting rod, top bearing is done and yep the rod is corroded too, so crank job.

I always use Shell Ultra that is suppose to be ethanol free. The transmission side is just as new, but the crankshaft side is very dirty with what look like rust mix with oil. There as been big change in temperature in the last few month especially in the period I was out of town, but I would have never believe that it could such damage. When I change the oil in the transmission I only ran it for a few minutes, probably long enough to '' spread the disease ''.

Guy

[feetupfun]

Sad to hear, Guy 

[guy53]

There is always a bright side to a story: I was smart enough to buy all the parts needed for the job since the last rebuild, connecting rod, piston kit, seals etc., so hopefully I'll have it running like new for our big annual meeting with Mick.

Guy