ZK. The Ducati 350 Desmo Racer

Posted 8/19/2014

This story is about the construction of my 350cc Ducati Desmo GP race bike. It will be of interest to Ducati fans in general and Ducati singles fans in particular. It also illustrates the sorts of tasks required to turn a street bike onto a GP-style road racer, so anyone curious about that might enjoy this tale.

After racing the 250cc Ducati Diana for a year I decided that was enough. I needed a new race bike for the 1974 racing season. If I just wanted to race the smart thing would have been to get an up-to-date bike to race in the production classes, like a Yamaha RD350 or maybe a Honda CB750. But I didn’t want to just race. When I started racing one of my objectives was to ride fast in a safer environment than the street, but I also wanted to learn wrenching and tuning skills. I had learned a bunch but there was still more to know, so I decided to develop a Ducati 350cc GP-style race bike. I had made a racing pal, Terry, who was doing that and it looked like fun. I was also fascinated by the idea of Ducati’s Desmo valve gear, where the valves are closed mechanically instead of with springs. I couldn’t imagine how it worked.

I found a 1968 350cc Mark 3D. The bike had been left out so its running gear was pretty beat up. It had a rusted tank, torn seat cover and weather-beaten rubber and plastic, but the engine didn’t have many miles on it so it was a good starting platform. I went to work on it over the winter of 73-74.

First work was on the chassis. I stripped the bike down to the bare frame, hack sawed off all the parts that held street gear and had the frame nickel plated. The nickel-plating looked good, wiped clean and any frame cracks would be easy to spot. Unlike paint the nickel was impervious to brake fluid or fuel spills.

I read a book about fiberglass fabrication and created a racing seat and fuel tank. The seat was acceptable; the tank was really ugly but it didn’t leak and weighed less than the original steel tank. Eventually I redid the tank and the second attempt was actually attractive. I found an old one-piece fairing and fabricated some mounts for it. It had an open belly but the bottom of the Ducati motor was pretty smooth so no problem there.

I learned about automotive painting and bought a compressor and spray gun – no more rattle can paints. I wouldn’t say I became a good painter but good enough for a racing bike. I painted the bike blue and silver with red pin-stripe tape, then painted the fairing to match.

I installed rear set foot pegs and foot controls, bought from Vic Camp in England, and clip-on handlebars that allowed me to get into the proper racing crouch. The rear fender was replaced with a small rock guard to keep pebbles out of the carb. I used the stock Ducati forks for a while, but they were kinda wimpy so I sprung for a set of Ceriani road race forks. That was probably the most expensive single item added to the chassis. They were stronger than the Ducati forks and a lot nicer looking. I bought a new pair of clip-on handlebars to fit the new forks and constructed a new clutch cable. For the rear suspension I replaced the rusted Ducati shocks with either S&W or Mulholland shocks. I tried both and both worked.

The stock Ducati rear brake was a single leading shoe drum that was good enough, so I kept it after drilling some holes in the backing plate to let heat out. The stock front brake was too feeble for a race bike. I replaced the front wheel with one that carried a hydraulic disc brake. I had found a used wheel-truing stand, bought an Akront aluminum rim and a set of spokes and laced up the front wheel myself – that was a learning experience. I learned to let an expert do it next time. Both wheels were fitted with quality Dunlop KR race tires.

That completed the chassis modifications. Next up was the motor.

The Ducati singles had been raced for a number of years and there was a lot of knowledge about what modifications worked. I started with the intake/exhaust system. The stock Dellorto carburetor was replaced. It was hard to find jets and needles for the Dellortos, and its 29mm throat was too small. I hand-carved a special manifold from billet to allow the fitment of a 36mm Mikuni. I was really proud of that little piece of aluminum. I was able to use some machine tools, like a mill to drill the throat, but most of it was hand work with files. On the exhaust side I used a Ducati racing megaphone designed for a 250cc motor. It wasn’t exactly correct for the lower-revving 350cc motor but it was better than anything else I could get my hands on and it had a marvelous deep bass boom. Getting the jetting dialed in with the different carburetor and no starting point took some time but I learned how to read spark plugs.

I did a lot of work on the head. I added a second spark plug, which improved combustion quality and let me retard the timing a little. I used a Honda twin lead coil from one of their four cylinder bikes to ensure that both plugs fired at the same time. The second plug required slight re-routing of the cambox oil drain line. At first I used a rubber hose, ugly but effective, but eventually bought a drain line from a Ducati 450R/T, which had the new route built in. Much more professional looking.
A close up of the head-to-carb area showing the second plug lead, the rerouted oil drain line and the hand whittled adaptor for the Mikuni carburettor.

