feetupfun

Which engine are you asking about?

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.

Hmmmm porky according to the specs. I reckon this calls for an official weigh-off of TL320 vs Jumbo

I don't know enough about your cylinder or rings to say

I haven't played with a Sherco primary drive but it would be very unusual for those gears to be worn. Some clutches have a cushion drive between the clutch gear and the basket and they can get a bit of inconsequential play. Go ahead and try acetone if you want. It didn't work for me and before you ask, neither did throttle body cleaner, methylated spirits, kero, turps, eucalyptus oil, melaleuca oil, methanol or WD40. I didn't try xylene on its own. An alternative to paint stripper that would also work but risk taking off some metal would be to degrease then bead blast.

M6 x 1.0 is what is on all my Bultacos with mounts like that

Yes that sounds like a good plan. Sounds like you found the source of the noise. Very light scoring is normal to find. If you can feel lines then there is a problem. If it was me I would also clean the rings and ring grooves (using non-caustic paint stripper and a tooth brush) and clean any carbon off the piston crown and exhaust port with either a scotchbrite pad or bead blasting.

Pete88 you didn't ask me but I'll put in my 2c worth 1 for main bearings any discernable movement axially or radially of the flywheel means they need to be replaced. 2 If a ring stays in its groove in any section of the groove rather than moving freely in and out the groove, it is stuck. If there is gum, you will see it. Sticking rings is very commonly seen on two stroke trials bikes. Worn rings are measured by fitting them into the bore squarely between the top of the exhaust port and the head surface and measuring the gap between the ring ends. The pattern of carbon deposits on the walls of the cylinder also is a guide to there being ring sealing problems or not. 3 There should be zero discernable movement up and down of the conrod relative to the big end pin. Slight left-right rocking movement of the top end of the rod is normal. The little end can be inspected with a magnifying glass after cleaning the rollers and pin and the eye of the rod. If there are any visual defects in any of the bearing surfaces the parts should be replaced. It should also have zero discernable movement in the direction of the rod axis but it is normal for the pin/piston to be able to rock slightly in the eye of the rod.

Is that when they went to 32 spokes from 36?

Something fishy here. When USDB magazine did a comparison of all the available trials bikes at some point, they said the SWM was the lightest full-sized trials bike. Maybe it was the TL230 they had and the Jumbo is lots heavier?

Yes that will work with some frames but on many frames (eg Sherpa T and KT250) the wheel is already close to the front downtube or exhaust header with the standard setup.

Thanks. I had just cleaned them for a photo session for a virtual classic bike show.

Snap! You have great taste in motorbikes

They are the same basic layout as every other motor of the era but there are features that some people haven't come across before. Here are some things that spring to mind: The conrod is held centred on the crankpin by spacers on the gudgeon pin. The primary drive crankhaft gear is mounted on a taper which needs a special puller to remove without damaging the teeth. The cylinder is held on with 7mm internal hex nuts. The positioning (and side play) of the crankshaft is set using shims behind the crank seal carriers. The kickstart travel stop is a weak design and very commonly broken which allows dust and water into the magneto casing. Take note of how the shift mechanism is set up before you pull it apart. Yes the side play and positioning of the gearbox shafts is important but if you use the same shims in the same spots and use the same thickness centre gasket 0.5mm it will probably be fine. Pistons are very expensive.

The ideal footpeg position depends on a lot of things and you won't be able to know for sure until you have test ridden it so yes bolt-on adjustable position pegs are the go for an experimental bike like yours. For a starting point, I'd mount the pegs so that the middle of the peg platform is in line with the front end of the tyre tread with the axle at mid slot.

Generally a trials swingarm should be as long as possible yet still achieve the wheelbase that you want. For a bike with something like your B250 engine (an engine that is long from front to back) you will probably end up with as short a swingarm as will fit an 18" trials tyre and the wheelbase will still be on the long side of ideal.

For comparison, this is a standard Kawasaki KT250 front end. I chose this photo because this bike has the most steering angle of any of my bikes.

Excellent. Thank you. That is fairly serious steer angle

90 degrees is an awesome design target. If you've got it, flaunt it. I'd love to see a photo.

Maybe try googling Leonelli instead of Lionelli. Have you looked in the owners manual for your bike?

The SWM Jumbo came with 38mm forks

I'm still working and may not even get any time off due to working at a power station. Plenty of things I'm ready to do in the workshop, mostly involving 1970s trials bikes

and if I could source IRC I would use IRC also. They are both very good.

People (including me) fit Tubeless X11 Michelins on Tube Type rims.

Who cares what people do on ebay? Isn't there a for sale section here on Trials Central? That would be a pretty good spot. There's also the various facebook Montesa pages which are 100% Montesa enthusiasts