I've been encouraged to share my experience turning a run-of-the-mill GB500 into a giant killing roadracer (and a little encouragement goes a long way). As usual, it has turned out pretty long. Sorry.
I had been dragging my Ducati out to Willow Springs for open track days. After a year of that, I decided it was finally time to either make a commitment or be committed.
I scheduled myself for the licensing classroom session and novice race.
The classroom session is an all day affair--listening to the instructor pontificate endlessly, with short, much anticipated track excursions.Friday, as usual, I took the Ducati out for open practice. I was feeling pretty good. Until my motor dropped a valve and destroyed a cylinder. Bummer. That meant I didn't have a bike to ride for the licensing session, or to race. Double bummer.
I left the track that evening very depressed. My fun for the weekend was over, and I had a long, dark, lonely drive home.
By the time I got home, my depression had hardened into resolve. I had two other motorcycles in the garage. One of them was a BMW. Several months previously, I had the good fortune to be hired by Hughes Electronics to ride the track while they did some filming; and I had taken my BMW along just because I could. It was a horrible beast to ride fast on the track, so I knew I couldn't race that motorcycle.
The other bike, I had bought for my wife. I thought it was a perfect beginner bike--beautiful styling, good handling, not at all intimidating. Apparently, I was wrong. She let it topple over twice.
Once in the driveway, and once in the street. She was stopped each time and just let it get away from her. She wasn't strong enough to pick it back up, though. As a consequence, she refused to ride it. That bike was the mighty GB500.Her loss was my potential gain. When I got home from the track that fateful day, I immediately pulled the GB from the back of the garage and set to work. Lights off, fenders off, safety wire everywhere. Grab some old plexiglas from the back of the garage (yes, my garage is full of stuff I *might* need someday), cut it to shape, cover it with yellow duct tape.... Finally around midnight, it was as track ready as I could make it.
But, still one big problem looming. I had left my Ducati and trailer at the track so my Duc mechanic could pull it directly to his shop after the races. So, I had no trailer to pull the GB out to the track. I wasn't about to be beaten by mere inconvenience, so I grabbed a couple of wrenches and headed out to the Mazda minivan.
After I ripped out the interior, I woke up my wife, "Hey, I need you to help load the GB into the van." She was a pretty good sport but told me it probably wouldn't fit. And some other words that I won't print here.
I allayed her misapprehension by showing her how I intended to load the bike. Pushing it in backwards through the side door, up a two-by-six plank from the garage. With the rear wheel against the rear door, the front wheel just cleared the dashboard between the driver and passenger seat.
At 4:00am, I left for the track. Driving like a sleep deprived moron, I made it to the track in plenty of time to push it through tech (accompanied by the guffaws of my friends).
To cut this story short--I finished the license school on Saturday, got a good nights sleep and returned to race on Sunday. I was made to feel so welcome by the other 500 Singles racers that I decided I should "improve" the GB and continue racing it. Since this has gone on long enough, I'll end this here and post a recap of the work I did to the GB another time.
The Novice race? Well, let's just say, 'I finished.'
I didn't win. In fact, I was lapped during that first 6-lap race.
But, I finished. And I had so much fun, I knew I had to race the GB500.During the drive home, I mentally mapped out a plan for improving the GB. It desperately needed some weight loss, more power, better handling, and different gearing.
I started by ripping off the head and cylinder. Then (I always seem to do things backwards) I made some phone calls and inquiries to the Thumper list.
What I learned was not encouraging. The GB motor is essentially an XL600 sleeved down to 500cc. It shares a 24mm wrist-pin with the 600, but bore size with the XL500. Since the 500 has a 22mm wrist-pin, there were no high compression pistons specifically designed for the GB.
I called Arias Pistons in Gardena. I spoke with their motorcycle products person, John. He was encouraging, "Sure, we'd be glad to make a piston for you." I'd be glad to make pistons for the money they wanted, too.
I couldn't find any other source. Venolia, JB, and Wiseco all wanted me to buy more than one piston. So, I went to Arias with my old piston, some careful measurements, and a resin cast of my combustion chamber. Six weeks later, I had a custom designed and manufactured high-compression piston.
