The 500 Yamaha single article

Dual caliper brake This photo shows the dual caliper/single rotor front brake I made on my TT500 road racer. I built this in the early 1980s. The two calipers were equivelant to a 4 piston Brembo. 72K jpeg file

A not very sharp instamatic picture of me on the very stock TT500 at my first road race at Oak Hill (Henderson TX) in 1978. 56K jpeg file

Side views of the KR framed TT500 with Yamaha 4 shoe front brake, with and without Dunstall tank. Circa 1979/80. 75K jpeg file

I was pleased to hear from Jean François Vicente in France, the current owner of the Kenny Roberts' framed TT500 roadracer I built in the late 1970s. JFV runs a shop that specializes in the TT/SR/XT bikes from Yamaha. More photos of my racer as it is now are on Jean Vicente's site:

Chuck McAvoy Chuck was AFM 600 Superstreet champ one year on his Dick Mann-framed 500 Honda single. We teamed together on my Yamaha single (shown here) for one of the Budweiser-sponsored 4 hour races at Sears Point. The exhaust valve adjusting screw backed out after a couple of hours and punched a hole through the rocker cover. 48kb jpeg file

This article is about the Yamaha TT500 I road raced for 6 years, along with various tips and modifications for road racing and observed trials (observed trials?).

The History

I had a 500 Matchless single I rode around on in the early 1970s (including a hare scramble and MX - I wasn't last either!). I also had a 350 Ducati cafe racer project, so I was developing an affinity for thumpers. In November 1976 I moved to northern Louisiana where I had been hired to work. I rented a storage space from a local Yamaha and outboard motor dealer, and got friendly with one of the people who worked there. I had read articles on the new TT500 Yamaha, and the 1977 "D" model looked to be a bit nicer than the first year bike. Since I was in a semi-rural area a big thumper dirt bike had quite a bit of appeal. I arranged to buy a bike from the Yamaha dealer and took delivery in December 1976. I immediately ordered a set of the then-new Works Performance shocks, and an aftermarket muffler/extension pipe kit for the exhaust. Otherwise the bike remained stock except for a 3.20x21 Barum front tire. I rode the bike for several months in the dirt and found it to be fun, if a bit heavy for tight woods riding.

A friend of mine in Houston had a Ducati 750 and a set of Bates road race leathers, and one time when I was visiting him I tried the leathers on, just for fun. When I found that they fit me, and Michael wasn't adverse to me borrowing them, I decided to give road racing a try. My 350 Ducati was the first bike I considered, but the lack of an appropriate tank and seat was a bit of a drawback. Then I realized that with minor modifications I could run the TT500. I bought a set of Honda 400-4 low bars, a bigger countershaft sprocket and an Avon 3.00x21 rib front tire. A Dunlop 4.10x18 K81from the back of my Ducati and the removal of the skidplate saw my dirt bike transformed into a road racer.

I raced the TT in this trim several times with the Central Road Racing Club at Oak Hill (Henderson, TX) and Texas World Speedway (College Station, TX). I was having a good time, but a stock motor, dirt bike brakes and wheels were obviously not the hot set up for road racing. I had a local machine shop weld some lugs onto the TT fork sliders so I could use the Yamaha TD2 four shoe drum brake off my Ducati, but they warped the sliders beyond recovery, and I had to come up with an alternative plan.

About this time one of the magazines had an article on the TT500 dirt track frame Kenny Roberts had developed and put into production. It seemed like the best thing available that would be a bolt together proposition (I didn't have any fabricating tools at that time), so I ordered up one with a Star quick change rear wheel and steering head bearings/stem to fit a set of Ceriani 35mm road race forks. The addition of the Yamaha 4 shoe front wheel off my 350 Ducati gave me a rolling chassis. I cobbled together a seat, and bungeed on a plastic dirt bike tank I had to make the bike rideable. I also purchased and installed a Vesco TZ250 fairing.

I had an engine tuner in Ft. Worth TX port the cylinder head, and I installed a Pro-Tec 1.75" head pipe, megaphone, high lift cam, and dual rocker cover oil line kit, a 40mm Dell'Orto carb kit from Axtell, and a Venolia piston. Much better! Now I had more power, and a chassis much more suited to the tarmac.

I finished the 1977 season as the high-point novice rider in the CRRC 500GP class - probably due more to the limited number of other novices in the class than any overwhelming skill on my part. The high point of the season came at a race at Texas World Speedway. I had entered 250 GP and was gridded on the 5th row. Freddie Spencer was still a club racer at this time, and was gridded on one of the front rows on his TZ250. When the flag dropped the thumper charged off the line (something TZ250s and TD3s weren't known for) and took the lead. I was able to maintain the lead almost to the first corner, by which time the two strokes were up on the pipe and rocketing by. It wasn't for very long, but I can honestly say that I've passed and lead Freddie Spencer in a road race!

