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Build TY350 Mono as TY250 ?

kevin j

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I have a TY350 mono that I rode several years before it blew a crank seal 10 or 15 years ago. I had the cylinder redone with NikaSil but never put the engine back together. I have started riding it again (on another engine) just for fun, but am thinking of building it as a 250 instead. I have never ridden a 250 as they were not imported here (USA) nor very commonly brought in privately. I understand they are peppier, rev higher, and more 'modern feel' compared to my old dinosaur. I have a well worn 250 cylinder that can be bored, but no head.


So my questions are:

1. Will the 250 cylinder fit the cases with the existing studs? If I recall (it is boxed up), one engine used hex nuts and one used collar nuts down into the cylinder or head. Maybe the studs themselves are the same.


3. Are head gaskets available, or do I have to make one? I have soft copper available. I am running the 350 without a head gasket.


4. Wiseco has piston sets, but I plan on using the Wossner IT250 kit from Motoduro in the UK. I figure buy their experience and know it will work.


5. Anyone done the engine work per Stan Stephens in Mar 2017 Classic Motorcycle Mechanics magazine article?   I will search out Jon Stoodley and (Curt Johnson?) the US gurus. I can do the work, just need the data.


6. Anyone used the OKO flat slide carbs? My old carb is pretty worn anyway.


7. Any other input, suggestions, or 'that's a dumb idea' comments?


thanks, kevin j   Minnesota USA



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Yes the 250 barrel fits straight on, with head piston etc. One of the main reasons the 250 feels a little more peppier is that they don't have the steel band on the magneto flywheel. You can obviously take the band off and run the 350 barrel and see if you like the extra revs.

Cheers Greg

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18 hours ago, ivsforever said:

Chris Johnson in OK is the rider you are looking for he is on FB also look for Jon Stoodley  in OK also on FB.  NEOTT is the OK club he rides with.

Chris Johnson is on the ADV Trials forums as Motobene.  Good guy, very knowledgeable, and fun to drink beer with.  :D


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thks all. Chris, not curt.....   I got his packet of info 15 years ago, talked with him then, long before he moved. and Jon is awesome also.  I will pursue.


I have I think three flywheels, I have run with ring, with 60% ring, and with 40% ring. I have not tried without, as at 40% I felt the power pulses were becoming too abrupt. Running 1.6 BTDC timing instead of 2.0 smoothes it out a lot, so last ridden with 1.6 and 60% ring.

Dug it out again last year (with the other engine), lowered the pegs, fixed a broken frame, and more interested in pursuing now. Put AJP/GasGas front disc on 10 years ago, love that.  Next step is rear disc, not  for braking reasons but to get to tubeless wheel. I first fought with bead locks in the late 1960's with British mc, and have no love for them.....


thks all for info. I am on track, now just to find parts.




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 Don`t bother to take the whole ring off as it is hard on the clutch and the 40% works just fine. Thanks for reminding me that I have a Beta tubeless rim I need to put on my bike next weekend.  Much easier just to use a 32 hole Morad rim than make a project out of it. new spokes can be bought from the UK.

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  • 3 weeks later...

Hi Kevin,

Re: your item #5 above:  I have a 1984 TY250S (red frame/steel tank model) which I have just finished a resto on & I also read the CMM article by Stan Stevens.

My TY was particularly bad in regards to the rattling & clanging these bikes did in the day so I was keen to see what difference these mods would make.

I only did some of his porting mods though - I already had a brand new Pinky piston with the reliefs on the crown (the "bodge" he refers to) so we didn't want to modify the transfer ports straightaway (bit hard to put metal back in once it's been removed!)

We did open up & straighten the exhaust port & we also machined the cylinder head as per the magazine article. We also cut open the exhaust system & removed 33 years worth of accumulated oily sludgey packing & replaced it with new stainless packing & made a new "straight through" aluminium tail muffler (similar to the DEP/WES type)

We fitted a new air filter element but didn't cut the airbox lid up or change the jetting which still feels fine. We also left the steel ring on the flywheel.

The bike still has a bit of a jerkyness on the over-run & because of this, it still wants to run on after blipping over logs etc but the power is much stronger & smoother in 2nd & 3rd gear situations. 

I hope this is helpful for you,

Cheers, Andy




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