feetupfun

section swept, the A model seat that T5avs1 is seeking is nothing like what you have shown in your photos. You have shown the seat for C, D and E models TY250. The original seats on the A (and B model TY250) were also a very poor design with a thin steel skeletal frame glued to a foam pad and a vinyl cover glued to the pad and wrapped around the underside of the steel frame. The thin steel frame had captive nuts for the mounting brackets attached and 6mm steel bolts went through the brackets and the fibreglass shroud/surround/fairing. Because these original seats fell apart quickly, Yamaha developed an improved design that had a moulded plastic base which fitted very well to the original fibreglass shroud/surround/fairing and were sold as a replacement part. I only know this because one of my TY250Bs came with this plastic base seat design, with part number sticker still attached. I do not know of anyone still making replica seat bases that fit on the A, B models fibreglass shroud/surround/fairing. It wouldn't be too hard to make from scratch a seat base that will fit against the fibreglass shroud/surround/fairing if you were really keen to retain the original look. However most people I know do away with the fibreglass shroud and buy an aftermarket (C,D, E) seat or seat base as shown in section swept's photos and weld on a couple of mounting plates to their A or B model frames to replicate the C, D, E model seat mounting plates. The brackets shown in the T5avs1 photo are seat brackets for A and B models seat design and their lower ends attach to the inner ends of the upper shockie mounts. The small hole in the end of one of the brackets is for a spring that pulls the end muffler forwards. I don't know of any supplier of the A, B model seat brackets, but they would be easy to fabricate.

That terrible noise may be big end (conrod) bearing failure

Are you asking about brackets to replace the original brackets to suit the original design seat or brackets to suit a new aftermarket seat or something else

and as done by Yrjo Vesterinen with his Bultaco in the early 1980s. Nothing new under the sun

Aftermarket shocks for your bike have become available in the last couple of years and probably have a superior action to the original Yamaha unit even if it was in perfect condition. You probably need to ask yourself how serious you are about suspension performance.

I've just bought a pair of springs from majesty and am keen to try them out in the forks that already have the majesty damper rods fitted.

Frame is a few years earlier than M198

Those "weld seams" on the white tank are present on all the original fibreglass MAR tanks I've worked on

I ran a BPR5ES for trials in a hot part of Australia in a Beta and it seemed to be the perfect heat range. If I was flogging it along tracks or on the road I would use a cooler plug

The window tinter who showed me how to put stickers on said that he used the cheapest brand of trigger pack window cleaner. He reckons it worked better than an expensive brand of window cleaner.

James (on the blue TY250) could do with stiffer fork springs

The TY250 springs I have bought from B&J are quite a bit stiffer than standard springs. Your springs may be a tiny bit shorter than standard but do not exhibit much wear so their rate should be pretty close to standard. If you use additional spacers with the standard springs, anything longer than 9mm additional to the standard spacers will cause the fork springs to coil bind on full compression.

If you are stuck, replacement crankshafts can be made by suitable machine shops

If you can still kick it over with only three springs then it will probably work OK. There are many ways to reduce the clutch pull force

Posting a video on youtube may take a lot of guesswork out of answering your question

Tony the B models in Australia came with the same engine covers as the C and D models

This is a series 1 M80 frame with the tank resting in place

"top of the frame mountain"? If you mean those non-standard mounting lugs, they are definitely non-standard and yes they would be in the way of an M80 fuel tank. Would it help for you to see what a standard series 1 M80 frame looks like? A stand mount on the swingarm is also non-standard for a series 1 M80 but would be a great improvement if done properly.

The kickstart shaft and the return stop for the kickstart shaft needs to be set up so that the return stop holds the ratchet teeth apart. Some people accidentally leave out the shim that johnjsy is talking about and the ratchet teeth may not fully disengage. This MAY be the cause of your noise Another possibility is that maybe there was a problem with the return stop or the shim was missing before you rebuilt the motor. You didn't say what the history of the motor was so I have no idea if this is likely or not. I know that I have discovered many mangled kickstart return stops and kickstart shaft shims missing when disassembling 5 speed Bultaco motors.

I see the messages about this on facebook and agree that the spring shape is not quite right

Second photo is correct fitment of spring. The arm should be pretty much vertical when it contacts the spring. Spring only has load when pedal is depressed

I don't know what Greg Harding did to his Fantic 240 forks but that is what I'm comparing mine to.

If your clutch spring posts are adjustable, set the spring preload so that the kickstart will just transmit enough torque to kick the motor over cold. Also set the individual spring preloads so that the pressure plate remains parallel with the plates as it moves in and out. There are so many variables with clutch hub design and plate thickness and number of plates and spring length and spring stiffness and nut thickness and post length on Bultacos that there is no logic in recommending anything for someone else's bike regarding where the nuts should sit relative to the posts.

Unlikely to be crank seals. Sounds like gunk in the pilot jet. If it is air ingress, check the cylinder base gasket before you go pulling anything apart. These bikes have a weakness in the seam of the main chamber of the exhaust which sometimes splits when there is a decent "backfire". If you had said that the exhaust sounded louder I would have suggested checking this out. If it revved high for a long time before you got it stopped it may have done some damage to the cylinder, piston or rings. You can check for damage by looking up the exhaust port with the pipe off. Sounds a bit odd that you say it stopped with a backfire when the fuel supply was shut off. It usually takes quite a while to stop an engine just by turning the fuel off. Next time I suggest you block the exhaust outlet or turn the bike completely upside down to stop a runaway motor.

Good to see it going. Very cool-looking header pipe. Did you make it? Also where is the beautiful fuel tank from?