198B Angine - Differences? Flywheel?
Posted 14 April 2012 - 03:58 PM
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?
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Posted 14 April 2012 - 05:16 PM
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.
Posted 14 April 2012 - 06:25 PM
Posted 14 April 2012 - 07:40 PM
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...)
Posted 14 April 2012 - 08:37 PM
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.
Posted 14 April 2012 - 10:19 PM
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
r: distance r from point mass
Will look up if I find some pic's of the different ignition rotors.
Posted 15 April 2012 - 09:21 AM
Edited by Nigel Dabster, 15 April 2012 - 09:21 AM.
Posted 15 April 2012 - 11:04 AM
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Posted 15 April 2012 - 11:49 AM
Posted 15 April 2012 - 01:18 PM
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, ...
Posted 15 April 2012 - 03:11 PM
Thanks for everyones help.
Posted 15 April 2012 - 07:04 PM
I haven't really ridden it much yet (other than a short session at Cown) so will be interesting to see how she goes.
Posted 15 April 2012 - 10:04 PM
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