DUCATI 100 SPORT
When you’re an experienced man of the motorcycling world like our Pip, you’ve seen it all, done it all and never make mistakes, right? Oh…
Pip Higham restores a beautiful Ducati tiddler…
Iknow we’re all perfect here at CMM and we never cock up, but it’s time to own up. Let’s wind the clock back a decade and a bit and I’m searching for something old, small and Italian to ride around the Dolomites.
In a chance social call to my mate Bill Hawker (general good egg and bike nut) Bill mentioned to me that there was a strange little Ducati on ebay: “It’s a 100 bevel, never seen one!” says Bill.
I eventually found the incorrectly listed scoot and bought it a couple of weeks later off a Minardi F1 mechanic. Turned out he’d found it lying in a garage in Napoli and brought it back to Blighty with a container load of battered titanium and carbon fibre.
I’d never seen a 100cc bevel before and I wasn’t sure that I could do it justice, but over the coming months I managed to tease it back into shape. Since then I’ve just enjoyed having it around, but I’d never run it: can you smell that Elephant? Now, after spending 12 months moving house I decided it was time. I recall struggling to
set up the piston and rings due to the zero availability of pretty much anything for a 60-year-old bike that most people said didn’t exist! So I pulled the top-end and reacquainted myself with the puzzle.
I carefully honed the cylinder, not with a bobble hone I hasten to add, but a proper Snap-on solid, three-legged hone; a superb bit of kit, been in my box for about 40 years! Cleaning the bore after is super critical. I favour lots of pieces of cotton rag, then a squirt of WD-40 on the bore and wipe away, now repeat, and keep repeating with a clean rag every time. When the rag comes out perfectly clean you’ve removed all traces of silicon carbine and cast iron swarf; you don’t want that stuff anywhere near the insides of your motor. I measured up the rings and called up Adrian at IMD to ask his advice. I wanted rings a shade oversize so that I could gap them to fit the bore nicely. From past experience I’ve learned that the best (only) way to promote easy starting, a sweet idle and minimal blue haze is to run ring gaps as slender as possible. I’ve stripped motors with gaps of over one mill (40 thou) and they are ugly! Get that down to five or six thou and the difference is spectacular; that was the area I was looking for.
Adrian consulted his book of words and within five minutes he had the answer: “C90 oil ring and the old C200 (ancient cast-iron, push-rod, 90cc Honda) uses 1.5mm top rings.” Thanks Adrian! Couple of days later and I have both sets of rings ready to be carefully gapped. I use a diamond file, plenty of light and crossed fingers! It’s critical that the filed ends are dead square, so you gotta be careful! The other critical area, prep of the head and valves, had been dealt with when I did the initial build. Again, I have distinct opinions on the right, and the not so right, way to prep the head. I’d made and installed new bronze guides and then had the seats carefully cut using John’s Neway seat cutter. This machine won’t follow a seat that’s out of true, it cuts the seat perfectly concentric with the valve guide and, with the right cutter, at the right combination of angles, that much discussed ‘three angle’ valve job, 15 degrees and 75 degrees create the throat and the top angle with a slim 45-degree angle to contact the valve.
I’d machined some fresh valves (the originals were knackered) out of some old GS850 valves, same 7mm stem and great quality! John also refaced the valves with two angles so that the face of the valve was an exact match to the seat. Sounds a bit extreme I know, but if you want anything four-stroke to run half decently the above steps will pay you back in shovels! Just a quick word on grinding valves in: my opinion, don’t do it. Do you really think that there are rows of bods at Yamaha central twirling valves coated with grinding paste? Erm, nope! If the valve and seat are prepared correctly they will seal perfectly. No amount of grinding can compensate for a bent valve or a distorted seat, end of.
With rings, bore, valves and seats dealt with, I blued and scraped the cylinder head face for a perfect seal to the top of the cylinder sleeve. You’re unlikely ever to need to do this, but the little bike doesn’t
use a head gasket! Time to pop it back together; simplicity itself on any bevel Ducati. Simply line up all the dots on both top and bottom bevels, then engage the Oldham coupling on the vertical bevel drive. Fits like a glove! With clearances set and head nipped up, it’s show time. Final check of ignition timing, clean plug and sufficient oil (I prefer a simple 10/40 basic mineral oil for initial running, as this will allow the rings to bed in quickly. If you use synthetic the rings will glaze and are less likely to bed in. I realise this is counterintuitive, but trust me, this is what I do).