Meanwhile, I called MegaCycle and asked for a fax of all the cam specs available for the XL/XR/GB motor. Having previously talked to White Brothers, I was expecting two, maybe three, choices. There were six. After some phone calls to trusted advisors, I settled on the 173-x12 with short valve guides and high lift springs.
The next task on the agenda was readying the chassis for the track. I removed everything from the frame, including the wire harness. I was incredulous at how much all that junk weighed. The wires alone weighed five pounds. The stock exhaust weighed a ton.
With advice from an electrical engineer friend of mine, I built a racing wire harness. Five wires. No battery. Total weight, less than a pound.
By the time I was done, I had removed about 60 pounds of useless bits.
I changed the soft rubber brake line for a braided stainless line. I tried to swap the shocks for a set of Ohlins I have on my BMW, but the mounting eyes are the wrong size. I changed the exhaust to a WB/Supertrapp system.I swapped the carb for a 38mm Mikuni I got from a friend. Unfortunately, the carb spigot was smaller than the stock one, so the carb went off to a machinist for an adapter. I called Sudco for recommendations on carb setup and had to change *everything*...it had been set-up for a CR500.
By the time I had the chassis bits worked out, I had received the piston, cam, and valve guides. Since I had done valve guide replacement for some old British cars in my misspent youth, I figured it would be no problem in the Honda.
Three of the four new guides popped in easily. The fourth went about halfway in and stopped fast. I gave it a good whack with the hammer.
It broke. When will I ever learn? Off to the machinist, again. I took him the broken guide and asked if he couldn't, please sir, make me another? He gave me the "what kind of idiot are you?" look. Just a few dollars later, I had a guide. When I got home, I put the head back in the oven to reheat.My wife came in about then and wanted to know why the house smelled like burning oil. She was a bit perturbed to find out I was putting greasy engine parts into her nice, clean oven. If you are married, learn from this. There is a better way. Use your propane barbeque, outside, to heat soak engine parts.
The new valve guide stuck, too. Just like the first. This time, with lots of bad words and plenty of force, I managed to barely get it seated. Then I had to ream the guides for the valves. Have you ever tried to find a 6.5mm reamer? It's not easy. I found a fractional inch reamer that was close enough.
After that, everything went together pretty smoothly. Which was a good thing, since it was now only days until the next race. The time spent doing these mods was about three months. During that time, I had continued to race the Ducati. The month after I broke its motor, I crashed at about 100 mph when I hit somebody's oil in a turn. Between the bill for the Ducati motor, and the bill for the crash damage, I was more strongly resolved than ever to race the GB.
By race day, I had put the motor back together and installed it in the frame. I left the kick starter on (when was the last time you saw a racebike with a kickstarter?), so it wasn't a big problem to start it up in the garage and at least make sure it ran.
I trailered it to the track with high hopes. I had not had a chance to meddle with the suspension, but it hadn't seemed too bad the time before. And I vaguely recall being told by the previous owner that he had installed Progressive Suspension springs in front and new shocks in the rear.
So, there I was at the track with my new baby. All my friends were standing around admiring the work I had done to change the GB from a wimpy street bike into a steroid-crazed racer.
Nervously, because of the audience, I put on the choke and used the kick lever to position the piston at TDC. Winding up, I gave a mighty kick. Nothing happened. OK, singles are hard to start, I told myself.
So, I kicked again. Still, nothing. Again. Again. Again. Now my audience was starting to whisper amongst themselves. More kicks.
Still, nothing. I was sweating in the early morning cold of Willow Springs. From embarrassment as well as exertion. As my audience began to leave to get their own bikes ready, I finally realized I had forgotten to turn the kill switch on.Stealthily, I flipped the switch. On the next kick, the motor fired and kicked back on the kicker. I had removed the anti-kickback compression release. Man, that hurt. I thought I had broken my ankle.
From that point on, as long as I remembered to turn on the switch, the GB fired on the first (usually) or second kick. If I wasn't paying strict attention to holding my ankle rigid, it would invariably kick back and I would be hobbling around for the rest of the day.
Anyway, throughout the day, other Singles racers stopped by and made me feel tremendously welcome to the class. On the track, naturally, they were intent on passing me. But, in the pits, they were friendly and helpful.
