WE BUILD A FULL-RACE HUSQVARNA WR300 MOTOCROSS BIKE:

July 6, 2010
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Husqvarna is back on the map in America. They have a super-powerful offroad team, led by stars like Ty Davis, Glenn Kearney, Nick Burson and Bobby Garrison, and a full line of offroad machines. So, when the MXA wrecking crew went looking for a competitive Husqvarna two-stroke to race in the 450 class, we quickly settled on the WR300 enduro bike. In truth, the WR300 started life as a bolt-on big-bore kit for the 2008 WR250 and CR250, but with the CR250 no longer in the line, the newly minted WR300 became MXA‘s obvious choice to turn into a big-bore motocross bike.

The icing on the cake was when Husqvarna factory rider Bobby Garrison used an identical WR300 to MXA’s project bike (save for Marzocchi forks) to win the 2010 World Two-Stroke Championship. Additionally,  Glenn Kearney campaigned the GNCC on a Pro Circuit-tuned WR300 (with former Jeremy McGrath mechanic Wyatt Seals as his crew chief). To build a full-race WR300, we needed the help of Husqvarna, Pro Circuit and Wyatt Seals. Pro Circuit agreed to build MXA a replica engine. We set aside $1500.00 and put the gears in motion.


Designed as an enduro bike, MXA had to stiffen the forks and punch up the powerband to make it a race-ready motocross bike.

WHAT DO YOU GET FOR THE MONEY?

The 2010 Husqvarna is a mild-mannered 293.1cc enduro bike. And since it was a kit engine that had become a production engine, MXA‘s goal was to turn it from a gentle trail bike into a full-blown racer. This meant porting the cylinder, milling the head, changing the piston, swapping out carburetors, ditching the enduro flywheel, revalving the Kayaba forks and mounting a special Pro Circuit exhaust. The complete project was labor intensive, and here is the list of mods:

Cylinder porting: Mitch Payton personally ported our WR300 cylinder. He had also done Glen Kearney’s GNCC race engine and knew exactly what to do. The cost? $329.95 (for cylinder and head mods)

Cylinder head: The stock compression ratio is a mild 6.9:1, so Pro Circuit milled the head to raise the compression to racing levels.

Piston:
The flat-top Husqvarna piston was replaced with the domed Vertex piston from a Gas Gas 300. The price? $250.95.

Pipe: Believe it or not, Pro Circuit makes a Husqvarna WR250/300 exhaust pipe. The cost? $229.95 for the pipe and $119.50 for the silencer.

Ignition:
We stole the ignition off a discontinued Husky CR250. The new flywheel was lighter and smaller for snappy throttle response.

Carburetor:
Wyatt Seals sent us one of Glenn Kearny’s 38mm Keihin PWK carbs (with a 162 main and 48 pilot) to replace the stock Mikuni TMX carb. The price? $250.95.

Forks:
Although the 2010 Husqvarna WR300 comes with Kayaba forks, they are old-fashioned, open-cartridge forks (circa 2004). They were very soft, so we simply looked up the settings that we used on our 2004 KX250 (a bike that came with these forks) and had Pro Circuit’s Bones Bacon revalve them and install stiffer 0.44 kg/mm springs. The cost? $179.95 for the revalve and $129.95 for stiffer fork springs.

Shock: We didn’t touch the Sachs shock absorber.


There were portions of the dyno curve where the modified Husqvarna WR300 produced ten horsepower more than the stock bike. Pro Circuit’s Mitch Payton did all the work himself. He ported the cylinder, milled the head, spec’ed the exhaust, swapped the Mikuni for a Keihin, borrowed a Gas Gas piston and used the ignition from last year’s Husky CR250. It was a job well done.


HOW MUCH WORK WAS INVOLVED?
 
Since every modification on our transformation from WR300 to CR300 required outside assistance, all we had to do was remove the forks, ignition, carb and top-end. Pro Circuit did all the leg work on the suspension and engine?and the MXA gang bolted it all back together.

WERE THERE ANY SPECIAL NEEDS?

The ignition and carburetor are off-the-shelf Husqvarna parts, but not off the WR300 shelf.


The handling was very good. Every test rider though the bike looked like it had a long wheelbase, but when measured it was actually a short wheelbase. Go figure!

WHAT WAS IT LIKE TO RIDE?
 
On the dyno, our full-race WR300 produced 47 horsepower. The stock WR300 made 42.4 horses. Better yet, at 7000 rpm, the Pro Circuit engine made four more horses than the stocker. At 8000 rpm, the race engine made six more horsepower than the stock engine, and at 9000 rpm the Pro Circuit engine was ten horses better. That is a massive improvement. For info on the engine call Pro Circuit at (951) 738-8050 or go to www.procircuit.com.

As it rolled out of the factory, the 2010 WR300 was very easy to ride. There was none of the rushing around like with a purebred motocross engine. After we got our Glenn Kearney replica engine done, things happened a lot faster. Gone was the mellow nature of the 300 engine, and in its place was a hard-hitting, more aggressive midrange and a large increase on top. The stock engine only topped 40 horsepower from 7300 rpm to 8900 rpm. The Pro Circuit engine topped the 40 horse mark at 6800 rpm and stayed up there until 9500 (that is a 1100 rpm gain at maximum power).


In stock trim the Kayaba open cartridge forks were pitifully soft. We had Bones Bacon revalve them?(to 2004 KX250 specs, because that was the last bike we remember this model of fork on). We added stiffer fork springs, but didn’t touch the stock Sachs shock. The suspension mods improved the overall feel of the cross-country-specific Husky.

We weren’t all that surprised by the performance gains, because we ported it, raised the compression, pumped up the carb, lightened the flywheel and mounted a high-performance expansion chamber. The real surprise would have been if we had not gained a significant amount of power.

The biggest single improvement we made to the 2010 Husqvarna WR300 was to stiffen up the forks. With new valving and stiffer springs, MXA test riders were able to push the WR farther and faster than we ever could have with the stock, marshmallow, 0.42 fork springs.

Were there any issues? Gearing presented some dilemmas on a motocross track, because “WR” stands for wide ratio. We eventually geared it down enough to make it work.

For more information on the Husky WR300 kit, go to www.procircuit.com or call (951) 738-8050.

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