In the spring of 1978 I moved to Los Angeles and started planning my Southern California race schedule. I was lucky to get friendly with Barney Tillman, of Tillman Sports Center in east LA. Barney was the Dunstall importer, and dealt in Norton, Ducati, and several other European marques. Barney's mechanic installed a Dunstall Command tank and a replica KR750 Kawasaki seat, and neatened up the rough fairing brackets I had made. The bike was much more presentable now.

I rode the latter half of the 1978 season and first half of the 1979 season with the LA chapter of the American Federation of Motorcyclists (AFM). These races were at Ontario Motor Speedway, Riverside, and Willow Springs. I acquitted myself with honor if not distinction, except for crashing in a slow corner at Riverside the first time I took my girlfriend to the races! The crash was entirely my fault, as I had gotten a good start and was ahead of one of the fast thumper guys. I figured I'd just gas it up coming out of the corner and stay ahead of him, and ended up lowsiding into the tires on the outside of the corner. A lesson learned - ride your own race, not the race of the people around you.

While I was in LA I had a Carrillo rod installed. The stock Yamaha rod is copper plated in the small end of the rod, and this plating doesn't hold up under road race use. The Carrillo rod is much stronger, and has a real bushing. After the new rod was installed and the crank shaft rebalanced I never had rod troubles again.

In late August 1979 I transferred to San Francisco, and started racing with the North chapter of AFM at Sears Point and Laguna Seca. My girlfriend and I rented a house after a year or so in a apartment, and I finally had a reasonable amount of garage space. I bought a lathe, milling machine and gas welder, and started learning to use them. I switched the 4 shoe front brake to a Suzuki 500 single disc front end that I mounted two 32mm twin piston Brembo calipers on, though it didn't work well because I had too large of a master cylinder piston. I then switched to a 38mm Grimeca caliper and Kosman 13" floating cast-iron rotor on an aluminum carrier I made. I also put a floating-carrier Brembo caliper on the rear disc. A local exhaust builder named Tony Williams built a nice high pipe, and the days of dragging the down pipe were over. I also put a Megacycle 5180 cam and new rockers in the engine.

When the bike was fitted with the Suzuki/dual Brembo front end my buddy Kurt Bickel (a former singles racer and then racing a TZ250) entered the bike in the FII class at Sears Point. Even with the handicap of an inadequate front brake Kurt managed a 1:55 lap time which was one of the faster thumper laps put in at Sears Point in the early 1980s.

The Dunstall tank was a bit wide at the back, with sharp corners that made nice bruises on the inside of the thighs. I was going to ride the bike with Chuck McAvoy (AFM 600 Superstreet champ on a Dick Mann-framed 500 Honda single) in a Budweiser sponsored 4 hour at Sears Point so I had a local fiberglass guy build a TZ replica glass tank for the bike. That and a TZ replica seat dressed up the bike's appearance a bit more. We made about 3 hours of the endurance race when the locknut on the exhaust valve adjusting screw backed off, letting the adjuster screw itself out. This greatly increased the valve clearance, and punched a hole through the rocker cover too. The next time the bike was run, both of the adjusting screw locknuts were safety wired to the rocker!

My best placing with AFM was a fifth in the singles class, and after 6 years of racing the 500 I was ready to try something new. I had raced my 750 Ducati for a while (until the rod broke in turn 10 at Sears Point) and decided I wanted to keep racing in the Battle of the Twins. I traded the 500 for a stock 750 Laverda, and the Yamaha went to live in New Mexico.

A few years later my bike resurfaced in an issue of the Four Stroke Singles National Owners Club (FSSNOC) newsletter. The new owner told a glowing story how the bike held the lap record at Sears Point, etc, etc. I set him straight on the bikes history, and the Yamaha disappeared again. In the early 1990s I met a fellow at a swap meet who told me the bike was living in Arizona, and that he had almost purchased it. Again the bike disappeared. The most recent surfacing was in, of all places, Classic Bike magazine. The new owner, a 500 Yamaha fan in France, had just purchased this prototype racer that Kenny Roberts had built and raced. I recognized my bike, and got to write another letter telling the true history of the bike. I'm curious how much extra money the new owner had to pay for the spurious history. I expect the next time I see anything on the bike it will be how it was used by KR to win all three of his World Championships!

Modifications for 500 Yamahas

The 500 Yamaha is basically a very strong and reliable motor.

