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DickyM

Sherpa power curve

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Hi All

Just joined the forum. I've made plenty of visits as a guest and found it very useful so thought I'd join as I think I'm going to need plenty of advice with my project.

Bought a 198A Sherpa T 1979/1980 vintage over the Christmas holiday as a retirement project. It's a bit of a wreck with everything falling into the 3 B's category. Bodged, bent or busted. However that's fine and going well so far. But I have a question that no one seems to know the answer to. So heres hoping.

Before I bought the bike I did a bit of online research. I discovered, on this forum somewhere actually, that max power output of a 250 Sherpa was around 20 bhp compared to around 21.5 bhp for the 350 version but with the 350 having much higher torque. As soon as I bought the bike I also bought myself a Workshop Service Manual from In-Motion Trials. This covers all models up to model number 200 but actually excludes the 198 and 199 series. In this manual there are power and torque curve graphs for 250 and 350 Sherpas showing max bhp as 20 and 21.5 respectively and torque as 2.72 and 3.15 M/Kg. Exactly as expected.

However the bike came with an owners manual specific to the 198 and 199 model series. This is a proper printed glossy booklet printed in Barcelona in November 1978. When I got round to reading it several weeks after acquiring the bike I found there are power curve graphs shown in this book also. The 350 is much the same with max power shown as 20.8 bhp but with a lower torque of 2.76 M/Kg. The 250 however shows a maximum power of only 14.1 bhp and torque down to 2.04 M/Kg. IE Power down 30% and torque by 25%. These figures are also repeated in the general data section.

What goes on? Can anyone shed any light on this? I find it hard to believe that Bultaco, faced with ever increasing competion from Japan, would take a proven bike with impressive history and track record and suddenly decide to lop power by 30% and torque by 25%.

It seems that they did but what on earth did they do to the engine?

I have not seen any reference to this anywhere and believe me I have dredged the internet. I am aware that the cc of the 250 ws dropped to 236 cc but surely this didn't lead to a 30% drop off in power.

The outcome of this is irrelevant to my project bike as I will refurb it as it comes. I'm not chasing power but I like to get under the skin of a bike and I'd just like to know.

Does anyone know what happened here and what they did to the motor to strangle the power.

Thanks in advance.

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The later 238cc 250 has different porting and compression ratio to the early 244cc 250. The later "250" motor has very different characteristics to the early "250" motor. While the later motor in standard trim does feel less powerful, it will keep producing useful power to lower RPM before dying than the early motor. The later 250 motor also will rev out better than the early 250 motor.

Neither of the Bultaco "250" motors is what you could call powerful compared with many other "250" twinshock trials motors. The early 250 is strong in the mid-range but signs off early while the late 250 is fairly weak everywhere but has a useful RPM range that is about as good as you can get.

Spanish power numbers of the early 1970s are usually derived from using an engine dyno. While the early 250 Sherpa T is declared as being 20 HP, a TY250A (which is declared at 16 HP) will leave a standard 250 Bultaco Sherpa T way behind in a drag race. How they work in trials sections though is the most important thing and is also why the 325 is only 21 HP.

As for the 325 motors, again there are two different motors. They share the same bore x stroke but in the mid 1970s the motor was redesigned and became a much more refined thing to ride than the early 325 and is very highly regarded for competition use.

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Thanks you both for yourPower Graphs.pdf comments and your comprehensive reply feetupfun.

Could be that the 238cc unit was tested at the back wheel but that would be a big loss through transmission for a chain driven bike and it would be odd to put the 250 and 350 on the same graph, one measured at the back wheel and the other at the crank.

Understood all you said feetupfun and you can pick up the torque reading at lower revs from the graphs. Clearly having useful power at lower revs is good for a trials bike. It's just that 14.1 bhp  is so low and the 350 doesn't seem to have suffered in the same way. 14.1 is even lower that the 1959 BSA C15 I used to own which was rated at 15 bhp if I remember correctly. For a Bultaco 238cc made 21 years later 14.1 just didn't seem right.

Interestingly, to me anyway, I also noticed that the power curve given for the Alpina 250 (again 238cc I believe) in the Workshop Service Manual is almost identical to the 198 graph included in the Owners Manual. 

I have attached a copy of the power curves in question so you can have a look for yourselves if you like.

All the best

 

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4 hours ago, DickyM said:

Thanks you both for yourPower Graphs.pdf comments and your comprehensive reply feetupfun.

