Motorcycle Repair: 62 yamaha, bottom stroke, exhaust ports

Question
Hi Gary,

Your profile shows that you have extensive knowledge with race bikes.  I hope that you can help me with my 2 stroke street bike.

I have a 62 Yamaha TD3.  It is a 250cc twin with a single carb.  This bike uses premix only.  I am not sure what premix ratio to use.  Previously I used 32:1.  Todays oil bottles recommend 50:1 and 66:1.  I have even heard that some recommend 100:1.  What would be an appropriate oil and ratio that I should use for a bike of this vintage.

Also, is there any way to test if a 2 stroke engine has good crankshaft seals that are working properly?  I have heard of a pressure test for this, but do not know how to do this.  Do you have any suggestions?

Any assistance that you can give will be appreciated.

Thanks for your help.

Oscar

Answer
Oscar, 32:1 is correct but the oils have come along way and 50:1 wont hurt it.You can use a ratio/rite cup appros $5.00 to aid in mixing it properly. The pressure test is quite extensive but not hard. A compression tester can be rigged to a air line and regulator to put air back into the cylinder. The intake port and exhaust ports have to be plugged up so as not to allow air to escape.Place the piston in bottom stroke allowing the ports to be uncovered. Pressure up the cylinder to about 50 lbs and you can spray a liquid on the seal on the flywheel side and see if any air bubbles appear. The other side will suck in tranny oil and really smoke if the seal is bad and really smoke. The flywheel side is usually the one that dries out and goes bad and the bike may even rev up and not idle because of the vaccum leak.