My friend and tuner, Craig Hanson, says the Yamaha crankshaft is much stouter than a comparable 500 Honda crankshaft. Once a Carrillo rod is installed to prevent the small end wear the stock rod is prone to it will last for years of racing. The 500 does have a very short rod, and the rod to stroke ratio is also short - about 1.6:1. Most race engines try to get a 2:1 or higher rod/stroke ratio. I was told that Mert Lawill was involved with building a 500 dirt tracker, and they weren't having luck getting the horsepower they expected until they put a spacer under the cylinder and installed a longer rod. Then the engine responded to tuning. Craig built several short stroke 500s in the mid 1980s. I think they were 92mm bore by 75mm stroke. He destroked the cranks by welding up the outside of the big end pin hole in the flywheel, and then boring a new hole at the shorter radius, finishing up with a grinding operation on the inside of the pin hole. The shorter stroke and a longer rod increased the rod/stroke ratio quite a bit.

Craig also welded up the combustion chamber to increase the compression as well as the squish band area. Big valves, a downdraft intake port and welded exhaust port were also added to the head. One drawback of the downdraft intake is that the motor and carb probably won't fit back into a stock chassis any longer. This may not be such a bad thing (see below).

Venolia and Wiseco pistons are available now, and I think I'd probably run a low dome Wisco piston, and use Craig's method of welding the combustion chamber to get the compression up. This yields better flame propogation, increased turbulence from the extra squish, a lighter piston, and an efficient combustion chamber that should require less ignition advance.

The stock Yamaha sends oil to the intake rocker shaft only. This is odd, since the exhaust is the hot side, and it needs lubrication more than the intake. Several different companies have offered dual oil line kits. These use a long banjo bolt at the case to feed both oil lines, which then connect into the left ends of the rocker shafts.

At Craig's suggestion, I modified the rockers to accept adjusting screws from a Ducati single (same as a non-desmo 750/860). These have a ball bearing with a flat in it that bears against the valve stem. The increased surface area of this type of adjuster prevents pocketing of the top of the valve. The underside of the rocker arm had to be relieved to make clearance, but as long as you don't get the arm real hot when grinding and take pains to smooth any potential stress-raising roughness away it shouldn't make the rocker less reliable.

I ran the Megacycle cam for a season with new rockers, and some wear was present at the end of the season. I then coated the cam and rockers with KalGard's GearKote, and recoated every time the engine came apart. No further wear was ever apparent. The Megacycle 5160-2 is the usual cam of choice for road racing, but it came out after I had purchased the 5180, and as I had good luck with that cam I didn't bother to change to the shorter lift longer duration 5160-2. I also ran Megacycle's vernier cam sprocket, which allows precise timing of the cam. The cams, sprockets, R/D spring kits, hardfaced rockers etc are still available from Megacycle, and can be obtained by your local dealer or through me.

The stock ignition isn't up to race use, and the SR500 electronic ignition isn't any better, and is probably less reliable than the points set up (as well as heavier and more expensive to replace). Craig developed a modified version of a Lucas RITA electronic ignition for the unit BSA single that has worked quite well. I can provide this ignition, but I do need the clutch cover and points shaft to do some modifications to them. A new ignition coil and conversion to 12v DC is also needed, but I can supply this too. For roadrace use you can eliminate the flywheel and run total loss from a battery. This conversion can be put on an SR500, but the XT/TT side cover and points shaft are needed, along with machining the points shaft receptacle in the right hand center case.

All the serious road racers used TT or XT frames instead of SR frames. The dirt bike frames were more rigid and handled better. The SR frame is prone to bending the backbone upwards under heavy braking. If you have the head steady installed this pulls up on the cylinder, and cracking of the cylinder may ensue (this can be a problem on any pumped up 500, SR frame or not). The trick here is to take the small cylinder to head stud next to the spark plug and convert it to a long bolt that screws into the base of the cylinder. Details are a bit fuzzy now, but I think that a similar modification may also be performed on the right side of the cylinder as well.

The 500 really needs a properly designed frame for road racing. Craig has de-raked some standard frames, and a longer swingarm from an XS650 can be installed as well. The KR frame I used handled quite well, in fact Chuck said he liked it better than his Mann-framed Honda. There are some other dirt track frames for the 500, but the ones I've seemed were a bit spindly for road race chassis loads. Special road race frames have been made by people in England and Japan on a limited basis. Craig thought about doing a batch of frames, but by that time many of the thumper racers were going to Rotax and other 4 valve head engines. With the current product liability/sue everyone climate here in the USA Craig and I are not real comfortable with doing customer frames for road race. Not that we wouldn't like to do them, we are just concerned that we'll get blamed/sued for someone else's mistake.

TT500 Observed Trials (?!) Modifications

Yes, the TT500 can be run in AHRMA vintage observed trials (Modern Classic class). I had heard of a Yamaha TT500 trials special that had been built in Northern California. I tracked the owner/builder down and spoke with him about the bike. I've asked Kim for some photos, and I'll add them here when I get them.

