Honda XR-75 with forward mounted Turtle suspension, J & B pipe ported head and manifold, CSCE cam and cam housing.

By David Coffin - In 1972 the most popular bike at the time was the Hodaka 100-B trail bike but it was the European CZ's, Maico's, Bultacos, AJS's, Ossa's, and Greeves that took command of the tracks of Southern California as they brutally pounded the hot, dry and dusty motocross tracks as riders fought for CMC or AME points.

In the kids leagues the early 70's saw the Yamaha Mini Enduros and Honda SL-70s as the king of the track. Most of the major riders rode on either one or the other at the time unless you rode in the PeeWee division where the micro sized Indian Bambino with its Minarelli engine and automatic clutch was the instrument of choice. In 73' the Honda XR-75 took over followed in 74' by the Yamaha YZ-80B. Other notable minicycles during this era included Chaparral, Bird MX, and Gemini's.

Mini's raced all over Southern California with organizations such as the CMC and AME at tracks like Saddleback, Carlsbad, Escape Country and Ascot, but the granddaddy organization of minicycle racing was National Minicycle Association or the NMA and was headed by Ron Hendrickson. (Today the NMA is now known as the National Motosport Association). If you didn't regularly race with the NMA you had no idea how competitive minicycle racing really was. The NMA was the AMA and the FIM all rolled into one. NMA races were regularly held at Indian Dunes on Friday evenings and they also ran the NMA Grand National Championship circuit and the NMA World Mini Grand Prix. Many of the well known rider's at this time included Jeff Ward, Brian Myerscough, Flying Mike Brown, Bobby Jones, Gene McCay, AJ Whiting, Shaun and Kelly Moran, Bobby Tocco, Lance King, Brad Roberts, Renee Payne, Kurty Hendrickson, Zack Barrett, Mike Koemans, and Lance Moorewood. Minicycle racing was as serious as the big guys and many of these kids went on to national and international prominence when they grew older. 

Yamaha Mini Enduro

When they first came out they looked very much like their larger 125 and 250cc Enduro series brothers which gave them that "big bike" look and feel. The Yamaha Mini Enduro was a 60cc rotary valve two strokes with a horizontal cylinder.

The Yamaha Mini Enduros were light weight and better suited to flat motocross tracks with wide turns like Indian Dunes. Other tracks such as Saddleback Park with steeper hills and sharper corners needed the torquer four stroke engines such as the Honda SL-70. The Yamaha also did well on the smaller oval track racing which was quite popular at the time. Lance King, Shaun and Kelly Moran who were among those that specialized in oval track and TT and who both went on to later race the brakeless 'speedway' bikes professionally in the United States and England. The small Mini-Enduro oval track bikes they rode usually were equipped with low slung expansion pipes.

The first thing to go when setting up a Mini Enduro for the serious motocrosser was to toss the flywheel points ignition and replace it with a pricey button ignition (still with points as there were no electronic ignitions at the time). Button ignitions had a smaller rotating stator that gave the engine instant response and quick acceleration.  Next was to replace the stock exhaust with an expansion chamber. A high pipe if you were racing motocross, a low pipe if TT or oval track was your specialty. Other racing kits available for these small bikes that included larger Mikuni slip-on carbs, racing cut rotary valves, and the usual piston kits, shocks, plastic handlebar levers and foam or pleated air filters. 

Honda SL-70's

As noted above, the Honda SL-70's were the bike you had to have if you raced the more demanding motocross tracks. Minicycle giants Jeff Ward, Brad Dutoit and Brian Myerscough cut their teeth on the SL-70's. General modifications to the SL-70 included racing cams, exhaust headers, machining and balancing the cranks, button magneto ignitions, larger Keihin (Yoshimuri) carbs, flow benching the intake and exhaust passages and often fitting the motors into new frames.

Honda XR75 

In 1973 the Honda XR75 turned the whole minicycle world upside down. The Honda XR75 was a vertical single cylinder twin valve overhead cam four stroke bike. The first time I had seen an XR75 was at the NMA World Mini Grand Prix up at Indian Dunes and it had everybody talking. These were real motorcycles scaled down and not another dressed up trail bike but out of the box racers. The XR's at the NMA race were piloted by Brad Dutoit, Chad and Terry McQueen (the last two were the children of actor Steve McQueen) and the bikes outperformed everything else. 

NMA Championship YZ-80 with a YZ 100 MX engine planted in  it. The engine was ported by legendary So California minicycle  tuner Ted Moorewood of CycleTown.

Racing modifications made to the XR-75 for most people was replacing the stock cam for a racing cam and replacing the stock exhaust pipe with a racing exhaust header. The most predominate pipes were the long tapered twin cone 'bazooka' design that I believe Murphy designed and were originally marketed by J & B Racing and later copied by everyone else. The next step for most people was to install button magneto ignitions and Lockhart oil coolers. The major Honda teams such as J & B Racing (Jeff Ward, Brad Dutoit) and Hacienda Honda would also flow bench the intake and exhaust passages, machine (thin and pork chop) and balance the cranks, larger Keihin carbs. The ultimate carburetors were the Keihin smoothbores only available from Yoshimuri but they were very pricey. The 100cc XR's had Powroll kitted stroked crankshafts.

