Rick’s Fixes

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Classic Bike (UK) - - Workshop -

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.

FLOAT EX­TEN­SION

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…

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