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The standard Yamaha rebound springs will work fine in combination with the aluminium damper rods that In Motion sells and with aftermarket fork springs. I'm 95 kg and have three bikes with TY250 twinshock forks and all have aftermarket fork springs in them. Two of the bikes have the aluminium damper rods and the third has modified Yamaha TY250 damper rods. Two of the sets of fork springs are Magical brand and the third set is from France Trial Classic. All three sets of forks work great for my weight with the standard quantity of 10 weight fork oil and 31 mm spring preload. All three bikes have standard Yamaha rebound springs in them. I haven't tried them without rebound springs. The Magical TY250 springs (dual rate) have an initial rate 0.36 kg/mm and a final rate of 0.54 kg/mm. Spring travel to reach the higher rate is 134 mm. Standard TY250 springs (dual rate) have an initial rate of 0.26 kg/mm and a final rate of 0.42 kg/mm. Spring travel to reach the higher rate is 137 mm. I also have some B&J Racing TY250 springs (single rate) that I shortened by 8mm for use with Gold Valves. They are 0.39 kg/mm. The France Trial Classic TY250 springs (dual rate) share dimensions with the Magicals springs.

You'll need to say which Betor shocks you are referring to if you want fitment help on here

Yes 360 Montadero. It's a very rare trail bike with a Pedrables engine design which is quite different to the common 5 speed Bultaco motor and hard to source parts for. What do you need help with?

Probably because of the cost for photo hosting. I think if you become a Supporter of Trials Central you can post large images

Yes it works. Fit them in opposition. Two narrow seals was the original setup.for lots of Bultacos

Sorry, I didn't look for stampings when it was apart last, but will next time

Bultaco cylinders are usually stamped with the model ID of the first model that came with that design. I would have thought however that the 137 had a protruding sleeve cylinder while the Sherpa T 250s went to a soft metal head gasket/flush sleeve before the 198B. My 198 has a flush sleeve and soft metal head gasket. They did do lots of strange stuff to get bikes out the door towards the end and a 137 Alpina type cylinder and head would be fine to use on a 198B. As for the large diameter bore, yes it's fairly common to bore a 238cc motor much bigger than standard to get more grunt rather than because the bore is worn.

Width at pivot including end caps 178 mm Length from centre of pivot to centre of axle slot 380 mm

Be aware that some 2 stroke ignitions have a curve that advances the timing as the RPM rises and at some point then start retarding it as the RPM rises further. You will be able to see what your ignition does when you put a strobe on it. Yes make your marks somewhere near where you think you want the spark. If it's got a flywheel similar OD as an MAR I would think that 22-24 mm measured around the rim of the flywheel BTDC would be in the ballpark. With a trials engine, the spark timing at low RPM is probably the most important thing and trying out different timing settings for low RPM will yield the best result.

There is nothing new under the sun. Here is what a 1970s Maico MX clutch spring looks like. When stacked up top to top and bottom to bottom, these are used in Maico clutches in a similar way to the helical compression clutch springs of other brands. Maico were onto the unusual spring rate benefits of diaphragm springs for motorcycle clutches over 50 years ago https://maicoatchippys.com.au/clutch-spring-cupped-belleville-washer-maico-all-to-81-and-82-gs/

Is it possible that the eye of your conrod has had some mechanical damage that has deformed the little end of the rod, causing the hole to no longer be perfectly round? To answer your question, if the hole is still round, the hole can be honed larger. If the little end has been deformed then it is time for a new rod.

It's fairly normal on motorbikes to spring the frame and cut through the swingarm axle to get the swingarm out. The inner bushes can then be slid out of the outer bushes and the whole thing done without risk of damaging the frame or swingarm. The issue is usually corrosion causing one or both steel inner bushes to stick to the steel axle

There are plenty of mistakes in the diagrams in the Yamaha TY parts books

Lowering the gearing will help in the tight stuff because you can ride slower with the clutch out but it won't help the engine braking/power surge when you are on and off the throttle. That is a normal 4 stroke thing. If you have only ridden two strokes in trials previously it can take a while to get used to the characteristics of riding a 4 stroke in trials. Riding technique can also help as in using rear brake to control the bike in the tight stuff. Increasing the idle RPM will reduce the engine braking effect.

No, the level is not critical. Just drain it as well as you can through the drain plug hole, add 700 ml and you will be doing what Yamaha intended. If you want to check that there is not too much oil in there, park the bike level front/rear and vertical side/side, leave it for at least 24 hours like that and then take out the level screw and see if oil comes out.

I don't remember having that drain plug on my 1993 TY250Z. I just looked up some parts fisches for later TY250Z models and from those it looks like that plug was a change that was introduced after the first model TY250Z. Both drain plugs are in the gearbox section. Two possible reasons for that second drain. 1 It could be for where a switch is fitted to some other Yamaha model that shares the same engine casing and has a neutral indicator (perhaps TY250Z Scottish). 2 It could be so you can drain the oil without removing the bashplate.

I have not noticed any drain plugs in the clutch section. From what I've seen, the clutch compartment has the filler hole and the level screw hole. Maybe share a photo of the drain plug you are seeing

Leave the oil level screw in place while you are adding oil and put the whole 700 ml in. The reason for this is that you are adding oil to the clutch section of the transmission and it takes a while for the oil to flow from the clutch section to the gearbox section. So the oil level in the clutch section will be higher than normal until the oil levels have equalised in both sections.

Have you found a manufacturer of conrod kits for Bultacos other than Italkit?

I don't have a non-points MAR flywheel for sale at the moment. Aren't there any OSSA specialist parts people in the UK? There are some in the US who would probably have what you want. What's wrong with your points ignition? I've never seen an MAR with points ignition to know what other bike shared the same stator setup. Maybe it's something common

option 1. Buy a non-points OSSA flywheel and the In Motion ignition option 2. Buy an ignition from somewhere else https://www.francetrialclassic.com/en/ignition/5010-ossa-250-350-electronic-ignition.html

In Motion Trials (UK)

There should be clearance there. Having new parts does not mean that you are guaranteed the correct clearance. It would be risking damage to run it without clearance. If you need to, make the spacers a bit thinner. Aim for about 0.5mm total clearance.

I didn't mention the key or keyway on the left end. Yes it's common for the key on the right hand end to fail and chew out the keyways and the shaft because it's a dodgy design and people sometimes don't tension the nut properly or they sometimes fit a key that's too long. Depending on the amount of damage, you might be able to get away with machining the keyways wider in the crankshaft and the sprocket and fit a wider key.

So it's just the drive end of the crankshaft that is damaged? It's probably more common for the magneto end to be damaged by the bearing seat wearing the crankshaft down. I've got some with damaged magneto ends and good drive ends so you might be lucky and find a model 80 crank with a good drive end somewhere near you. My comment about the balance holes in the crank wheels also applies to 325 crankshafts being different to 250 crankshafts. Another thing is that early 325 crankshafts have the same diameter crank wheels as the model 80 but after about 75/76, the 325 got bigger crank wheels which will not fit in the M80 cases.