So. You want to race your GB500? My first advice is, forget it. Sit down, have a nice glass of wine, and wait until the urge passes. If it won't go away, sell the GB. Seriously. You'd be dollars ahead selling your street GB and buying an already developed and proven race Ascot from someone else. Besides, wouldn't you rather crash an ugly Ascot than a beautiful GB?

If you just *have* to race a GB, and nothing less will do, here's what I recommend:

Buy a clapped out 1986 XL600. I just checked and found two for sale locally for less than $750. Personally, I'd look for one around $500. This bike has parts you will need and could save you far more than it costs. It needs to have a decent motor, but otherwise it can be bent and rusty. Once you've stripped it of useful parts, you can rebuild it with leftover bits and re-sell it.

Rip your GB down to the bare frame. Put all those shiney bits into a couple of boxes that you can trip over in your garage until you decide to make it back into a street bike.

Put the motors from the GB and the XL on the bench. It's time for the fun to begin. One of the first things to notice is the XL motor is much lighter than the GB motor. That's because the GB has a walloping huge flywheel, an electric start, and massive side covers. But, you're going to either transplant the GB transmission into the XL, or move the XL crank into the GB.

The two crankshafts are the same except for the flywheel spindle on the left side. Since you want to ditch the heavy GB flywheel and electric start, you need the XL crank. The transmissions are different in two major ways. One, the gear ratios in the XL are more widely spaced than in the GB. And, two, the output shaft is longer in the GB. So, choose the best looking center cases and start putting pieces together (XL crank, GB tranny). Also, by using the XL flywheel, you get to use those really cool magnesium side covers off the XL! Strip the paint, clear-coat them, and none of the Ascot pilots will know you don't have stock side cases.

I almost forgot. Before you put the crank into the engine case, take it to your favorite machine shop and have them sleeve the small connecting rod hole down to 22mm. This is because there just aren't any good pistons made for the GB. There are, however, several good ones made for the XR500 which has the same bore/stroke/deck height as the GB. Unfortunately, the XR500 uses a 22mm vice 24mm wristpin. Wiseco makes a pretty good 93mm (1mm over) XR500 piston with a 10.5:1 compression. Others are available.

If you look closely while you reassemble the motor, you'll discover there is a difference in the Primary gears inside the motor. The GB has 29/72 and the XL has 32/70. Which one you choose will affect your rear sprocket size, I'd choose the 32/70 gears myself because of the limited selection of rear sprockets available for the GB. Sprocket Specialties will make up anything you want. You'll probably want to use a program like "Redline Gearing Calculator" (available on several Web Shareware servers--do a search on "redline" and "calculator") to determine the sprocket sizes you need for your particular track.

Also, look at the shift drums. The GB and XL shift drums are mirror images of each other. If you want your race GB to have "GP" (reversed) shifting, put in the XL shift drum.

If you want, you can have your friendly lathe operator shave about 3 pounds off the XL flywheel while the motor is apart. You can also remove about 4 pounds of useless steel if you delete the kick start assembly. You'll have to plug the oil passageway that normally lubricates the kick start gears, and plug the holes in the right side cases. This will also mean you either get to push start your GB like a real racer, or use rollers. It's your choice.

With the head, pretty much all you need is a little cleanup of the ports and match them to the inlet and exhaust. The port shapes are really good from Honda. Once you look at the XL head, you will see it's pretty much junk. You will need the XL rocker cover, however. Order a RS600 cam from HRC. It's way cheaper than anyone else, and they spent lots of money making it right. They offer two grinds, it's between you and HRC which one you get. Both are very good. The HRC cam is of the earlier non-automatic compression release design, hence the use of the XL rocker cover. I removed the manual compression release and closed off the hole. If you want a compression release and it's added complexity, have at it. You will need to use the high lift valve springs from R&D, available from Megacycle, LA Sleeve, and others, to prevent coil-binding the springs with the high-lift cam. While the head is off, plug the two smog system holes in the exhaust ports.

