Domed piston in a flat piston bike?

[cheese]

I'm pretty sure its the wrong piston, get another one.

[noobi]

Iv emailed wossner asking for a please explain,
the box says 8097DB for 03'-06' EC125, wheras the 2009 wossner catalogue says 8097DB for 03'-09' EC125
which makes sense as the engines havent changed much/at all since ~2001 ish

[theblacksmith]

Ive just put a flat headed piston in a kx 125 motor that used to have a domed piston and she goes sweet! Starts 1st kick.
I have noticed that it runs a bit rich in the higher rev range(but this could be down to a mixture issue maybe). Power is the about the same Id say.

[SS90]

Ive just put a flat headed piston in a kx 125 motor that used to have a domed piston and she goes sweet! Starts 1st kick.
I have noticed that it runs a bit rich in the higher rev range(but this could be down to a mixture issue maybe). Power is the about the same Id say.

A bit rich, or a bit "flat"....... lower compression that the engine is designed for will lower the MSV (maximum squish velocity), and, at higher RPM slow the combustion rate down.

also, depending on the "compression height" of the piston, alter your port timing as well, which is why in some cases people say "wow, it improved bottom end"...simply, because a taller piston lowered the trapped compression (and exhaust port opening time)

If, for some reason you wanted/needed to run this set up, you would need to alter your ignition curve to compensate (in this case less retard in the top end.....remember two strokes retard the ignition in the high rpm, unlike a 4 stroke)

all in all, false economy....if the engine set up (head, ignition, pipe)is designed to run a certain design of piston, and unless you know better than a Japanese manufacturer (in all likely hood you don't), it would make sense to stick with what is meant to be.

[Reckless]

Agreed, SS90 makes a lot of sense in the things he says!
Just to add when increasing our compressions in our race karts (ie machining the head).
My good karting buddy used to do it when blue printing our engines. When re machining the correct squish back in, we always reproduced the angle/shape and width of the squish band as close to factory as possible. I'm know he experimented with this on his own karts (125cc watercooled 2smokers) and never got better reading on the dyno or track than with the factory internal head/bowl/squish shapes/angles.
Bearing in mind in that environment a 1/10th on the track or a very slight drop in performance is much more obvious than on our Enduro bikes.
So my preference would the to replace domed or flat topped pistons with the same and not to mix them.
I remember putting one of my race engines back into its 125 MX bike chassis as I was changing to a Cagiva engine. The bloody thing was lethal and now that I have ridden trail for the last 3 years pretty much unridable in a dirt bike except for maybe beach racing. LOL!!!

[theblacksmith]

[QUOTE=SS90;1129628889]A bit rich, or a bit "flat"


Rich Id say - as if choke was still on. And she actually starts first time with no choke, which might suggest its a wee bit rich. I dont know what the previous owner had done with the carb-but judging by the amount of air the motor sucked in(both crank seals were rooted and leaking air) I would say that they d most probably chose to give it more than standard just to get it to run lol!
Im off to Thunder X tomorrow and that will be the first decent run for the newly built motor - I will have a shot at tweaking it then I think.