Cynic

Personally i don't know any Yamaha's since the 70's that have the model name as part of the engine/chassis number. As an example the 1980 pw50H was prefixed 260, the following year it was prefixed 4x4. (From a 3rd party so may be incorrect, is correct re my dt) The font looks ok though but it is missing the * that's normally present somewhere. But that's all i got.

You need to supply more information, one post question about the viability of security issues. You could, emphasising 'could' be attempting to verify a corrupt engine number. This falls in the same category as first timers asking how you wire up the ignition. Can you even say the age and where in the world you are......

As far as i know the CS3 came first and became the RD in 72/3. but i may well be wrong. The same thing happened with the DT monika taking over from the CT prefix on the trials bikes at about that time 72/3. There is confusion with the ct1 as they kept the ct in the ch/engine number after becoming dt's Not sure about the coffin tank, could be a refit from a later bike after damage/corrosion or a changeover model. Changeover more than likely considering the age. Wouldn't swear to it but i don't think I'm far off. What else does the bike have, spokes and front drum brake, spokes and a disk or alloys and a disc. How lairy is the colour, purple or gold or a more restrained red or blue. Could well be a US bike but that would put it back as US bikes were generally 6-12mths behind their spec, shipping times. So a 1974 US bike could easily be 1973 spec model. If it was subsequently sent over here and re registered it could get even sillier.

Sure its a T and not an F. Normally its green (N), yellow (T)turn) and red (C) for charge with an oil level warning in the spedo.

Cone filters......... OMG that's where your 20mph went, if you can get the proper filter back on do it. Those things are only any good for turbo's and drag bikes that spend their entire lives at 100%

Worth asking, we don't know. Used to ride with a guy rode a 1200bandit, (he's even bigger now). I had a 600 and the bikes were equal. There was one time we swapped, wow his bike was fast, tuned and everything, without his extra. He was 20+ stone, I'm only just 14 now and was nearer 12 then. It was a proper wheelie monster flying machine. Funny though after i stopped and waited for him he never wanted to swap again. 8 stone is worth a LOT of horsepower, i bet there was 40-50 hp difference between the bikes.

Its a pretty rudimentary rule of thumb but as far as it goes you would expect .015 for the inlet and .025 for the exhaust. An ohc engine is what it is and they all share the same tech. Just measure them cold and see what they are, with shims normally you only need 2. If at all. Unless its been neglected in the oil department there should be little to mess with. I had a 600 Suzuki, put 35000 miles on it with good oil and changed the shims around once. That was .01-15 and 0.02-25 as well.

Yep, but you got the 400 tommy, the 250 was.... well...not. Had the same hp as a dt175 (17). you have the 16v 400, that's 45hp. Tdr is little more than that but weighs nowt. Not wanting to rub salt in......... well..... just realised reading TDR forum that my bike being unrestored original(ish) is prob worth around 4k now. Ropey ones are up for 2 and a half. That got you a minter 2 years ago.

Calm down, stop thinking 1980's fsie or 50m, that thing is liquid cooled running a upsized version of the am6 engine that's in the daughters 50cc. Out of the box that will do 50 at a push, they can get that engine to 20hp at 50cc so if you have the know how and a few bob (were not talking big numbers) those guys could be running 15 to 20hp. Plenty for those antics. Sounds good too.

Quite possibly, you might want to say hi in the new members area. Short questions with no info don't get much of a response. What is it you need to know.

Wow. Takes me back. I had one of those. Cost 200 quid in 1986. Practicly lived on it till it got nicked. The m was only token restricted. Pipe and a rejet got you 50mph. A decent big bore and doing a superman you were over 60. Proper 60 too. I could outperfom rubbish like cb100's most days. Took loads of maintainance though. Points were a bugbear an condensers. They are such a pain i took to fitting them outside the mag. Lot of laughs on that bike.

Double check there is no cack or old gasket left in there, some are 2 rings with a material centre, the outer may still be in place. Got a part number, we can double check for you.

How do you mean too big. Too round or too thick. It is supposed to crush to a degree to form the seal.

Yep leave them a dummy and just use a kill switch somewhere under the tank. Friend used a remote thing from maplins. Press a button and the bike was dead. Or you could be really cute and use the tank cut off switch for your own use. Only have to splice in a basic switch and fit it where you like. Fellow at work has a power socket on the bars for a 'sat nav'. Nope, needs his modified power plug or the bike wont start. Kills the ig to so it wont bump, but looks like a regular power socket. +1 on the ground anchor too, just adding to the list of reasons why i love living out in the country. Any scrotes kicking about in the streets everybody knows.

Nice one, glad its fixed. People hardly ever come back and say what the problem actually was thanks. Have you nailed it yet, eh??

Fat lot of use that is.

Strip the sump and check. Clean it up and if it is cracked get it welded. Ally welding can be done for ok money. I had some lc cases welded a long time ago. After grinding and a bit of elbow grease you would never know. As for the plug, pinni.... phinny.... phhimr... see above

Changed the plug and cap recently? Always, ALWAYS job number 1, keep the old ones too. You may get duds out of the box. If you ever change a plug keep it till the next time you do as a known 'good' one. Makes diagnosis much easier when you have components that are certain good.

Be honest. When was the last time you or somebody : New plug New filter Decoked the pipe (can off and shoved a hosepipe up the pipe, dry of course). Time since last rebuild. Any of these will affect the way the engine runs and ultimately the amount of smoke. A long run (20-30) miles with the bike held at 8500-9000 rpm even if you need to do it in 5th, but better with wider throttle will burn a lot of the softer gunk right out. May smoke quite badly if you do get the gunk to light off. If you do keep going till it stops. Looks dramatic (especially at night as you get loads of red embers/sparks) but its not hurting. Now get yourself into the new members area and say hi, this is a first post question or i may well be the only response.

Quite, the one i saw at the tt had 2 underslung pipes (front pair) and the rear cyl came straight out the rear similar to the std cans.

Lol Saw one at the TT. Looks cool, sounds better but a 500 stroker and a 2 gallon tank. 250cc is bad enough. Worse problems to have though.

Look something like this then, https://yamahaclub.com/forums/topic/28667-my-dt-125/

That my friend is a perfectly healthy 2stroke doing what a perfectly healthy 2stroke should. Its what they do. Maybe let it get a little warmer before moving off and use the gears to get through the smoke. If yours has a problem i have 3, 2 strokes in my garage need the same remedies. What is actually happening is:- A 2 stroke uses some of the exhaust gas as the charge, wonderful thing 2 stroke engine. Trouble is the oil condensates out when the engine goes cold and some will puddle in the cases (normal only actually a tiny amount). When you start up initially the oil starts to burn off, when you ride off and give more throttle the oil burns off roughly, this smokey rubbish gets in the way of the charge and gets spat into the front of the exhaust. As i said, its how they work, letting it tick over for a bit and not revving the engine before you switch off will help as there will be less residual oil.

And the signs of needing a decoke are?

The flaw in that statement is somewhere between, "buy a cheaper 350 or 500" If its a general 350 rd your after the earlier 350b is still out there for sensible money but your into the sharks for anything water cooled or a 400 (350 is probably a better bike in truth but hey). Do the work you might get lucky..