Bigger isn’t always better
With the current interest in smaller capacity bikes for pre-65 trials, we have a look at a rather special small capacity bike for twinshock trials.
If anyone had said to me in the Seventies “what you need son is a 125…” I’d have laughed them out of the place, I knew what I needed, it was the red and black Model 199 325 Bultaco Sherpa, because that’s what ‘Vesty’ and the Lampkins were riding and winning on, or so it appeared. The thing is with the best will in the world I’m not a ‘Vesterinen’ or a ‘Lampkin’ nor I suspect are a great many other riders, but the bigger bikes were doing the winning and that’s what we wanted.
In some respects this has always been the case in the competition world and despite a smaller capacity machine being available the rider would always convince themselves the bigger one would be better. The evidence to the contrary was always there as Bill Lomas beat all the big bikes on a 200cc James in the Travers Trophy Trial in 1951, just to rub things in a bit Arthur Shutt took the 1953 Scott Trial on a 200cc Francis-barnett. Then Jim Alves started winning awards on small Tiger Cubs, Roy Peplow won the 1959 Scottish on a Cub too so the argument small bikes weren’t up to the task held little truth.
Fast forward a little to the early Eighties and Fantic’s 200 model – actually 156cc – which showed small could work quite well. At that time I’d been persuaded into the chair of an outfit based on a 200 Fantic and of the two of us in the team I was the small one.
The bigger bike wasn’t finished though and Finnish superstar Yrjo Vesterinen was developing a 340 Bultaco at Comerfords in Thames Ditton while the factory was sorting itself out and what a fabulous bike the resultant machine was, as long as your name is Vesterinen. Problem was not every trials rider is a Vesterinen and having ridden it in a trial some time ago I learnt the true meaning of being ‘over-biked’, as that particular Bulto was built to Vesty’s riding style and not mine. As he revealed later though, not everyone was convinced of the bigger bike’s superiority and a close run thing at a trial where fellow Comerfords’ rider Steve Saunders gave him a run for his money while riding a 250 Sherpa set a train of thought going in Vesty’s mind, which resulted in this neat looking 125 Sherpa being campaigned by another Vesterinen… daughter Hanna.
Vesty felt the 125 Sherpa had some potential if the under powered and lazy engine could be pepped up a bit and some weight shifted off the bike. A 125 Model 185 came his way but it was a bit underwhelming to say the least. “The problem is the 125s didn’t have much development at the time, Montesa had one on the market and Bultaco thought they’d better put one out. I picked up a barrel from the 125cc Streaker model thinking it might prove better, it was at the top end but any torque the original Sherpa had vanished with the road bike barrel.” The idea of the 125’s potential didn’t go away though and when Hanna got a modern 125 Beta and her dad had a ride on it he couldn’t believe it was just a 125. All of a sudden there was a catalyst for the 125 Sherpa.
“It was so much better than the 125 Sherpa, yes it was a lot newer but still, I contacted John Lampkin – Beta importer – and asked for some information about the engine specification which he got for me, then talking to Mick Grant he pointed out we should strip the Beta and compare to the Bultaco to see where the differences were and if the 125 Sherpa could be modified to the same.”
What followed from this initial ride on a Beta was an intense period of development, which by his own admission became a steep learning curve for the Finn. “When I was at Bultaco I was primarily a rider, yes my opinion was wanted but I didn’t have to do the work so a lot of what has been done to this bike was new to me and I’ve learnt a lot along the way, such as measuring port timings and that sort of thing.
“This is a good point to say without Mick's help I couldn't have done the work, I was pretty
good at doing the ignition timing on a Bultaco but the rest of the work I had no idea about,” said Vesty.
Delving in to the Beta engine the first surprise was the shape of the cylinder head. “It was like nothing I’d seen before and the port timings were nowhere near what the Bultaco used, in fact initially we felt it couldn’t work, yet it did.”
He went on to say the first task was to weld up the Bultaco combustion chamber and reshape it to the same as the Beta, this made a significant difference to performance but measuring the compression ratio showed it was nearly 16:1.
“Mick and I were convinced it wouldn’t work at such a high ratio but the Beta was the same and it did.” The bike was reassembled and tested and what a difference. “I knew it was on the right track but development wasn’t happening fast enough and I forgot an important lesson ‘modify one thing at a time’ then you know if it is good or not good.”
