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198B Angine - Differences? Flywheel?


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Ive got a 198b and the flywheel seems to be about half the thickness of my older 190 model. It may have been machined down but if so, its a good job as it looks 'factory' if that makes sense.

Does anyone know if the 198b engine was any different than earlier models?

thanks Stu

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Which flyweel - the ignition or the clutch side crank weight?

198b is a standard 198a engine as far as I've seen.

The ignition flywheels didn't really change throughout the years but the 250 models are lighter than the 350 models.

Same for the clutch weights, the 350 has a double weight and the 250 a single weight but again, the actual weight of them wasn't radically altered. The shape changed sightly around the circumference to suit the inside moulding of the different clutch covers, so they aren't necessarily interchangeable as the clutch cover can foul the wrong one.

Is it a standard bike or one of the Vasquez or Puma modified Spanish bikes? On those, they lighten the flywheels to quicken up the engine response. This can cost the engine's ability to lug right down low and stall easier, but they ride them on the clutch so it isn't critical.
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Hi Woody, its the drive side crank weight. Its about half the width of the one on my 190 - so seems like its been modified then. Only bought it fairly recently so Im not sure if its been breathed on by Vasquez or Puma but I doubt it.

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I've not seem many Pursang weights, so don't know how many variants of those there are. Possibly one from a Pursang?

As the 198b was thrown together by the workers at the factory in its last days, it's possible that they just used up any bits lying around to finish the bikes. Maybe it came from the factory like that?

However, skimming the weights on most older bikes was common(ish) back in the day if the rider wanted quicker pick-up, so this is the more likely option. I know a good few people who've done it with older twinshocks over the last few years - I had an Ossa flywheel in the lathe last week experimenting (frightening for a non-engineer...)

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Pursang models had a duplex chain (double row) so had double row teeth on both the clutch basket

and the crank weight,

so no it is not a pursang one as all sherpas (apart from one model in the early 70's) used a single row configuration.

There were however a few different weights of various bultaco bike/models but sounds like it has been machined/modified.

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The flywheel weight on the clutch side is completly inductive hardened, so it is very very difficult to modify them, (like milling or grinding).

The difference in weight between the 350 and 250cc model on the clutch side is around 1kg, here a pic of the 250cc flywheelweight in the background and the 350cc flywheelweight in the foreground.


The weight of the 350cc model is about 2,85kg

The weight of the 250cc model is about 1,95kg

The biggest differences you can get using different rotors on the ignition side:

Femsa rotor for the 340cc Sherpa: 3,4 kg

Motoplat rotor for the 325cc Sherpa/Alpina: 3,7kg

Femsa rotor for the 250cc Sherpa/Alpina: 1,4kg

Modify the rotor will have a lot of effect as the diameter of the "flywheel" is a influence variable to the moment of inerta:

J = m * r^2

J: moment of inerta

m: mass

r: distance r from point mass

Will look up if I find some pic's of the different ignition rotors.

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Yep the clutch side flywheell looks like it came from an older Pursang or a Matador as shown here:


Anyway biggest differences/effect will beto add weihgt or reduce weight to the ignition rotor side as the diameter is twice as big there.

I don't know if this a really big improvement to get the bike as "snappy" as possible, the Bultaco is in my personal experience much easier to ride using the torque and forget as much as possible the clutch while riding. When the bike is continous in movement the bike handles best, with the loss of too much flywheel weight the risk of stalling the engine is higher, ...

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Yes, it looks like that one - but Im on a single chain. I wonder if half of the sprocket has been machined of if its just been left and run with a single chain - ive re-fitted the clutch cover but will take it off again as Im curious.

Thanks for everyones help.


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Ive checked and it looks exactly like the one pictured by PSchrauber but only has on set of teeth. I suspect its a Pursang with half the sprocket machined off to suit the Sherpa single chain.

I haven't really ridden it much yet (other than a short session at Cown) so will be interesting to see how she goes.

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