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10 January 2001
I have been checking some top gear sets from an CB250RS and
Motorsport that Martin Roberts sent me, that would close up the gap between 4th and
5th for racing.
The rotating gear is a direct swap, but the splined sliding gear has 2mm
larger spline diameter, and I think a 2mm larger diameter selector fork
rubbing diameter. I am investigating this further, and am likely to get a
batch of sliding gears with Motorsport splines and RS teeth made.
This
would allow the use of the std RS rotating gear and would provide for a
cheap close top gear conversion. I need to check that shaft centres are
the same, but strongly suspect they are.
I am busy porting the head with the Dremel at the moment, and am
attempting to set my Bridgeport Miller up to open out the valve throats,
and enlarge and cut back the seats, I opened up the inlet port back to the divider to 34 mm
with a milling cutter, taking the centre diagonally up and left to straighten
and downdraught the port as much as possible.
I will send some zipped JPEGS taken at the initial stages of the project.
Regards
Dave Smith
BTW, the front end is a Yamaha disc and hub off of an LC,
early TZ forks with RD bottom yoke, and a Lockheed alloy
racing caliper off of a racing outfit (sidecar), the back is
machined and polished to clear the spokes. The caliper carrier is
care off my milling machine!
18 March 2001
Attached are two
pictures. The bike as is at present with a CB77 race tank fitted.
Also showin is the crank between centres in the lathe being checked for true running. This was in
the end a problem as the tapered flywheel end of the crank was bent on
the outer taper, and I eventually set the crank up in its bearings in V
blocks to get it running true.
I have now got the modified 4th sliding gear, and am about to machine barrel
and head to get the compression and squish bands correct. The intention is to
set the squish clearance at 0.040," as last years engine at 0,060" showed no
signs of contact. It will then be getting dangerously close to assembling the
engine time!
8 April 2001
Have been in the garage this weekend, the engine is coming along. I have built the bottom end with
the close ratio 5th gear set, the only unexpected bit was that the RS (250S) rotating gear was about 1mm thinner than the M/Sport one, and I had to shim it on the splined bush to get good mesh with the sliding gear dogs.
I have fitted head and barrel, had to shorten all the dowels top and bottom by the same amount the head (1.5mm) and barrel (3.4mm) have been skimmed. The only problem I have is that the wider cut out in the head above the bigger inlet valves has encroached into the head gasket fire ring by about 1.5mm. A copper gasket will cure that while allowing a gentle radius of the liner edge to reduce turbulence. The size was OK before I skimmed the head, forgot that the hole would widen as I reduced the deck height. I think it will still seal OK enough to run the motor in and try it at Aintree in May anyway.
I will need to modify the tensioner blade as before as the chain will not tension up. Taking 4.9mm out with 8mm pitch links leaves you with not quite enough for a 2 link shorter cam chain, mores the pity. I have drilled the cam sprocket at 30 deg intervals to give vernier timing adjustment, and turned
the central part of the plastic facing away, but left a rim on the periphery to damp the chain.
The compression ratio measured out at 11.4:1 which is about where I need it. I have also got some heavier ignition cam advancer bob weights from a CB250 that I will use to give a more rapid advance curve, After that I have some nice shiny stainless header pipe that I will start cutting and shutting to
get the shape.
Hours of fun left yet!!
25 April 2001
Some more pictures to catch up with the progress on the engine.
I have machined 3.4 mm off the base of the barrel, to allow use of a later
twin port piston, nominally a 10.5:1 forged item from Wiseco. and 1,5mm off
the head to get the compression up to 11.4:1. The piston interestingly was
approx. 40gms lighter than the standard cast sideport item.
Crank balance has been left alone as the Carrillo rod is heavier than standard, and who knows what the correct balance factor is at the intended
revs. Experience of big Weslakes vibrating will probably help here. Double
vision, white finger (arms, and shoulders too), loose fillings will probably
put it into perspective!
I had pocketed both inlet and exhaust valves back while porting the head for
the 350 size valves, 38mm carb, and 1.75" exhaust. The pocketing was to keep
the valves clear of the piston during overlap, but increased chamber volume
somewhat. Squish clearance was set to 0.040" on this engine, which conversations with Swiss had indicated as being better than the 0.060" of the
previous motor. The Megacycle cam and rockers installed well, but the tensioner blade,
lengthened as was the previous one and shown in the picture, snapped like a
carrot when I turned the engine backwards. This was solved by modifying a CB250G5 blade that is the right length. The eyes at each end are narrower,
and no shoulders are present mid blade. This was sorted with dowels, spacers,
and a riveted on set of "shoulders". The gearbox is standard apart from top, which gas a twinport rotating 5th
gear, and a made sliding gear (silver item in the picture), as the twin port
splines are of a bigger diameter than the sideports. The rotating gear was
about 1mm thinner, but this was shimmed. This gives a two tooth change and
brings the gap between 4th and 5th to 0.070.
If this is too close, intention is to make a pair of gears with one tooth difference, next winter!
The cam was timed up by drilling mounting holes at 30 deg spacing, giving 1.76 (camshaft) deg difference between 1 hole space and three sprocket teeth.
The newtronic CB125 ignition was fitted with a made light beam chopper, that
had a bob weight abutment face 1mm larger than the standard ignition cam, and
one standard advance spring was changed to a softer CB250 item. This restricts advance range to 13 degrees, and will give a faster advance. Timing
is initially going to be set at around 30 deg. Fitting involved a bit of machining as the XL obviously differs a shade from the CB125, despite using
the same points!
Stronger clutch springs from a Suzuki GS1000 after-market kit are fitted to control the expected power.
The engine is now in the frame to allow construction of an exhaust to be undertaken.
Best Regards
Dave Smith
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20 June 2001 Susie
and I spent May 23 through June 11 in the UK and Paris. Part of our trip
was a visit with our good friends Dave and Jean Smith, who reside in North
Wales, UK. On the morning of May 28,
Linda, Dave and Jean's friend, collected us at our B&B in Llandudno,
a sea-side resort in the NW corner of N. Wales, and took us to Tye Croes,
a road racing course located on the Isle
of Anglesey. Dave had carted both
his Honda XL250 road racers to the track, and was anticipating a good
showing on his recently completed, and second XL road racer.
Pictured above is Dave aboard his #10, 2nd
generation XL250 road racer. Also shown is a neat example of a starting rig,
about to fire up an exquisite example of a Honda CB450 road racer.
The next day, Dave and Jean took us around on a
castle tour of N. Wales. These pictures were taken at Conwy
Castle. D & J live within a few miles of this historic and beautiful
area. "It ain't Kansas, Toto".
4 August 2001
The new 250's head after welding,
following the dropped valve
Bill Pilling's (Pie Filling's) Vincent
500 Comet at Aintree. Bill who is 68 going on 28 and still racing,
had a vintage win on this at Daytona just a few years ago.
Dave Wylie and his boys at East Fortune
in Scotland, a really nice guy.
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