feetupfun

This is a fairly eye-opening and entertaining thread To answer the original query you should stick with the fuel that Honda recommend because "race gas" burns too slowly for a standard 4RT. The problems with burning too slowly are that it causes high exhaust gas temperature and the motor produces less power

I find with the standard Ty250 frame you do need to be over a certain height to be able to lower them by much without it becoming a problem. I'm 1760mm 5' 10" and have a Ty250 with standard frame that I mounted them in the position you have done and found that my coccyx sometimes hit the bike behind the seat when unweighting the rear on obstacles. I figure I would have gotten used to how far I can unweight if I only rode the one bike. It did change the handling a lot

248 is a much better motor for mortals

If everything is perfect it will be no better or worse than a 1985 TY drum-front-brake mono was when it was new. If you had to take lining material off to get full contact, it means that the drum is probably a bit big in diameter and you probably now need to pack out the pivot ends of the shoes to minimise the cam angle. If the drum surface is grooved or uneven they will never work well no matter what you do. It's normal practice with people wanting the best front drum brake they can get to machine the drum smooth, have oversize high friction lining material attached to the shoes and then machine the shoes down to match the diameter of the drum, using the backing plate to hold them in position in the lathe

lean the bike over

Yes a fine line with the backpressure. Too much and it will be RPM limited, too little and it will have less torque than ideal at low RPM. It should go heaps better (for trials) with an end muffler or a stinger compared with having nothing on the end of the main chamber

I can say that TY250 TS is sensitive to exhaust backpressure and you can also have fun experimenting with the length of the header pipe (just like any other trials two stroke) You should know what you like before you start experimenting. The standard Yamaha TY250 design works well but is pretty heavy and the end muffler makes a terrible sound Some people prefer the 74 model engine over the later ones, so again it would be good to know what you want

I'll have a go at interpreting these metaphors from the other side of the earth "A degree of marketing?" = writer is indicating in a sarcastic manner that they would attribute a low probability of being able to convince potential buyers about the benefits of buying a side-valve trials bike "What like if this pig can fly you'll need a 1/4r of a ford pop for trials?" = If this unlikely concept were to succeed in the market, then potential buyers would need to believe that it was a good thing for a trials bike to have similar engine characteristics as an obsolete British road car or van that had a four-cylinder side-valve petrol engine

Need a better photo if you are serious

The KT end muffler now has a perforated tube that runs the full length of the casing with loose wool packing attached to the tube with steel wire and has a long radius 90 degree bend of the same diameter as the perforated tube welded to the outlet, pointing downwards. The perforated tube was cut from a 2 stroke go kart muffler core.

Yes the WES is noticably quieter than the standard TY250 muffler, and it is a much more pleasant tone as well to many people's ears.

I'm like that with the noise level too. I've a nice practice area at home but there are houses within 50 metres, so I have been quite persistent in getting my bikes as quiet as I can so I can practice without annoying people. One of my bikes is a Cota 348 with standard (repacked) exhaust system and it is blissfully quiet to ride. Late model Bultaco trials bikes are also amazingly quiet with the standard exhaust. With a bit of work you can even make Japanese two stroke trials bikes quiet. One thing I noticed about a WES muffler I have on a TY250 Yamaha with standard main chamber. It is nice and quiet both through the walls and at the muffler exit, but if I put the same WES end-muffler on my KT250 (also with standard main chamber), there is a lot of noise at the exhaust exit of the KT. Because of this I have re-modelled the internals of the standard KT250 end mufler to make it quiet enough to ride at home (and so I can hear the whining primary drive gears better) One of my TY175s is a Whitehawk with a standard Yamaha front exhaust section and the end muffler is from the super-quiet TY175JC that was fully road registerable after the noise-limiting Aussie vehicle design standards came out in 1976. The TY175JC also had a rubber snorkel on the inlet of the airbox which further quietens this bike. The engine in this bike has just been restored so there is no mechanical noise to speak of and overall it is a very stealthy bike.

In answer to your question about the tailpipe insert. Don't do anything to the exhaust that will change the backpressure because it is fairly critical for how well the motor runs. Too little and the low end response will be poor. Too much and the motor may develop detonation problems when it is hot

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.