Kim Proctor sounds like he has been riding for a good while, as he mentioned that he used to ride a Yamaha CT-1 175cc in enduros, which would place him riding competitively in the late 60's/early 70's. He told me that he had purchased a 1976 TT500C that went through various dirtbike incarnations before being turned into a cafe racer which was then sold. He said he was very fond of the bike, and had often regretted selling it.

Kim had been riding Class A trials, and has as a neighbor Derek and/or Clive Belvoir who has a Velocette engined vintage trials bike that I believe was built by Dick Mann, using an Ariel HT frame (I've seen the Velo and it is a lovely bike - but then that seems true of pretty much everything Dick builds). Kim found the idea of a big thumper trials rig appealing, and the memories of his original TT500 prompted him to get another one and convert it to trials trim. As an aside I'll mention that Mick Andrews had done work on a prototype TT500 trials bike for Yamaha (per "Four Stroke Finale" the book on the Honda trials bikes), but I've never seen any indication that the bike saw competition.

Kim obviously has a well-equipped shop, with a mill, lathe and welding equipment being mentioned in our conversation. Since the bike runs, and runs well, he must know how to use the equipment too.

Chassis: Kim wanted to avoid cutting on the main frame section, since the Yamaha carries its oil in the frame tubes. He trimmed off the rear subframe, and jacked the back-end of the bike up until the fork rake was around 24-25 degrees (stock is in the 30 degree range). This process also increased the ground clearance, and reduced the wheelbase several inches. He then built a new subframe modeled on the Ariel HT frame. He fitted a sprung Wassell solo seat to give a bit more suspension when riding between sections. The stock forks have been raised in the triple clamps, also helping with the steepening of the front end. Kim purchased some Gas Gas footpegs and tried them in several positions until he found the one that he liked best and then welded them on permanently. He cut the fuel tank into seven pieces with a band saw and rewelded them into a slimmer shape. He is still using the stock swing arm, wheels and forks. Some trials bars, a 70 tooth rear sprocket and reworked chain tensioner pretty much finish the chassis mods.

Engine: The TT500 can be a bit snappy at low rpm, so Kim worked to fix that. The engine proved to have a fairly fresh piston, so he chucked it up in his lathe and dished the top, reducing the compression. Next, he slotted the cam sprocket so that he could advance the cam about 12 degrees. A new pipe in the stock size tubing was made that was a bit lighter and better tucked in, and a J & R aluminum silencer was attached. He tried a 28mm Bing carburetor, but ended up returning to the stock carb which he felt gave better performance. He told me these modifications calmed the bike down a great deal, and that it trickles along in a very controllable manner, yet will accelerate sharply when needed. I had thought that some extra flywheel weight would have been helpful, based on the days when I ran a TT500 in the dirt, but Kim thinks the bike works fine with the stock flywheel.

Craig feels that the long rod (see above) would be of benefit to a trials bike too. He thinks the short rod will make the engine jerkier at very low rpm, due to the way the piston is rapidly accelerated/decelerated at mid-stroke. This is reduced by the long rod, and should smooth things out even more.

Since the bike is largely stock components, it still weighs in at a pretty stock weight - in the mid to high 280 pound range. Kim agreed with me that substantially lighter wheels are not that hard to come by, and certainly wouldn't hurt things. He has his eye on a couple of $25 125 MX bike carcasses in a nearby yard, so I would expect that different wheels might be fitted shortly. The Yamaha frame isn't a particularly lightweigh bit of ironmongery either, and a Miller High Boy- style chassis could be built that would probably save 30 pounds. A bit of attention to lighter forks etc could probably see a bike in the 220-230 pound weight range. While this is still noticeably above the 180 pounds of an RTL250, it is still less than the 242 pounds of GOV132, Sammy Miller's heavily modified Ariel HT.

Kim says he is very pleased with the handling of the bike, and feels that it has better balance than then TL250 Hondas he has ridden. At a recent vintage trials he was only a couple points behind a 348 Cota in the Modern Classics Masters class, so while he doesn't claim that the TT-trials is as good as an 80 pound lighter Cota, it isn't all that much worse either. Then again, he may just be a very good rider!

I know that running 500 Yamahas can be purchased pretty cheaply, as a friend of mine just picked one up for $350. There is no reason why similar modifications could not be performed on a XL250/350 or one of the Kawasaki 250 four-stroke singles, creating a fun to ride and not too expensive vintage trials mount.

Congratulations to Kim on building an interesting bike, and I'm looking forward to seeing it at a trials soon.

I've borrowed an empty 500 engine from a friend, and after I get it measured up I'm going to draw a real trials frame for it. Just what I need, another project!

For those of you interested in offroading Yamaha's single you'll probably enjoy Les Tinius' web page about the different versions of Dick Mann's TT500 chassis

Les includes information on the renovation of his own DMS TT500. Les also runs a suspension tuning business in the Pacific NW and you can get more information about that at his site.

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