With the inlet valve cover removed I can check on oil supply to the top-end. I really like to see that liquid gold appear through the cross drillings in the rocker arms.i know, I’m sad. But I can still remember buying new rocker arms from Kings on Deansgate in Manchester for my mate Chas’ Elite (200cc Duke single) 50 years ago! The price nearly made me faint back then. They’re even more expensive now, but harder to find!
In second gear I flick the rollers into life and drop the clutch. To my surprise (it never leaves me) the little bike burst into life and five seconds later spluttered to a halt. It sounded fine if a little loud, but it also consumed about 50cc of fuel from the small test tank. I repeat the process: same. At this stage I’m looking for oil leaks, fuel leaks, all manner of checks and balances so the carb glitch is something I don’t need. The solution is sitting on the shelf, a mint 22mm Mikuni flat-slide that is a known quantity. I machine up a short adaptor out of Delrin (engineering plastic, tough, easy to machine) to install it. A few minutes later I crack the bike up again; it instantly sounds strong after ejecting a slug of unburnt fuel from the exhaust. There’s no sign of oil leakage. In fact there’s no sign of oil at all.
I shoot some oil on to the cam through the open valve cover access out of my oil can just to keep things safe while I wait for oil to show out of the inlet rocker arm. After 20 seconds I realise that I’m barking up a very dry tree, stop the motor and have a cup of tea while I consider my options. I decide to check the oil-pump which, thanks to Fabio Taglioni’s great design, takes about five minutes. The gear-type pump is super simple but incredibly effective, consisting of a pair of meshing gears contained in an aluminium housing with a ground steel back-plate which has four equally spaced mounting holes plus an inlet port and a delivery port.
As I pull the pump apart I can see the problem, and that’s me! I’ve assembled the pump with the back-plate 90 degrees out of whack. Not only can the pump not deliver oil, it can’t even pick it up Within a few minutes the pump is reinstalled correctly and the cover is refitted. Crank up the motor again and wait for oil to show and – again nothing! Pump off again, all
Emulsion tube, main jet and pilot jet.
perfect, and now I have oil supply to the crank, just nothing up to the top-end. I put my pressure oil can on to the oil gallery that supplies oil across the crankcase, up through the base gasket, up through the cylinder and then into the head where it feeds both rocker spindles and the camshaft, but there’s lots of resistance, just no oil!
Okay, head off, time to check on the route for the slippy stuff. As I lift the head and barrel I take a gander at the base gasket. The hole in the gasket which should allow oil to pass up from cases to cylinder is not actually a hole as when I made the gasket I neglected to remove a piece of gasket material about 3mm in diameter. I know, I know, but that’s the purpose of this, I messed up, and I’m admitting my crime, so punish me!
It takes about 20 seconds to trim out the hole in the gasket, and about 15 minutes to stop berating myself for being such a dick! But with the top-end back on I’m elated to see oil appear instantly emerging from the rocker pivots. A quick check on the clearances again and I can refit the valve cover.
The motor is sweet, doesn’t leak and with every minute of run time it settles to a stable idle with instant throttle response. Now all I need to do is sort out the problem with the original little gas guzzler. The UA19S fitted to the bike is in great shape considering its age, and extremely simple, so what have I missed? Let’s face it, if I can balls up the pump like that, anything’s possible. It doesn’t take long to find the problem. On the larger SSI style of Dell’orto carbs there is an externally mounted pilot jet. This allows easy access for cleaning or to change the jet size. I looked around the body of the dinky Dell but couldn’t find a pilot jet, or location for one, but on closer examination, sneaked up at the side of the emulsion tube in a
well concealed spot was a drilling that didn’t appear to have a purpose. A quick scan of an ancient parts manual showed a jet in this location. This explained the reason for the ravenous appetite for fuel I’d experienced at first start-up. Instead of a pilot jet with a 13 thou diameter hole in it I had a 5mm tapped hole that supped fuel like an Astley miner sups bitter at a free bar! A quick call to Travin at Eurocarb came up trumps, jets in stock, what size do I want? With the correct 35 (0.35mm about 14 thou) jet installed the little bike started up instantly and ran like a good ’un. I never cease to be amazed by old Dell’orto carbs; my old 125 Ducati, with a 25mm SSI fitted would return over 120mpg! They occasionally present a few set up issues, but everything can be fixed, especially if they have the required quota of jets fitted! So there you have it, lots of mistakes, and a few lessons along the way. You’re never too old to learn!