feetupfun

OK I'm going to ask a silly question. Is it possible that your other big 2 strokes have been water cooled? The 348/349 motor is well-known for having significant piston rattle and if you are not used to the sound of a noisy air-cooled motor the noise you are having trouble eliminating might be piston noise (not being deadened by the water jacket)

Bultaco big ends on 250 motors can be refurbished by using a new pin, honing the hole and fitting slightly bigger diameter (imperial sizing) rollers (Harley Davidson part). I don't know if the M92 (325) big end bearing uses the same size rollers though. I would have thought new rods kits are available for M92

I now think I got that the wrong way around in my previous posting. I now reckon that they used leftover Cota 348 frames to make the Cota 348 Trail after they started making Cota 349s, so no, I don't know of any 348 model that came standard with an alloy bashplate instead of tubes under the motor

The Cota 348 Trials models all had frame tubes under the engine but later there was a Cota 348 Trail model that used a frame very similar or the same as the Cota 349 Trials model which has the alloy bashplate instead of frame tubes

At least we now know it is a flange mount Mikuni, which narrows the field a lot. Most Mikunis are spigot mount. A photo showing both sides would make it easier. To measure the "size" of a carby you measure the venturi diameter, which requires taking it off the bike

Which plates are you talking about?

Yes you were complicating it. The piston position when the points open is critical. The 0.3 to 0.4 gap is not critical. Just to explain why the Yamaha does not have an adjustable (slotted holes) backing plate. It is to make sure that the magnet in the flywheel is in the correct spot relative to the stator coil when the points open, to maximise the spark energy at the spark plug. Some other bikes are less sensitive to having the magnet position exactly right and have slotted holes so you can change the timing without changing the points gap. Working out exactly when the points are opening can be done by just looking at them with a torch (least accurate), inserting some very thin paper (roll-your-own cigarette paper), using a battery and low voltage light bulb across the points or using a timing buzzer across the points. The light bulb and buzzer methods give the best accuracy. Because points ignitions sometimes have problems with sparking between the points (condenser failure) or mechanical problems with the points mechanism, it is a good idea to also check the spark timing with the engine running by using a strobe light that is triggered by the spark (timing light method). If the points and condenser are working properly, the spark timing should not change as RPM changes so if you see it move using the strobe, you will know you have a problem

Front wheel has Grimeca hub (Fantic, SWM)

sounds like a bent clutch pushrod

Nowadays (aged 58), 20 trials competitions and 70-100 practice sessions per year. Been riding trials competitions for 41 years and I've never come near being an Expert. My skill level has always been somewhere near the middle of the bell curve.

Great photos and advice. Thank you

Thankyou and the tank looks great. I have always liked the idea of being able to buy a new plastic tank to replace the rusty or dented originals but am not keen on buying unless I can see what they look like on the bike.

Thanks Zee I have been wanting to see decent photos of what that Clark tank looks like on a TY250 and it sure does look good from the side. How wide is it at the rear end of the tank? I also see you have an aluminium kickstart lever on Rob Halford and would love to know more details about that

A doctor at the local hospital told me why I ride trials A few years ago I got a burn on my leg worth seeking medical attention when I landed on top of the exhaust, went to local hospital A&E dept and when being checked by the doctor he asked "how did you do this?" and he started reading my details on the chart. "Ah... motorbike" I reluctantly replied, knowing that doctors don't like people riding motorbikes. Dr then says "I understand. You are 55 years old. I'll bet you fell off your Harley" Due to my dislike of Harleys I get a bit rattled and say "no way would I ride a Harley, it was a trials bike" Dr says "Ah, you haven't grown up then yet hey?"

I have kept an orange/white Ty175 tank with (well aged) factory paint if you need photos for comparison of the shape of the graphics

As far as whether it has had the tank repainted, a side view usually gives it away more than looking at the filler neck, because the shape of the black line and the location of the YAMAHA letters are very difficult to get exactly right. Yes the orange colour in your photo looks wrong. It is a standard Ty175 tank and all Yamaha Ty175 tanks were made of steel. TY175 frame welds were done by MIG process and TY175 frame welding does seem to have a low cosmetic standard. That era was a time of rapid development in production line welding processes and the Japanese manufacturers were early adopters of MIG process while others persisted with flame welding until later. TY175 frames were designed to be cheap to make in large numbers and cosmetic appearance did not seem to be a high priority at the time. Another good reason for people to buy beautiful aftermarket frames made in the UK. Another weaknesses on the TY175 cosmetic front was the painting of some of the shiny aluminium components with a clear finish that chipped off when the bike was ridden off-road, leading to poor appearance very quickly. They looked great in the brochure and on the showroom floor and could be warehoused for a long time without losing the visual appeal, unlike the uncoated polished Spanish bikes at the time which soon developed a white dusty appearance on the polished aluminium bits if warehoused. As far as mechanical build quality, the Ty175 was of a very high standard for the time

I see in your earlier photos the top shock mount is in the bottom hole of the gusset plate and in this photo it is in the middle hole. Is there still clearance between the shock body and the shock mount gusset plate with the mounting pin in the middle hole?

With the axle in the position as shown in the photo where the front edge of the tyre is too far towards the left side arm, is the side of the rear wheel parallel with the front wheel? (use a straight board and allow for the difference in widths of the front and rear tyres)

Sorry for the seemingly stupid question but is the hollow side of the dish in the sprocket facing outwards?

I just of thought of some things I have found with TY250s that might be relevant Someone had respoked the rear wheel and it was offset relative to the hub. TY twinshock rims should be centered on the hub spoke flanges Another one had a swingarm that had both swingarm arms bent slightly sideways

What do you mean tyre centered in the swingarm? Centered between the axle tabs? Centered between the pivot tube ends? Tyre edges same distance from the swingarm arms at the front end? I'll measure some TY250s if you tell me what you are measuring. Please also confirm that you have the first type swingarm with the simple flattish plate gusseting rather than the later type with the formed, curved gusseting between pivot tube and arms

About the tab washer, yes this mode of failure is more likely because the tab washer connects the sprocket with the nut, making it more likely that the nut will move relative to the shaft. However even without a tab washer, the nut may still rotate with the sprocket when it slips on the taper

If it was me I would let TALON know about you issue. If the problem is the coating on the bore of the sprocket, it would only cause problems with bikes that use a taper mount sprocket, and there are very few bikes that use a taper to mount the gearbox sprocket, so they may be unaware of the problem.

I think you are right about the nut becoming tighter when you ride, causing the taper to break the sprocket. The only thing that can cause the nut to rotate relative to the shaft while you are riding is if the sprocket spins on the shaft, so maybe 50 foot pounds is not enough to prevent slippage. I see that the sprocket came with a yellow coating which may affect how well the sprocket locks onto the taper (the angle of the taper is designed for steel/steel contact)

I have that gearbox output shaft arrangement on my 348 and have not had a problem with the sprocket splitting or slipping. My sprocket is a TALON purchased in the 2000s. Some thoughts came to mind when I read your post: Maybe faulty sprocket (possible incorrect heat treatment process) Maybe a lubricant was used on the taper during fitting of the sprocket A clear photo showing the fracture surfaces of the sprocket should show if the problem is a faulty sprocket.