Do I have a crankcase air leak? (1974 T500)

[craddojc@yahoo.com]

Hello everybody-

I've had this bike since 1997. Been a great ride but it always has had a random problem with excessive smoke on left hand side exhaust. Smoking comes and goes, particularly at freeway speed. After the crank center seals failed a couple years ago (41,000 road miles), I pulled engine and split the cases. Crank, pistons, cyls, and gearbox were great shape, but crank center seals shot. Sent crank out for a rebuild with all new rods, bearings, and seals.

Just completed careful rebuild that included all new engine seals, rings and cyl hone. Road tested it for about 50 miles - Bike runs SUPER, starts easy. But smoking is unchanged - exactly the same.

I pressure tested left crankcase - found that with 10 psi air pressure, left crankcase will leak down to zero in about 1 minute, and I found air flowing slowly out the gearbox vent port.

Is the vent port air flow normal? I'm thinking new cranks' center seals may be bad, or the crankcase casting is porous.

Any ideas much appreciated...
Jeff in Colorado

[karl pa]

There are several reasons they can smoke, but with air leaking out breather seems like something in that cylinder is not right. If the smoke is from burning transmission oil, the oil level will drop.

[craddojc@yahoo.com]

Yep, somethings not right. I removed both of the crankcase drain plugs and pressurized the gearbox breather with low pressure air to see if there was communication between the gearbox and crankcase. Nothing came out of the right side crankcase drain. Red gearbox oil (I run ATF in the gearbox) came out of the left side crankcase drain.

Just a few drops - but it made clear there is a leak from gearbox to left crankcase. May have been there since I bought the bike in 1997, cause I've seen left side exhaust smoke since I got it. Since I bought it I have split the case twice (once in 2002 to address a failed kickstart pawl, and when I had to rebuild crank last year).

Any ideas how to fix?

Jeff in Colorado

[karl pa]

I never split a T500 cases so I am unfamiliar with the T500 crank.
My 75 T500 has 10,000 miles and sat for many years, it has compromised center seals, I run seal repair in my gearbox oil,otherwise it smokes out the left pipe and drops gearbox oil level, trying to prolong a rebuild.
How long have you run ATF in gearbox? I used ATF in my lawn tractor gearbox till I heard it didn't have the lube quality as motor oil, just curious.

[craddojc@yahoo.com]

Hi Karl-
Thanks for the post. Originally, I ran 75W-90 gear oil in my gearbox. I have run Dexron ATF for about the last 15 years in the T500. In a 2-stroke, the gearbox is isolated from any combustion (just like a typical automatic transmission), so its lubricant doesnt need any of the additives used in motor oil that keep piston rings and valves clean. Also- Dexron is colored red, which may be helpful when identifying the source of a leak, and ATF has extra seal conditioners to extend the life of automatic transmissions.

Based on the condition of the shift forks, clutch and geartrain at the last teardown, ATF works great - everything looked like new at 41K miles.

I'm interested in the seal repair you mentioned in the post - what is that, and where did you find it?
Thanks!
Jeff in Colorado

[karl pa]

Thanks for response, when I was told that ATF doesn't have the lube quality I was skeptical because automatic transmissions have gears and bearings, so I would think ATF lubes.

My T500 and T250, I add blue devil stop leak from advanced auto to the gearbox, both sat for many years and center seals dried out. Not sure if it would help in your case or not.

[craddojc@yahoo.com]

Interesting- I read up on blue devil oil stop leak, and its probably worth trying. On your bikes, how long did it last? Did it completely stop leakage?
Thanks
Jeff

[karl pa]

It stopped the leak for the most part, but if I change oil and don't add repair again it will start leaking again.

[craddojc@yahoo.com]

Hi Karl, I have a bottle of the Bar's transmission stop leak on order. Will try out next week. At least its an easy fix to try out!
Jeff