hello all can someone explain to me the dynamics of the 2 stroke engine against that of the the 4 stroke unit hope im not being to thick but i understand that the 2stroke runs with a petrol oil mixture not sure how this works and want to understand more and also would i be able to pick up a 4 stroke yzfr125 for under 2.5k price wise in anticipation of any reply s 
  many thanks 
    caitlin                                                                                                                                                                              

3 main types of piston engine (forget diesel and petrol) The 4 Stroke engine has more moving parts but tends to be more reliable than a two stroke  http://www.youtube.com/watch?v=V-z-R8Mv_HM&feature=fvst   1 Piston moves down and sucks air/fuel mixture 2 Piston moves up and compresses this mixture 3. Spark plug ignites mixture and the explosion forces the piston down - hard 4. burnt gases pushed out of the exhaust by the piston going back up.   The engine is lubricated by oil which is kept at the bottom of the engine by rings on the piston so it doesnt mix with petrol   2 strokes tend to be simpler, and produce alot more power for their size, but they tend to be less reliable and a bike like the Yamaha TZR-125 would need a rebore every 20,000 miles.   http://www.youtube.com/watch?v=LuCUmQ9FxMU   The oil has to mix with the fuel in a 2 stroke engine, as it needs to lubricate the engine too, but cannot be kept down in the crankcase, as the fuel / air goes down there too. So 2 strokes burn oil all the time.   The piston rings on a 2 stroke wear over time, along with the bore (a nikasil lining on bikes like the NSR don't wear as quick). They are seen as more polluting than 4 strokes so are being slowly pushed out.   However if you could properly fuel inject a 2 stroke you may be able to get round this oil loss problem (no one has managed it yet)   The only other real alternative right now is the Rotary engine such as the Notorn rotary, but is very rare and burns fuel at a massive rate.   Alternatives to the YZF-R which is also 4 stroke  (if you cant get hold of one) Hyosung GT125R - For me the only other 125 which can cut it next to the Yamaha in looks, not too bad on reliability but probably not on a par with Japanese - 4 Stroke   Aprilia RS125  and Cagiva Mito 525 (it is a 125 though) - Both are gorgeous bikes, but are two strokes. and Italian, and like most Italian people, are gorgeous, desirable, and likely to explode with the slightest provacation.   The older Cagiva Mito which looks like a Ducati 916 was also a gorgeous little race rep - if you could find a very tidy example I know I would be very jealous.   But still the Yamaha is one of the best looking little sports bike out there - I know I wouldnt be emarresed in owning, or riding one myself                                                                                                                                                                             

A 2 stroke engine is designed to use the engine oil as it runs, so the oil is mixed in with the petrol, burnt with the petrol and kicked out of the exhaust pipe with the petrol. This is bad news emissions-wise, which is why they are being phased out.

As a rider, you must keep an eye on the oil level. If you run out, the engine will overheat and probably seize up.

The advantages are that they are simpler and therefore cheaper to build, and give (almost) twice as much power as a four stroke - the entry class Grand Prix bikes are 125cc 2 strokes (250cc four strokes from next year); the next class up was 250cc 2 strokes but has been replaced with 600cc four strokes; and the top class was 500cc 2 strokes but are now 800cc four strokes. PM if you want the techy nerdy reasons.

They seem to get a generally good write-up Caitlin. Register with a few of the local bikes shops, flea-bay, MCN, etc, to see what comes-up. Bought through a Yamaha dealer will be a bit more expensive, but at least you have better comeback if there are any problems.

Duh ... 2 stroke 125s are no more powerful than 4 stroke 125s, as they're both limited to 15bhp