Could be that the 238cc unit was tested at the back wheel but that would be a big loss through transmission for a chain driven bike and it would be odd to put the 250 and 350 on the same graph, one measured at the back wheel and the other at the crank.

Understood all you said feetupfun and you can pick up the torque reading at lower revs from the graphs. Clearly having useful power at lower revs is good for a trials bike. It's just that 14.1 bhp  is so low and the 350 doesn't seem to have suffered in the same way. 14.1 is even lower that the 1959 BSA C15 I used to own which was rated at 15 bhp if I remember correctly. For a Bultaco 238cc made 21 years later 14.1 just didn't seem right.

Interestingly, to me anyway, I also noticed that the power curve given for the Alpina 250 (again 238cc I believe) in the Workshop Service Manual is almost identical to the 198 graph included in the Owners Manual. 

I have attached a copy of the power curves in question so you can have a look for yourselves if you like.

All the best

 

When I said that the early 250 Sherpa T would not pull to as low an RPM as the later Sherpa T 250, I'm actually talking about low RPM as used in trials riding, not the 3000rpm that is the lowest RPM on those power curves. 3000 RPM is mid-range RPM on a 250 trials bike.

When you get your 198A going, find someone with a round barrel Sherpa T that runs well and have a ride comparison. They are very different motors and in my opinion the numbers on the curves reflect reality.

The power curve for the late model Alpina 250 looks like the late model Sherpa T 250 curve because apart from the gearbox ratios, the motors are the same design.

 

 

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I think you are worrying about nothing here as every year the bikes were better, with a wider spread of more usable power. I wouldn't read too much into the graphs published in the manuals as for example the M92 325 and the M125 325 have the same graphs published in their respective manuals but the M125 has a larger 27mm carb in place of the M92 25mm  and is a much better motor. In 1975 the 250 was reduced in size to 238cc to allow for cheaper registration in Europe and I wouldn't be surprised if the factory downgraded the power output 'on paper' to assist in this cheaper registration process, just a thought anyway!

Cheers Greg

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Thanks for the replies guys.

I'll do as you suggest feetupfun and cadge a ride on an earlier bike for comparison when I can. And your thought Greg about downgrading the power output on paper is a very real possibility, one I hadn't considered to be honest. An earlier version of the VW emissions scandal!

I'm not at all worried about this power output thing and as I said before the bike will be rebuilt to it's original spec (or as near as I can get it)

All I was querying really, admittedly being a bit of an anorak about these things, was to find out what the actual engine differences were. I dare say I'll find out more when I split the engine. Dealing with the cycle parts first.

Thanks again.

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The 250 is more ‘peppy’ than the 325, this larger engine has much more pulling torque and in my opinion is easier to control. The porting obviously will have an effect upon engine response. Mine was an ex Comerfords bike and was really soft and easy to ride, where as my mates 325 was a bit of a handful both the same model bike but chalk and cheese in the tune department.

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3 hours ago, DickyM said:

Thanks for the replies guys.

I'll do as you suggest feetupfun and cadge a ride on an earlier bike for comparison when I can. And your thought Greg about downgrading the power output on paper is a very real possibility, one I hadn't considered to be honest. An earlier version of the VW emissions scandal!

I'm not at all worried about this power output thing and as I said before the bike will be rebuilt to it's original spec (or as near as I can get it)

All I was querying really, admittedly being a bit of an anorak about these things, was to find out what the actual engine differences were. I dare say I'll find out more when I split the engine. Dealing with the cycle parts first.

Thanks again.

Apart from changing the way the 250 Sherpa T motor performed, Bultaco made multiple internal and external changes along the way to your M198A, for reasons including styling, frame design change and mechanical durability.

One of the reasons I love Bultacos so much is that by working on their bikes you can learn what they changed when and try to understand why they changed it.

A recent quest on this has been understanding the sequence of changes made to the external engine casings around the time that the Sherpa T frame design under the motor changed from a single tube to two side tubes during the 3 years of the M49 production. Just when I thought I had worked it all out, I find there is a third type of M49 primary drive cover that must have only been produced for a very brief interval.

One thing that surprised me about my M198, which is the latest model Bultaco I have, is finding a copper head gasket much like what is in Japanese engines of the era. I've yet to work out the reason for why they changed this part of the design when the previous gasket-less design worked so well.

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One major difference between the later 350 and the 250 engines is the 250 had lighter primary drives as well as a light ignition flywheels. That is why they rev quicker.

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I forgot to say David, the main reason for the head gasket was the head gasket way was much cheaper to make than the extended barrel liner.

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