The helpfulness was a double-edged sword, however. I had not had a chance to work on the jetting until I was actually at the track. Up to 3/4 throttle, the bike ran fine. On the main jet, it stuttered and hesitated.
Every one of the Singles racers I asked advised me the bike was jetted too lean. I scrounged up a bigger jet, and installed it between sessions. Sure enough, it ran better. But it still wasn't right. And if I backed from full throttle to a partial throttle, it hesitated enough to throw me over thehandlebars. Not exactly confidence inspiring.
I thought to myself, "If I want to win, I need to keep the throttle pinned anyway." So, I raced it that way for a couple of months trying increasingly larger jets. The hesitation would get slightly better, but just wouldn't go away.
I asked everyone what they thought. I was always told it was too lean.
One successful Single racer told me, "These things need a 350 main. They just won't run unless it's a 350."So, I put in a 350. It still didn't run right. Between races, I hauled it to my friend's shop and put it on the dyno. Another friend, an electrical engineer, helped me. The dyno trace looked as though the main jet was too lean. I bought more jets. Nothing worked.
My electrical engineer friend was as puzzled as I was. He suggested it might be an ignition problem. Fortunately, he works for DynaTek Performance Electronics. They make racing ignition systems. I felt very confident that he could fix the ignition, if anyone could.
We tore into the electronics. We tested everything. We replaced everything with known good parts. We tested everything again. Still, no progress. It had been four hours of dyno runs, and I was no closer to solving the problem. I was, however, closer to meltdown.
When my friend declared himself completely baffled, I gave up. I admitted defeat, and decided that the only way I could ever sort it out was to go back to the beginning and start over. Hopefully, I would find that I had missed something crucial.
I put in the leanest jet I had. Even smaller that the original jet I had used at the track. I dejectedly kicked over the motor again. It was absolutely incredible. The motor ran perfectly and pulled hard to redline. Talk about Jekyll and Hyde personalities.
The dyno registered 45hp. With the bike finally running right, I did some "development" testing. I borrowed a 41mm Keihin carb and reverse megaphone exhaust to test. With those on (and no changes to the carb which had been jetted for a Ducati 900) the dyno showed 50 horsepower.
If I ever had any money, I knew what I needed to buy for that bike.
But, money is always short, so I never did.Following the extensive testing on the dyno, I began doing well on the track. I could out accelerate all but one 500, and even some of the 600s. It was a locomotive.
With all that new power, I found the front wheel wanted to wobble and wander around in turns. That was pretty disconcerting. When I unscrewed the fork caps, I discovered the reason. Out popped a couple of 3 inch pieces of PVC pipe. The poor front springs had been so severely preloaded that the coils bound in the turns. With those nasty plastic chunks removed, the front settled right down.
In August, I was ready to move into second place, 500 Singles class; and 3rd Top Rookie Expert. During race day practice, I was passing everyone. Some novice on a Honda F2 came over in the pits to ask what kind of motor I had that I could pass him so easily in the turns. I was on top of the world.
Then, while running at redline down the front straight--on what was going to be my last practice lap before the race--I felt the clutch slip. Instantly, I had a complete loss of power, and the nasty sound of metal bits in a food processor filled the air.
Back in the pits, I pulled the motor out of the frame, and took off the head. I was sickened by the carnage. Three of the four valves were broken, all valve seats were shattered, the piston was completely gone, the cylinder was deeply scored, valve springs were shattered, rocker arms were broken in half. There was even a thick layer of aluminum swarf choking the carburetor.
It was over. Lots of fun while it lasted. But, finished nonetheless.
I gave the motor to a friend opening his own shop, and ordered lots of replacement parts (everything from the base gasket up). Months went by and I never got the motor to race.
Finally, I managed to reclaim parts of the motor so I could use them in my TZ/XL project. But the once mighty GB still sits engineless in the dark depths of my garage. Someday, perhaps it will again roar to life and terrorize competitors while women protectively draw small children closer as its thundering voice of doom reverberates through the hills.
I hope you've enjoyed at least some of this. If you are contemplating building a GB into a 500 Single racer, I'd be glad to correspond. And, I wish you luck.
randall zempel
rzempel@ix.netcom.com
01/29/97