Gemini SST gear oil leak

Mini Bike & Go-Kart Parts

#1
So i've got this 1970 Gemini SST that is fairly complete, but not running. When I picked it up it would just about start, but only fire once or twice before eventually flooding. I did a the usual checks on the ignition to make sure there was spark, fuel lines clear, etc.. and after that, it would sputter a few more times than before, but still no dice. It ends up flooding.

but here is the new problem for the day. So as part of getting this on track I emptied the gear oil completely and then re-filled it using 400 cc of Yamalube 20/40 gear oil as recommended in the manual. No problem when I was fiddling with it yesterday.

The next morning I come out to find a puddle of oil underneath. Prior to changing the gear oil, no oil was leaking (at least from this spot). Why would it suddenly leak oil just from changing it? It's not coming from the oil plug, but appears to come from around the flywheel.

To make matters worse, I also found oil inside the exhaust pipe, which makes me think the rings are shot, which may be why it won't start, but that wouldn't explain the oil underneath when its not running.
 
#2
If somebody used starting fluid to get the engine started it may have backfired into the crankcase and that can blow out a crankshaft seal, especially true if the seals are old and brittle from age. This would also explain why the engine sputters and quits.
 

ugmold

Active Member
#3
It's a 2 stroke, oil in the pipe is nothing to worry about. The gearcase is also seperate from the combustion area. Might just be an outer case gasket. Wipe it off and see where it is coming from.
 
#4
If somebody used starting fluid to get the engine started it may have backfired into the crankcase and that can blow out a crankshaft seal, especially true if the seals are old and brittle from age. This would also explain why the engine sputters and quits.
That seems possible. The guy I purchased it from did try starting fluid on it, but that was well before the oil started to leak. It wasn't until I replaced the gear oil that the leak started. (at least not to the degree it was).

Why would a blown out crankshaft seal be related to it sputtering and quitting?
 
#5
The crankcase must be sealed up because the changing volume as the piston moves through the cylinder is what draws vacuum through the carburetor and pushes the fuel/air mix through a transfer port to the combustion side (top) of the cylinder. At low speeds any leakage will have a huge effect on the fuel mixture (usually rich or flooding) and it will reduce the ability to transfer fuel mix to the top of the cylinder for proper combustion.
 
#6
The crankcase must be sealed up because the changing volume as the piston moves through the cylinder is what draws vacuum through the carburetor and pushes the fuel/air mix through a transfer port to the combustion side (top) of the cylinder. At low speeds any leakage will have a huge effect on the fuel mixture (usually rich or flooding) and it will reduce the ability to transfer fuel mix to the top of the cylinder for proper combustion.
Thanks. Sounds like the next step is to pull off the cover and see if its there. Do you know if its even possible to get a crank seal for the Gemini 50cc (or the equivalent Yamaha rotary valve) engine? And do you think I can just replace the seal, or would I also need to open up the crank case and clean it out if
 

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