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Perry Barwick brought round his repainted BSA flat tank frame last week. The plan was to fit the rebuilt engine and gearbox and then maybe start on the wheels, but first there was the horizontal top head fin to attend to. It was broken and jagged; you expect the odd broken fin but this one’s in plain view and looked horrible. Instead of attempting to fix it I had another barrel with better fins that just needed an oversize valve guide, so why not use that?
I set about making and fitting a guide and soon after we were grinding in the valve. That’s when we found out why the cylinder was on a shelf not a bike. The valve wouldn’t seat and the stones for my grinding tool were all either too small or too big to fit through the access hole in the head. I spent the next hour reducing a large stone to fit, only to find that the seat is past saving. So I dug out yet another cylinder. This one’s seats look okay but it does have fins missing and a broken inlet stub, or maybe this one – it’s spent a long time outdoors and needs reboring but the seats and stub are good... I realised just in time that I was headed for a whirlpool.
So I did what I should have done in the first place; shaped some fins cut from a scrap cylinder and welded them to the original. Sometimes there are too many options and you end up going round in circles.
Back on track I started grinding my welds and guess what? My super reliable 20-year-old angle grinder started belching smoke. Some days you just can’t win!
‘THAT’S WHY THE CYLINDER WAS ON A SHELF, NOT A BIKE’
CV OR SLIDE?
Jim Lloyd is having carb problems with his Kawasaki Z250-engined special. The original CV30 Keihin carbs were worn out and impossible to get so he has fitted a pair of 28mm Mikunis, recommended as a suitable replacement. After making a few alterations to the jetting the bike runs well up to 6000rpm but above that it splutters in all gears.
Jim asks, “Could it be an air speed problem created by the CV carb having a varying venturi, where the Mikuni’s is fixed?”
I don’t think so Jim. The choice of CV or slide carbs doesn’t affect the design of an engine, so it should work with either type, but Japanese engines are pretty precisely tuned and aren’t as tolerant of swapping carbs about as older British bikes. I had a quick word with Dave at Mikuni specialist Allen’s Performance (01949 836733) and he agreed: “The trouble is you need to know what jetting to fit – Keihin and Mikuni jets are marked differently. Also, if the carb is secondhand, it could have come off anything, even a two-stroke – latemodel RD250S used VM28S for example. We can supply a pair of new VM28S jetted to customer requirements for £281.88 – but on a non-standard fitment you still need to work out what you want and the best way really is to pay for time on a dyno. It may seem costly but will save a lot of time.”
If Jim’s carbs are secondhand they may need ultrasonic cleaning although I’d expect that to affect low revs more than high. At the revs Jim’s talking about I’d blame the needle and jet rather than the main jet which only takes over at full throttle.
Steve Taylor asks whether the 1960s tune-up float bowl extensions made for Amal Monoblocs are actually worth having as a performance boost: “After all, they don’t make the fuel flow any faster so what’s the point of them?” he asks. You’re right, Steve, all the extension does is increase the amount of fuel in the carb – the amount of fuel coming through the feed remains the same.
I suspect the theory behind it was that on snap acceleration, where a sudden increase in fuel consumption may cause a brief drop in fuel level, increasing the volume of the chamber should reduce this drop, reducing a tendency for the engine to run momentarily lean. It might make a difference on a racing bike but on the road the most worthwhile feature (apart from looking snazzy) was that being pretty solid, it wasn’t prone to the distortion that plagued the original flat cover and produced irritating fuel leakage.
BIGGER AND BETTER
Paul Wheeler is planning to build a Triton: “People tell me I need to get a ‘big bearing bottom-end’,” he says “but how will I know one without stripping it and is it important?”
In Triumph terms, ‘big bearing’ refers to the larger diameter timing side (right hand) main bearing on the pre-unit construction engines. Introduced in 1955, it went hand-inhand with larger diameter big-end journals that took conrods with detachable shell bearings.
Previously Triumphs used the Hiduminium alloy of the rod as the top bearing surface with a corresponding coating of bearing ‘white metal’ on the steel rod cap, but this meant that the big-end was not replaceable.
So with the big bearing motor not only is the bearing more robust but the crank is stronger and you can fit rods from any later 650 twin.
The casting was enlarged to accommodate the new bearing, so the easy way to identify big bearing cases is by the lip that runs around the underside of the timing chest (see above). But be warned, some of these crankcase castings were machined to take a small bearing so you need to measure up the diameter of the bearing bore.
Nearly fin-ished: Rick welded some fins he’d cut off a scrap head onto the original. But only after much faffing about…
Tuning mod or just fashion accessory?
Mikuni VM28 carb should work - with the right jets
That lip means it’s a big bearing case. Probably…