The head was pretty easy to modify, it’s an air-cooled two-stroke and not exactly difficult to remove. Late one Saturday afternoon CDB happened to be taking some scrap… er… valuable classic spares to Granty’s for some welding and arrived just as the Bultaco cylinder head was being modified. In the space of a few minutes the head was lifted, piston greased, head tightened, liquid poured in, calculation made, head lifted off, fitted in the lathe and a fraction skimmed off, head back on and Vesty was testing the bike.
Prior to the cylinder head work Vesty told me the balance holes in the crank had been plugged by Max Heyes and the crankcases had been welded up around the transfer ports so when the cylinder barrel was dropped in it could seal the air gaps. So now the timing of all five ports in relation to each other could be measured properly and experimentation could begin. Engineer Bill Beveridge was happy to modify cylinder liners but this started to become time consuming and expensive as an experiment – the Bultaco cylinder is alloy, the liner is more likely iron of some form – the barrel is heated up and a liner dropped in or pressed out. A suggestion from Mick Grant to use a Loctite plastic metal for experimenting on the inlet port speeded things up a lot and improvements were noticeable but still not right… at least if the viewpoint is a from a three-time world champion.
Looking back at the Beta engine, Vesty’s attention was on the exhaust and the port shape plus the pipe it matched. Having an alloy block made to replicate the Beta port shape made a vast difference, “all of a sudden
there was torque at the bottom end of the range and it still revved cleanly. The exhaust on the bike now is different to the one used at the time of your photo shoot,” said Yrjo, “with the port making such a difference I measured up the Beta exhaust and tried to replicate it as best I could within the framework of the Bulto.” At one point apparently the pipe was modded on a daily basis by Barry Chamberlain – Barry also did the work on our Project IT frame up at Rod Spry's place in Rawtenstall – until the time was reached when performance went backwards and he knew he’d reached optimum level.
While all this was going on, in order to improve the zip from the engine, the flywheel was lightened, “maybe by a little too much,” says Vesty, “as I had to have a band put round it to stop it breaking apart and add a little weight to prevent it stalling.”
Though the 125 was a proper Bultaco it was a physically smaller machine than the rest of the range and needed attention to the riding position and steering. There was no need to – and we quote the man here – “reinvent the wheel, we measured the dimensions of the Beta 125 and made the Bultaco as close to them as possible but retaining the ‘look’ of the Bulto.” What this means is the dimensions such as rear wheel spindle position in relation to footrest and handlebar position plus fork height, fork trail and wheelbase were replicated to modern dimensions. The forks themselves are from a 325 though with some unknown sliders on the bottom.
“I was at Telford show and saw these Betor sliders on a stand, brand-new and longer than the Bultaco ones, I’ve no idea at all what they would have been for originally but they gave a useful increase in fork length but needed the steering head moved up the frame a little.”
As we’re talking, Vesty made it clear he’s had a lot of help from a lot of people and said Rod Spry made the frame alterations early on in the process. This involved Rod cutting steering head off completely and rewelding in the new position with a gusset to steepen the angle. This allowed Vesty to increase the swinging arm length while keeping the wheelbase the same size.
“It looks as though we have repositioned the bottom mount on the rear dampers but in actual fact it is the extra length in the swinging arm which is different though Rod did cut the frame at the mudguard loop and move the top mount forward to gain more suspension travel. I feel now this bike steers better than my own 280 and if I ever decide to make a new frame for the 280 I will replicate this one,” he says.
At my question “is that it?” Yrjo laughed and said “of course not, this is an on-going process and an enjoyable learning experience for me. For instance carburation has been interesting, in your photos the 125 has an Amal Mk11 on but now has a Keihin 28mm flat slide but I’ve tried all sorts of other ones.”
Knowing Vesty’s obsession with carving weight off his bikes I wondered if the 125 was featherlight? “Not especially, we cut the frame tubes off and used an alloy bash plate which saved a little weight, then the spindles are all alloy so there’s a little saving there.” No superlight titanium fasteners? “Not really, there are a few but the saving versus cost would be impractical, maybe I will reduce the weight at some point but it’s about 76kg – 167lb give or take an ounce – and it started at 84kg.”
To finish off, Vesty allowed the project was time consuming and not the sort of thing to tackle lightly, but from our point of view the result is spectacular. “You ought to bring your 250 along and have a comparison test some time,” he said. Okay, you’ve twisted my arm, look out for the riding feature next season.