Wiseco atc250r piston question?

[B18PWR]

Hi there, this is a fairly basic question but as I'm no 2 stroke expert may be some one on here may have some answers to help me. Basically I bought a 1982 atc250r in perfect condition from California and had it shipped to the uk, the engine incredibly had never been to bits and as I'm an engineer i felt it would be a good idea to check the engine over and re piston it with new rings gaskets little end bearing etc to keep it in good shape. I did use the atc on a couple of occasions prior to this and rode it hard to make sure it was a strong unit and a worth while proposition.
My problem is that since changing the piston from the Honda item , the engine has seized twice and I have in fact gone through two new pistons . What I can tell is that the inlet timing is changed with the wiseco piston as its windowed in this area , has any one on here had experience of this and does the jetting possibly need changing drastically . All the clearences are good and infraction not as tight as I would have it if the barrel was machined . It s on its std bore size . Any advice would be greatly appreciated from you guys.

[HondaRidr]

Did you do any heat cycles when you first started it after a new piston? The forged pistons need the the heat cycles before you ride it. Also when the piston expands faster then the cylinder it will sieze. Not letting the engine warm up before ripping on it and just getting it too hot can cause that.

[B18PWR]

Yes I have warmed her up before any use , also run it with extra oil to help, from what I have now found out the wiseco pistons are prone to this unless extreme caution is taken in break in. This process takes much longer than normal. Each time the piston has mainly picked up on the front - exh end but on the left side (genorator side) ? Seems odd it hasn't taken all the front out uniformely?

[RIDE-RED 250r]

Hello, and welcome to ya from accross the pond!

What you are describing sounds like it is too lean.

You stated that you are running extra oil.. I assume you are referring to your premix ratio?

If so, adding more oil to your pre-mix will in fact lean your fuel/air mixture for combustion. The reason is this: If the engine has been dialed in with jetting to run properly on a 32:1 fuel/oil ratio and you richen your fuel/oilmix to say 20:1 you are displacing more of the fuel with oil. When the fuel/oil/air mixture is pulled into the engine the oil falls out of suspension to lubricate the bottom end and cylinder walls with smaller amount of the oil that is left being burned during combustion. However, too little fuel will cause combustion temps to run too high. The first component to fail will be the exhaust side of the piston crown as this is where the hot exhaust gases pass out of the engine.

So, if you have richened your premix ratio by running more oil and have not riched the jetting, it is likely your problem is due to a lean fuel mixture. When you get a fresh piston in there again, set your jetting safely rich and break the engine in. After break-in is complete, you can incrementally dial in the jetting for performance being careful not to go too lean.

Hope this helps..

[manbearpig]

if youre trying to put a forged wiseco piston in a stock, unaltered bore, you're going to have issues with piston to cylinder wall clearances when the engine is at operating temp

the forged pistons heat up differently than the OEM cast honda pistons. you'll need to modify your cylinder to use wiseco pistons. it has to do with their piston sizes and shape.

notice how wiseco's are listed for sale in .020 increments while honda pistons go up in .025 increments...


call and speak to wiseco about this and they'll tell you the same thing.


if youre on your stock bore, theres lots of OEM STD bore pistons on ebay. pick up one of those, and a new OEM set of rings n call it good. if it isnt broken, dont fix it haha.

[B18PWR]

Thank you for all that info guys, peoples experiences are in valuable . The reason I chose a wiseco was simply that over here parts are very hard to find and these I know are a quality product and were in stock . It's a big learning curve I can assure you . I've now got it up and running at 30/1 oil mix and opened my main jet out 2 sizes -160' and 55 slow speed clip positition 4 . Hope fully I will have more luck , for testing I have simply used my least damaged piston , cleaned it up really well honed my barrel for extra clearence and built up with new rings . By the way I'm not a full retard the piston is fine for use lol. If all is good will bore the barrell and get the next over size , need to get all the jetting correct first . The break in with these pistons does seem more important than with the cast ones and from what I've read from the ring aspect to as the rin gap is tight till they have bedded which is better for durability, not good for break in lol.