I had the TY out today riding around the yard and it smokes pretty bad.The more throttle you give it the more it pours out white smoke and it will not clear out.The gas was mixed at 32:1 with a freshly cleaned and oiled filter.It,s only the second time that I,ve had it out since buying it back from a friend,but I know it didn,t smoke like this before.

I,m thinking that it could be a primary side crank seal? Could it be worn rings? Too much oil on the filter or the fuel/oil mix?It does sit and idle good and I had no problems riding over logs behind the shed.On the plus side,I think that it killed off most of the mosquitos in the yard.

Check your oil level, sounds like crank seal to me

Seems most run about 50:1 on modern oils. Might need a good blowout too!

I also think it might be the clutch side crank seal, and if it is, the reason why it may still run and idle OK with that problem is that while it is sucking oil through the seal, it is not sucking air. I also suggest you check the oil level in the gearbox before riding it, to avoid the risk of damaging the gearbox.

Also, worn rings don't cause exhaust smoke on a 2 stroke.

Just thought of something else. If you are burning gearbox oil via a seal leak, the colour of the smoke is usually a lot whiter than the smoke caused by excess premix oil, or the burning out of gunk in the exhaust, which usually is pale blue

Thanks for all the help.I don,t have a service manual for it,so I do have one more question.Can I change the seal externally or do I have to split the cases?

Does it still have the oil pump connected?

If on premix I would use a SAE40 2T oil at 32 or 25:1 Silkoline do one for these

Modern lean ratio synthetics are designed for watercooled engines with much smaller piston / bore clearances and lower operating temperatures.

I am 90% certain you can replace the crankcase seals without splitting the engine but its many years since I had one apart.

Crank seal can be done without splitting cases or removing from frame

OK,thanks again for all the help and advice.I really do need to order a shop manual.

You will struggle to find 1 for an A model but there is a manual by cycleserv for the B model on the cover which is what I have, quite often listed on ebay

http://www.ebay.com/itm/YAMAHA-Workshop-Manual-TY250-Trials-1974-1975-1976-1977-1978-Service-Repair-/360685551067?pt=Motors_Manuals_Literature&hash=item53fa88bddb&vxp=mtr

I hope this thread isn,t too old to re-start,but here goes.I haven,t had a chance to ride the TY again until last week.I put knew crank seals in and it still smokes the same.I can hold it at 3/4 throttle and it just pours out smoke and will not clean out.I pulled the reed block out today to have a look and they seem to be fine.The onle thing I can think of now is maybe it,s the old stock carb? I did clean it good and the choke doesn,t seem to be sticking.Any other ideas would be much appreciated.I,ve included some pics for your viewing pleasure.The last one is my custom TC decal made from red electrical tape. Also in back side of the rubber intake boot that meets the reed block has a wierd color to it that I at first thought was part of a gasket,but isn,t.

Just a thought, was there a o-ring on the crankshaft behind the primary gear when you did the seals? Most aftermarket kits seem to be missing this

It has been quite a few months since I did the crank seals and I can,t remeber if I replaced the O-ring or not,but that is a good call Tony.I ordered the seals from a place were you look at a parts break down and maybe I missed the seal.I went back out after posting this and checked the oil and it was low.I did not check it before I rode it,but I know I would have refilled it to the correct level when I did the seal job.

I pulled the drive gear off the end of the crank tonight to check if I had put the o-ring back on before I had put the gear on.The o-ring was there and still had some fresh grease on it from when I had put it all back together when I replaced the seal.I cannot remember if it was the old o-ring,a new oem o-ring or one I got from work.I really do have to start keeping a log book on my repairs.I took a few pics of it next to a coin for comparison to see if anyone would recognize it as looking correct or too fat.It is in good shape with no defects.I am out of ideas with the reeds looking ok and this looking to be ok.I guess why it,s apart I can order an oem o-ring too put in just to be sure.Any more ideas are welcome and appreciated.

^{There are still quite a few possibilities for what is causing the smoke. Did you work on the bike at all between your first ride (when it didn't smoke) and the second ride (when it did smoke)?}