125 piston questions for you gurus

wpark89 wrote:

This seems to be the only part of the forum with any intelligence so I'd like to get some of your feedback and thoughts. I've restored...

This seems to be the only part of the forum with any intelligence so I'd like to get some of your feedback and thoughts.

I've restored and have been racing an 83 Cagiva 125. Stock pistons are not available. I was told years ago that I could use an 85 yz 125 piston. I've been running one...but the yz is much shorter and the power is very flat and weak. Could the big cutaway on intake be a problem? I will pull a few bike lengths off the gate....then get passed halfway down the start stretch. It runs clean but once up on the pipe it just doesn't "go" or pull like it should.

Here is the stock piston compared to the yz.


Last fall i was told to try a 79-81 kx 125 piston. It's closer on skirt length to the cagiva piston...but has this window cut out. How is that going to effect power delivery?




Now some contacts in the uk have recommended a 83 kx piston that would not have the window.

My stock cagiva piston is worn beyond tolerance..but I do have a new set of rings for it. I'm l confused on my best option.

dazzlindino wrote:

...first it looks like the YZ piston is much shorter.... ...which means it could be opening the exhaust port on the bottom side... ...which if the...

...first it looks like the YZ piston is much shorter....
...which means it could be opening the exhaust port on the bottom side...
...which if the case, you are not getting a full charge into the crankcase...
...pistons are only a cpl of millimeters long enough to keep the exhaust port covered at all times...
...the windows / cut out the piston on the intake side will not matter much if it is a reed motor...

TTperra wrote:

..What i was thinking also.

And, do you get same squish with the different pistons?

wpark89 wrote:

Thanks for all the good information guys. The YZ is just a touch taller...I can't remember exactly but I tried to compensate with base gaskets to...

Thanks for all the good information guys.

The YZ is just a touch taller...I can't remember exactly but I tried to compensate with base gaskets to get correct squish. It was very close, but with only about an hour of racing on the bike the YZ piston does look very beat up. There was a slight amount of detonation (running Sunoco 100, Maxima K2 for years in all my bikes).

The KX when I measured the tops was nearly identical to the Cagiva piston. I can't remember numbers as it was about year ago I did all the calculations.

I'm using a newer Mikuni carb off my 08 Husqvarna CR 125 and the jetting is spot on. The bike sounds very crisp and responsive but it just doesn't pull like I believe it should. It seems like it's going to do something but it just doesn't. I have won all my motos on it....but I've gotten pulled by older air-cooled Husky 125's! I have to make up time into corners and out ride everyone. At my age I never rode early 80s non power valved 125's and I'm not really certain how they should run. But I do feel very underpowered when the Cagiva 125's should be very competitive if not run stronger than a lot of the Japanese bikes. I think it's something like has been mentioned with not getting the correct crankcase charge or volume.

I'm tempted to just put my worn stock piston in it with new rings and see how long it last. At least that will take the piston out of the equation.

This is as 83 model that was sold in the US as an 84. It was basically a 1 year only bike so there isn't as much parts availability as there is for the 85-89 Cagivas.