Late in 1973 I designed and fabricated my own camshafts for the XR-75 by machining them from bar stock, roughing them down, heat treating them and I had Luther Iskendarian (brother of  the legendary Ed Iskendarian) finish grind them for me. The new cams used needle bearings rather than the (ugh!) plain bearing and I could specify the opening and closing angles of the intake and exhaust valves. I showed Jim Murphy the needle bearing arrangement and he was soon using the same setup with his engines as well. Soon after, almost all of the aftermarket racing product companies offered needle bearing cam assemblies.

Powerhouse Aftermarket Shops

J&B Racing (Jeff Ward & Brad Dutoit) were the first to offer racing parts. Later Hacienda Honda, PK Racing and others sponsored teams and sold their own racing parts. These shops were always present at major races and were the guys to beat.

The Rebirth of Motocross Suspensions 

I think I saw my first Maico with forward mounted rear shocks in 1974. It was at a CMC race at Saddleback Park This new design changed motocross forever and every manufacturer quickly adopted the long travel concept.  This added significantly greater wheel travel to the rear end of the bikes and it gave the riders a huge advantage on the more demanding courses. As early adopters we did the same on our mini's by moving the upper mounting points forward and were able to achieve 6-7 inches of travel. The forward suspensions came in two flavors. The first type had both top and bottom mounts moved forward. The bottom mount was at about 50-60% point on the swing arm and kept the shock orientated upright. The other type leaned the shocks forward at roughly 45-50 degrees. We chose the later, leaning the shocks forward and replacing them with longer units and dual rate springs. 

Yamaha YZ-80

A year after Honda came out with the XR75, Yamaha answered back with the YZ-80B.  Like the XR, the YZ-80 was a real motorcycle scaled down. It had an 80cc piston port engine with a vertical cylinder. Unlike the earlier Mini Enduro, the YZ was much torquer and could compete on the tougher courses like that at Saddleback. This bike came out of the box race ready though it was a trail bike by today's standards. Brian Myerscough rode YZ-80's in the stock class, modified 80cc class and the 100cc class and he couldn't be touched. 

Stock bikes are actually harder to build than the modified bikes. Stock bikes had to be stock in that is each component had to meet the manufacturers specifications and tolerances. To build a fast stock bike you need to check lots and lots of cylinder, heads, pistons. Some components were on the high end of the tolerance, and others were on the low end. To build a fast stocker you needed to look for the right combination of parts.  You check point cams, you check the ignition advance limits, you look at the port mapping and compare then with the factory specs. When you were done then you juggled the various components to get the optimum performance. Finally the bike would need to be tuned and jetted and you had a stock bike capable of getting into the first three slots in the first turn. All the best stock bikes went through this meticulous process. It didn't buy you a lot but with a good rider it got you into a competitive position at the first turn. Good enough to let your rider do his magic for the rest of the race.

Myerscough's Machines

Myerscough's modified machines were awesome. The motors were developed by Ted Moorewood and they were fast. I built the chassis for both 80 and 100cc bikes.  I also planted the 100cc engines into the YZ80 frames. 

The World Minis

I remember one NMA World Mini where Myerscough easily advanced into the evening 100cc finals except for one thing. As the day wore on the chain kept coming off. Fortunately he was so far ahead in the preliminaries he could still roll it back on by hand and finish in first. 

But why was the chain coming off? We thought that the chain might be loose but nope. It was fine. The tensioner wasn't bent or failing.  We made adjustments and would send him out numerous times to try it out but it stayed on fine. Minutes before it was time to go to the gate for the finals we had the bike up on a box warming it up and we decided to throttle it a few times while it was in gear. Then it became terribly obvious. The stock swing arm had rubber bushings and when you cranked on the throttle that 100cc engine would pull the entire swing arm assembly to the left and derail the chain. 

It didn't happen on the stand but I knew that when Brian got on the bike and began using some serious horsepower those bushings were getting squeezed big time. It never happened during the hot dry day when the track was dry and there was a lot of slippage but the evenings were getting down right cold and damp. The track was getting wet and traction was much greater. Combine that with a bike powerful enough to routinely beat pros in the 125cc class and you had a problem. I told Brian and his dad what I thought was happening and we had to be at the gate in a few minutes. He would just have to race it as is. You not going to be able to drop a chain when you are racing against Mike Brown (Indian), Bobby Tocco (Bird), Jeff Ward (Honda), Brad Dutoit (Honda), Jim Holly and twenty other of the nations best riders. At this race they didn't have a prayer when it came to power of this machine, but if you didn't use it and you dropped the chain they would eat you up in the blink of an eye. All we could say was go ahead an race like you have all evening. You need a lead in case it comes off. 

Under the bright lights of this cold damp and very late evening, the countries best riders were revving their engines over and over again as the ref was walked from one end of the gate to the other checking the line up. You had an incredible collection of two stoke and four stroke bikes booming and whining away while warming up at the gate as their mechanics (including myself) stood by. It was a pretty incredible sound. I can still hear Bobby Tocco's unusual Bird minicycle with McCulloch engine and automatic transmission. Finally the flag began to rise and all those machines began winding up at 8-10 thousand rpm's and the gate drops...  Brian was out in front and for most of the race he steadily increased his lead to the point where you could not even see the second place rider after a long stretch of sandy straightaway. But then bang, on the last lap Myerscough's chain once again comes off. It was on a set of three or four foot tall damp loamy whoop-dee-doos. Brian frantically tried to get it back on but he ended up walking. I don't even know who won the race. 

Brian went on to race in the NMA Grand National Championship circuit that year and take first place. All of our bikes except for the stockers had their swing arm bushings replaced with steel and aluminum bronze bushings. We also built a second YZ-100 as back up.