old dude working on old honda mt250

A richer mixture generally means cooler operation, but at a cost of excessive fuel consumption resulting at more carbon buildup in the combustion chamber, higher compression from this and the tendency for excessive carbon to superheat causing hot spots on the head and more importantly on the piston crown. This will cause pre-ignition (and detonation) in the engine. Once this glowing ember begins on the piston crown it's not too long until the aluminum the piston is forged from melts causing engine failure.

The thing about two stroke engines to remember is that every time the piston comes a few degrees before TDC a power stroke occurs, unlike a four stroke engine where there is an exhaust and intake stroke between each compression/power stroke to help cool things off, a two stroke fires every time - with out that extra stroke to cool things off.

Although much lighter and more powerful they do have their set of problems, most of them stemming from excessive heat. A well designed under stressed two stroke engine is a wonder, simple, light and powerful, but when the engineers try to up the power they become much more temperamental - and I am afraid the MT-250 is one of these. The DT-400 Yamaha was also a pain in the butt to keep running properly without self destruction (the the 250/175/125's were better, with the smaller the best.)

This link has a good description of a two stroke engine in operation:

http://www.keveney.com/twostroke.html

..And a four stroke (or Otto cycle) is here:

http://www.keveney.com/otto.html

Now realize these anamations show the engine running at 60 RPM - so in real life things go much MUCH faster, a two strole engine idles at 1500 rpm and redlines at 7000 RPM so you can see how heat would become an issue fast...

Mr. Kevenely has a great website here - check out all the different engine configurations here:

http://www.keveney.com/Engines.html

Thanks John, I am rather green about two stroke engines and you help me understand precisely the chain of events and conditions that causes two stroke engines to fail. If such an engine engages into self ignition this is indeed a very serious problem.

Keveney's site is really the best educationnal site on engine types I've ever seen.

Thanks for your expert advise.

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There is nothing like riding a Honda MC !

you can try and decarbon the engine with seafoam before you pull the head.it will help clean the carbon off the top of the piston

Removing the head on an MT-250 is extremely easy, remove the plug and wire, remove 4 bolts and take it off. To remove the carbon use a wire brush. Although the gasket is technically reusable, plan to replace it when you de-coke the engine.

I have not had good luck with "Miracle Engine Elixirs" like Seafoam and the like, this is a job for my friend Mr. Wire Brush. By removing the head you will be able to examine the piston crown and bore for damage, which is almost more important than removing (Oh, it's important-alright) the carbon.

if you pull the head off be sure and torque the headbolts in an x pattern. also get all the loose carbon [that came off the piston] off the cylinder by lowering the piston in the cylinder. we never had carbon buildup as we would rering them 2 or 3 times a year when we were riding them hard.also don't use cheep 2 cycle oil

on mt 250's they run much better if you remove the nylon oil injection gear and pre-mix you're gas like a normal two stroke,they tended to run overly rich on the oil injection.if i remember right the only downside is the tachometer will quit working.the performance gain is pretty substancial with the oil mix ratio optimized and the points and timing optimized for pre-mix gas.the dwell on the points is really important on these bikes.it can move the power band up or down depending on the dwell,and it can help with starting also.everybody remembers points and condensors don't they.the points are located behind the flywheel on these bikes and are a real pain to change but can be adjusted without removing the flywheel.although these engines look identical on the outside to a cr250 engine they are very different on the porting inside the cylinder and in the engine cases and you cannot interchange the parts,they will bolt on,but won't run right even if you do get it started.i know,i tried,it was an exercise in futility,wasted most of a day tearing down both engines and reassembling.