At Croft 2004 the lower bevel gears, that drive the vertical shaft that turns the cam, stripped. It seems like a missing piece of circlip from the top of the cam drive shaft had broken of and may have started the bevel gear break up. I found it in the crank cases, but not marked in a way that would make me think this was the only possible explanation. The other theory is that I didn't get the adjustment of the bevel gears right in the first place. Hard to tell when there's so little left!
Further investigation also showed that the big end was worn, so I took the decision to scratch the rest of the season rather than risk more running time with the worn big end. Fortunately, the con rod eye was hardly damaged and did not need anything more than a slight grind to remove the wear. Tony Brancato did this for me as well as make a new crank pin and thrust washers, and find a new roller bearing cage. He also assembled it as the job requires a press, V-blocks and appropriate experience - all three of which I lack.
The wheels were a bit scruffy, with corroding spokes and hubs, so a trip to Central Wheel Components has resulted in a new WM2 rear rim, old front Borrani polished and as new, new stainless spokes and black powder coated hubs. Went down from a WM3 on the back because they are a bit large for the 250 - unnecessary un-sprung weight and the extra width results in greater twisting forces on the not very strong swinging arm
The exhaust has been ceramic coated by Camcoat and a shiny new alloy tank may arrive from T A Baker in time for the first meeting
I'm adding a rev-limiter to try to prevent damage due to missed gear changes
I've also spent some time trying to get the piston to fit the head a bit
better, as it was not shaped to match. I took a mould of the head which i cut in
half so that I could see exactly what was going on, and this gave me a general
idea of what needed removing from the piston.
Then a few hours with the lathe, and trial
assembly with pieces of solder between head and piston and I was able to
remove 1mm of packing from beneath the barrel, so the piston is now 1mm
further into the head at top dead centre.
I measured the combustion chamber volume as 27.7cc which means the actual compression ratio is 10:1 - more is possible. Back of the envelope says lowering the head another .5mm would give me 11:1...... later!
With the piston closer to the head, there was no room for the spark plug earth
electrode. I was using 2 compression washers last season, and with another mm gone I
needed a better solution - an EGP series racing plug from NGK (www.sparkplug.co.uk),
which has the earth electrode welded to the inside of the plug shell at 90deg,
so that it does not protrude into the combustion chamber at all.
Only trouble is that it's a long-reach plug, so an ally spacer was needed to get things right. Not quite back to square one!
The new arrangement has full engagement of the plug threads in the head and much more space between plug and piston
Slight worry that the reduced thread engagement will mean the plug runs too hot, and maybe I should have bought a 9 rather than an 8
Having worked out how to do it, I had to get the compression ratio up to 11:1. So after Mallory I stripped it down and took a tad more off the squish area of the piston to get it a little closer to the head. Above left shows some pieces of solder held in place with grease, just after I'd rotated the motor with the head on and only one 0.5mm base gasket under the barrel. You can see the exhaust valve has left a small indentation - and you can also se the mark left by the piston hitting the valve at Croft in 2004.
Middle picture shows the 4 pieces of solder that were in the squish area, with the pictures below showing some thickness measurements. This is how I left it.
At Mallory in April I was a little concerned that there might be some detonation going on. With the head off there was no evidence of it, but as iwas going to raise the compression some more, I decided to play safe and ordered enough Hiperflow 102 octane leaded to last the season.
Used it for the dyno run and it seems to work. If I have time I'll do a comparison run with 4star later in the year - it would be much cheaper as well as more convenient!
Did a fuel consumption check at Snetterton: 6 litres used for 3 laps of practice, 4 sighting laps, 23 laps of racing and 4 half laps back to the paddock, plus warm up. That's about 64 miles. Better than 10 miles per litre, or 45 mpg :)
First run, and it's giving 4bhp more than last year at the same dyno!!!
Spent an hour finding the least advance that will give the best and smoothest power and torque, and checking that the fuelling was right. Chris Adams at Road and Track Dyno in Aylesbury is very helpful.
