CR125R Airbox Modification
On the last generation of Honda 2-strokers, the airboxes received some modifications that resulted in dirty air being pulled in the intake boot. The biggest problem was small plastic bulges from the backside of the box that pushed the filter away from the sealing surface (pictured below.) Most the time, the problem went unnoticed until your bike ate a piston and ruined the bore. There are some VERY cheap ways to fix this problem, and I have documented it here. The pictures are from a 2002 Cr125 I owned. You can see the intake boot where it has pulled away from the air box here:
To remedy the issue, first remove the boot from the air box and scrape all the sealing goo off. Reseal this with quality gasket maker (RTV works great.) You must prep the sealing surface where the airfilter meets the airbox with engine degreaser, as to give the fix somewhere to bond to. What I used to fix the leak was double sided foam window sealer. This stuff is used to fix cracks in doors and other applications where an airtight seal is required. No, airfilter oil doesn't break this stuff down. You can get it in various thicknesses and widths from your local hardware store.
Peel the protective tape off and take your time installing it around the rim of the airbox. Remember this surface must be totally degreased! FIRMLY press the foam against the airbox, this is crucial to getting the foam to bond in. Finished product: