TWO-STROKE TUESDAY: 1991 KX80 CHASSIS WITH KX125 ENGINE

On May 28, 2024 Last updated May 28, 2024

What is it? It’s a KX80 with a KX14 engine and lot sof special parts.

By Mark Chilson

The project started off after I built my wife’s TTR125 with YZ85 front and rear suspension and a BBR 150 kit. It was a blast to ride and wanted something for myself that I could goof off on when she was riding it. I picked up a 1991 KX80 BW rolling chassis and decieded a 125 engine would be a blast to have, bigger is better right!

So after trying a few different engines I found that the 1991 KX125 engine had the right intake offset that I could still use the stock airbox and air boot. I found a blown-up 1991 KX 125 and used it as the donor. The KX80 frame was modified to accept the later plastic, tank and a Y-down tube replaced the single downtube to allow for the KX125 center exhaust spigot and pipe. The frame was gusseted around the subframe and footpegs and re-welded around crucial areas for added strength and then powder coated.

The KX125 engine cases were cut down half inch on the right side to clear the swingarm and put the engine back in the center of the frame, an aluminum spacer was made to adapt to the smaller swingarm pivot bolt. A 2009 aluminum swingarm off a KX100 big wheel was used to replace the 1991 steel one. New engine mounts were made welded in, and new brackets. The Boyesen intake was also cut down 3/8” to allow enough room for the larger KX125 carb to fit in and the airboot was stretched out to accept the larger carb.

A Pro Circuit pipe , which originally was built to exit on the right side of the KX125, was chopped up and turned around and new hand rolled cones were made to fit on the right side.

A new Ohlins rear shock was found on eBay. The 1991 KX125 front triple clamps and forks were used and lowered internally 1.25” and the forks raised in the clamps another 1.25” to get the proper rake and trail. The KX125 front hub, rotor and caliper were used, both hubs were turned down and hard anodized black and laced up to Excel rims with Buchanan Stainless steel spokes wrapped with Pirelli tires.

Custom one-off parts were used and made as needed. A Honda CRF450 rear master cylinder was used and a late model KX100 rear brake pedal was moddified to accept it. A 2007 YZ450F carbon fiber skid plate was modified and added on as well as a water pump engine guard. Jeff at Fontana Radiator fabricated two new radiators to handle the cooling. Handlebar risers were also added to help open up the cockpit for an adult rider.

A brand new Öhlins shock was found on eBay for the build.

A Pro Circuit pipe which exited on the right side of the KX125 was chopped up and turned around and new hand rolled cones were made and added from Randy Blevins. A custom silencer and end cap was also made. A Honda CR85 clutch and throttle cable were used since the 125 cables were too long. 2003 KX 125 lower fork guards were also modified to fit the 1991 KX125 forks. New brake cables were made from G&J aircraft in Ontario, CA. I used a 520 chain and went to Sprocket Specialists for the rear sprocket to fit the KX80 rear hub. The bike’s engine and frame is finished off with stainless steel allen bolts and new hardware.

From junk to a work of art. This is a one-off bike that took lots of patience, time, money and creativity to build.

Acerbis plastic and Decal Works graphics were used. The build hours are unknown since it turned into years. I tried to make it look as OEM as possible as if it came with the 125 engine from the factory.