350 Sherpa (325 Cc)

[trialsrfun]

Has anyone converted a 350 Sherpa (325cc) to 250cc.

I wonder if I would ride it better if there was a bit less power and what is involved in the changeover. Presumably 250 power would be softer and less agressive than the 350.

[twinnshock]

Bit hard to start on this one, as you do not state what year is your bike is therefore my comments will have to be very general. You are right in that the 250 is a softer less aggressive bike than the 350.

In the mid 70

[woody]

As Stuart has summed up I think you may encounter all sorts of problems trying to convert to 250 because of the variances throughout the two ranges. I have a 74 325 which is very soft and completely standard running on the MK1 Amal. I think it was from about 75/76 that the motor got sharpened up with a snappier delivery. Having said that I know someone with a later and standard 325 that is also soft so there are variances in performance too....

Have you tried the usual calming measures such as extra head gaskets, space the inlet manifold back, both of which will decrease cylinder/crankcase compression, retard timing as far as possible (when you go too far with this they do like to run backwards though...) These are cheap and quick measures to experiment with. If you know someone who knows what they are doing get a spare head - should be plenty around, and get the combustion chamber machined to soften it off.

Probably worth having a word with Dave Renham too at Bultaco UK.

[trialsrfun]

Should have added it is a blue 199a from around 1980 and has the Bing carb.

[trialsrfun]

Thanks for the advice fellas I have spoken to Dave Renham who confirmed that it is impractical to make any capacity changes so I will look at the things you have suggested including getting a head and modifying the combustion chamber, also possibly lowering the gearing from the 11/39 sprockets that were fitted as std..

The bike is very nice and original and oozes that Bultaco magic but all that power can be just a bit much.

[toofasttim]

As far as I remember the 250 produces more power than the 350 but the 350 produces much more torque.

[bob abell]

From my bultaco owners manual for models 198A 250cc and 199A 350cc:

250cc max power 14.1 C.V. @5500 rpm max torque 2.04 m-kg @4000 rpm

350cc max power 20.8 C.V. @6500 rpm max torque 2.76 m-kg @4000 rpm

1 C.V. (cavallo vapore) = 1.01387 HP

[twinnshock]

As far as I remember the 250 produces more power than the 350 but the 350 produces much more torque.

<{POST_SNAPBACK}>

Again this is a bit of a large subject. The early 325's on paper produced the same power as the 250's but did produce more touque. As time went on the 325 was made sharper. My own opinion is that the 250 is a better bike for us southern softies here in East Anglia who ride mainly on mud and sand and the 325's are better suited to a Yorkshire rocky burn. Incidently the bigger bike was initially called a 325 and later called a 350. This name change took place with no actual change in capacity.

A 250 engine is for sale on ebay at the moment. But be carful with this engine because the description states a "199" engine number (ie a 350 from about 1977/78) but the pictures show a much earlier 250 engine despite the black paint.

Stuart

[toofasttim]

From my bultaco owners manual for models 198A 250cc and 199A 350cc:

250cc max power 14.1 C.V. @5500 rpm max torque 2.04 m-kg @4000 rpm

350cc max power 20.8 C.V. @6500 rpm max torque 2.76 m-kg @4000 rpm

1 C.V. (cavallo vapore)  = 1.01387 HP

<{POST_SNAPBACK}>

I yield!