Some of the motor changes were for weight savings. I removed the kick starter shaft and gearing and made an aluminum plug to fill the hole in the side cover; it would be a push-start only bike. I took off the charging system from the end of the crankshaft so the motor could spin up more quickly and replaced it with an aluminum spacer, saving a few pounds. For spark I went to a 12-volt constant loss battery ignition using the stock points with the twin-lead Honda 12-volt coil.

It was well known that until the oil warmed up the oil pump would overwhelm the stock cambox drains and overfill the cambox. I added auxiliary steel-braided drain lines on the left side of the engine to drain oil from the camboxes directly into the main cases.

I bought a red/white desmo racing cam from Ducati. It was a higher lift, longer duration camshaft that allowed more fuel and air into the cylinder on each intake stroke. In fact the higher lift on the opening cam meant the closing lobe was also taller, and the cam would not fit through the normal cam installation channel. I had to grind away at one side until the formerly round hole was sort of egg-shaped before I was able to get the cam installed. To go with the hotter cam I did a mild port job on the intake port and put in slightly larger valves. The intake port on the singles was not well designed. It had a very slight downward slant but then made a sharp turn just before the valve. Fast moving air doesn’t really like to turn corners, so I raised the roof a tiny bit to try to make the curve smoother, less abrupt. I was very conservative because there’s not a lot of extra aluminum there and it holds the valve guide.

Another modification to the valve gear involved the valve springs. The street desmos had a light set of hairpin valve springs on each valve. Their purpose was to hold the valve closed if the closing rocker clearance got too loose so the bike would start. I wasn’t going to be that sloppy with the clearances so I tossed the springs and ground off the two little “wings” on the valve keepers that normally held the springs. Again, only a few ounces saved but they were taken right out of the valve train.
This shot is early in the development. It shows the extra oil drain lines, the stock Ducati forks, a dual-disk front end and the ugly 1st fuel tank. I quickly realized it was way over-braked and went to a single disk.

I did some work in the bottom end as well. I already mentioned removing the kick-start gear and shaft and the alternator charging system. In addition I drilled a bunch of holes in the clutch housing, saving another few ounces. The biggest change in the bottom end was replacing the off-side main bearing. The Ducati single was an all-roller and ball bearing engine. The crankshaft main bearing on the drive (left) side was good sized but the main bearing on the other side was smaller. This made sense for a street bike since the drive side had to power the clutch and gearbox while the other end of the crank only had the alternator and drove the oil pump. This smaller bearing was OK on the street but was a known weak spot when stressed by racing, and the standard fix was to put the larger main bearing on the right side as well. I couldn’t just enlarge the hole in the crankcase, however. In addition to being larger diameter the bigger bearing was wider and it interfered with the bottom bevel gear of the cam drive shaft. Not sure what to do I mailed Berliner Motor, the Ducati importer, and asked. I got back a memo reading as follows:

“Dear Sir: Mr. Rino Leoni, our Ducati technician has requested that we forward the enclosed diagram to you indicating how the bevel gear should be machined for the bearing conversion.”

Ducati people ought to recognize the name Rino Leoni, although in later years he changed the spelling of his first name to Reno because Americans kept calling him “rhy-no.”

The diagram was pretty clear; just trim the bottom bevel gear to a diameter of 50.2mm. There would be a little less meshing with the crank’s bevel gear, but what was left would be enough. Success.

It was a lot of work. I ended up with good looking, quick but slow race bike that was really a lot of fun to ride, handled beautifully, and sounded great. Its good torque and light weight allowed it to jump out of the corners, but its top speed was well below the Yamaha TZ350s that dominated the 350cc GP class in 1974. In truth it couldn’t even keep up with the TZ250s, but it was fast enough for me at that stage in my development. I started to learn racing skills although no one really noticed as my finishing positions were downfield. I couldn’t keep up with the TZs, even at Sears Point. I got to the point I could run with them in the esses or in the hill section, but as soon as we approached the start/finish area or the run up to turn seven they would disappear into the distance.

I also learned about working with fiberglass, automotive painting, wire spoke wheel construction, how to read spark plugs, how the Ducati Desmo system worked, and a little bit of oxy-acetylene welding. All in all it was a very productive two years.

The Final Product.

If I had built this bike in 1969 instead or 1974 I believe it would have been a contender but by 1974 it was too late. The water-cooled two-strokes ruled. Even though I really liked the bike I kept trying to get more top speed and toward the end of the 1975 season I tried some aftermarket parts with, frankly, disastrous results. I concluded that attempts to get more power would be at the expense of reliability, and I sadly put the 350 Desmo racer away. Eventually I sold it to my Ducati friend Chris and I understand that it’s still out there somewhere doing vintage racing. That makes me feel good. That bike belongs on a racetrack.