Find yourself a nice flatslide carburetor. 41mm is probably best, but I used a 38mm Mikuni with 4.0 slide, Q-2 nozzle, 6FJ41 needle, 250 main, 2.0 bleed, and 35 pilot. You can buy carbs pre-set pretty closely from Sudco or White Brothers if you don't want to fiddle with finding the right parts for used stuff. The flatslide will give much better flow and better throttle response than a CV type carb.

Now that your motor is done, look at your naked frame and try to remember what it was that started you down this path of madness. You need to build a new wiring harness for your fire breathing 500. Fortunately, you only need a couple of wires and a few other bits. Start with the CDI unit from the XL. (the GB CDI doesn't have enough advance for this motor any more--21 degrees vs. 31 degrees from the XL) There are five wires: black with white, black with red, blue with yellow, black with yellow, and green.

Wiring Diagram

Connect the black with white wire through your kill switch (taken from the XL--ditch the crappy GB switch gear) to ground. The black with red wire goes to the pulse generator (this wire comes out of the left side of the motor behind the flywheel. There are three useless yellow wires with it.) Connect the blue with yellow wire to the pick-up coil (right front of motor--there is also a green with white wire from the pick-up which needs attaching to ground). The black with yellow wire goes to the coil (the other coil wire goes to ground). The remaining green wire grounds to the chassis.

That's all the wires you need except for a tach. I ran for a year without a tach, but finally bought an Autometer tach designed for dragracing Briggs and Stratton lawnmower motors. It wires in pretty easily, a 12 volt source (I use 8 size "AA" batteries) one wire to the coil and one to ground.

Now, the chassis. At Willow Springs, the chassis modifications are highly restricted, so I didn't do many mods. But, some of the things others who race GBs have done are: Lace the rear rim onto the front hub. Get a wider rear rim and lace it up. This lets you use bigger rubber at both ends. You don't have much room at the back, so be careful. I used Pirelli tires because they were available when I needed them, but others have reported good results with Dunlop, Bridgestone, Metzler, etc. My next set will probably be Avon because they still seem to have race compound in 18 inch sizes.

If there is *any* slack in your swingarm bearings, take it to a shop and have new bearings fitted. The orginals are nasty plastic which wear out really fast. Do yourself a favor and have proper metal ones fitted. Just about any competent machinist can do this easily.

Change the fork springs to heavier straight rate Progressive Suspension springs. Drop the forks about half an inch through the triple clamps. Get a nice set of shocks from WP, Koni, etc. I never did, but you should probably fit a steering damper to the GB. I had pretty good luck setting the front oil level at stock weight and volume. That will give you a good starting point for adjusting to your own riding style.

For brakes, there's just not a whole lot you can do. The stock brakes are pretty marginal for racing, but if you're using the brakes, you aren't going fast enough. You'll need to ride this thing like a 2 stroke to win. Stay on the gas and never use the brakes. I've used both the stock Honda pads and Ferodo. They both work about the same, but the Honda pads are cheaper. You should probably also get rid of the rubber brake line and fit a proper braided stainless steel line.

Once the engine is back in the frame, haul the GB down to a good welder/fabricaor and have him make you a decent exhaust. The HRC RS600 exhaust is a much better starting point than the White Brothers/Supertrapp exhaust. For the 500, the tubes should probably be a bit smaller diameter all the way to the megaphone.

Exhaust specs

Note: click here for a full size version.

Airtech make a fiberglass seat in the same style as the GB stock seat. I haven't found a really good replacement fuel tank. Nothing that I've found is both lighter and fits. The search continues.... I used the stock clip-ons, levers, and footpegs. For a lightweight bike like the GB, they proved adequate. If, however, you want "trick-ness" the early foot pegs from the 900RR look like they would mount directly on the GB. Likewise the adjustable brake lever from the 600F2.

Hopefully, this has been enough to get you ready to go racing. Good luck, and let me know how it goes.