The chart shows May '05 in blue and May '04 in red
Off home, somewhat pleased!!
Couple of shots of the bike in the garage. I'll do something about the cobwebs one day!
I bought it in a fit of irrationality in spring 2002, to do track days, or for restoration, or something.
It's a 1968 "widecase" Ducati 250 that started
life as a SCR semi off-road machine, made under licence in Spain by
Mototrans. On the right is what it looked like in 2002 when I
bought it, after it had languished in the previous owners garage for
several years
It's now a little tidier cosmetically, and I've had to do much les than I feared mechanically. These two were taken at Pembrey during Easter 2004
The previous owner bought the bike at an auto-jumble as a direct import from Spain, so it's never been registered for the road in the UK.
The motor has had the head ported by Mez Porting, and is using 40mm inlet and 36mm exhaust valves, and it's had a second spark plug fitted to deal with the wide, thin combustion chamber of the Ducati's hemispherical head.
The cam is from Megacycle - a 21mm base circle grind they do for Syd's Cycles in Florida (owned by Syd Tunstall who was a notable Ducati rider in the UK in the 60s, and his son Malcolm who's had success with singles and twins over the years in the US).
I had to replace the piston in 2002 as one of the lands between the rings was becoming detached from the piston! Didn't help compression! The new one was nominally a 11:1, bought with an exchange barrel from Paul Klatkiewitz at Ducati Technical Services. I had to machine the crown slightly to get the squish band to fit the head. It worked OK but more work was needed - see above.
Con-rod is by Carillo.
An aftermarket straight-cut primary drive had been fitted by the previous owner.
Gearbox is standard - Ducati used the same one for the 350 and 450, so it has plenty of strength for a 250 in any state of tune, but first is rather low, with a big gap to second. Good for starts if the first corner isn't far away!.
Ignition is a Lucas Rita with two 6V coils in series feeding the twin plugs.
Carb is a 32mm Mk2 Amal
Exhaust is a custom made slow expansion rate megaphone with an end can to get below the 105dB limit. Made by Pete Gibson
Fairing and seat are from TGA Racing
The previous owner had replaced the forks with a mixed Honda set. The sliders are from an early CB750/4 and the stanchions from something unknown (shorter than the 750). Yokes are original, although the stem seems to have been welded at some time. Probably should change that one day
In 2003 at Lydden I had the front end let go at the fastest corner of the circuit, and it hurt. This was due to poor damping in the forks allowing the wheel to patter - as it does in a car when it's (past) time to replace your shocks. So, for 2004 I had Maxton Engineering replace the internals with a modern cartridge damper unit. What a difference!
Front wheel is Honda but I don't know what from - possibly a 350 K4, as the front calliper is. That's for eligibility with the Classic club - the same calliper was fitted to many machines throughout much of the 70s, e.g. 400/4, but the K4 is known to have had them pre 1972 which is the Classic club cut-off year.
Because the 750 sliders were expected to have a larger calliper, the disc rotor it came with didn't quite reach all of the pads - they overlapped the edge of the rotor. So for 2004 I had a new cast iron disc rotor fitted to the Honda carrier and the result was an immediate improvement to the brakes. I then went to the next level and replaced the Honda 16mm master cylinder with a new Grimeca 12mm unit - having first checked with the club for eligibility. It now stops reasonably well, although the pads wear unevenly because the calliper twists on its pivot and flexes slightly when used hard.
Can't afford to change the rear shocks till next year, so I'm making do with the pair of Hagon copies of the old Girling gas units that the previous owner had fitted.
It came with a WM2 x18 Borrani rim at the front fitted with an Avon AM 20 90/90. The WM3 at the rear was standard for the SCRs, but it was an aftermarket Akront rim. For 2005 I had the rear rim replaced with a WM2, new spokes fore and aft, and went for 3.25 Dunlops front and rear.