feetupfun

What was the carby that didn't fit?

To work out the spring rate the coil diameter and number of active coils are also required. If they were well sized at the time for the ROKON they will be in the ballpark for rate for that bike for trials. If the pistons aren't worn and you tidy up the tube bores without losing metal then you don't need to put rings on the pistons. I've got a few Spanish bikes with Betors that look like yours inside with not a huge number of hours and those forks work brilliantly. It's only worn tube bores and pistons that benefit noticeably from having rings. But if it makes you feel good, do it! Teflon sheet cut into a strip works well for rings. I don't know what micarta is.

There are countless different ways to modify a TY175 frame. There are copies of the first 175 Majesty frame but otherwise there is no generic name for any variation. If you want to make one you need to work out what you want to achieve and decide how to get there. TY175s have been getting frame mods since they were released in 1975 so there is no "year". There is also a very wide range of opinion for what the ideal TY175 mods are. I'll have a go at some safe generalisations. The Ty175 wheelbase is very short standard so lots of people make the bike longer. The steering rake is large standard (which suits the short standard wheelbase) so it's common to reduce the rake, in many different ways, to get a better trail dimension that will suit a lengthened wheelbase. The TY175 is short between the footpegs and the steering head so it's common to move the pegs back but there are other ways to fix this too. The pegs are high too so it's also common to make them lower. The forks are quite skinny so they flex a bit unless you are a light person. Most twinshock forks are 34, 35 or 36mm diameter and it is fairly common for the original TY175 forks to be replaced with something bigger. The TY250 twinshock triples and forks fit on without modification and can be set up to work very well. The swingarm pivot is a long way from the back of the motor so it is fairly common to move the pivot forwards which allows the use of a longer swingarm to improves handling and rear suspension action. The rear suspension travel is a bit short when compared to much later twinshocks like TLRs and Fantics and it is fairly common for one or both shock mounts to be relocated to increase the axle travel. The swingarm is fairly flexy so handling is improved by using something that resists twisting better.

Sorry Simon I'm not a Honda person. I just had a test ride on that one many years ago

Not sure what you are calling the spray tube cutaway. If you've got a semicircular shield that sticks up into the airstream where the needle comes out of the needle jet, the open side of the shield always faces the engine. No idea about that new type of slide. Your main jet holder looks like what was in the MK2 AMAL concentric on my M198. Having a bigger float needle and orifice doesn't matter but make sure the fuel height setting is not affected by the change. I figure you must be pretty keen on originality to be asking all these questions and you may well know already that this carby has a reputation for rapid wear of the moving bits. You didn't mention wear of the slide bore but they are well known for that. The fastest wearing bits are the needle and needle jet. There are many excellent alternative carburetors available.

I was thinking about that afterwards. I reckon it would be prudent if using 2 single lip seals to fit them so they were the same setup as a dual lip seal, which I think is springs out, but I haven't looked at a crank seal lately

They get very heavy due to the oil soaking the sound absorbing material and the buildup of carbon deposits. I've reduced the weight of a main chamber by over 1kg by cleaning it out. The sound absorbing material in the main chamber stops absorbing sound when it gets oily and when the mesh holes get blocked. On some exhausts the performance of the motor relies on the harmonics inside the main chamber and this can be affected by the holes in the mesh getting blocked. There will be deposits inside the header which might not stop you pushing a tube through but it means that it is no longer the ideal diameter and surface drag for ideal engine performance.

No way. It is very likely just the single lip seal causing it. If the seal diameters are 25 mm ID and 40 mm OD like I suspect they are on your 247, you can use a Yamaha TY175 magneto side crank seal. I discovered that the Cota 348 seals are the same size seal as the left TY175 seal when I changed the crank seals in my Cota 348. The seal would also be available from a Montesa twinshock-era parts supplier. The reason you need a double lipped seal is that it has to seal with the high pressure side alternating from crankcase to drive casing every time the motor turns. So if you want to use single lipped seals and can fit two thin ones in the holder, make sure you fit them in opposing directions.

If the seal spacer is worn it will have grooves worn in it, not a reduction in diameter. Have you got the right sort of seal there? It should have two lips in opposition. Looks like a single lip seal in your photo. It's possible that as suzuki250 says, one or both main (crankshaft) bearings may be shagged which can cause the seal spacer to move up and down enough for the seal to leak past the lips even with a nice new soft seal. You should be able to feel that degree of play in the mains by trying the move the end of the crankshaft up and down by hand. Another thing is that it is normal to find that much oil in the bottom of a two stroke, but it should look and smell like your brand of two stroke oil. The definitive symptoms for oil getting sucked into the crankcase from the primary drive are weird smelling exhaust smoke, too much exhaust smoke, weird colour (white) exhaust smoke and having the oil disappear from the primary drive casing. You have only mentioned one of these symptoms.

The top ring of the filter is normally the only thing used to seal the top side of that banjo joint. There may be a minute defect in the filter ring or either of the metal surfaces. There is no law against using a bit of sealant there. Other possibilities include a tiny hole or crack in the bowl or the banjo bolt thread bottoming out. A protruding tip plug is better for that motor. I use BP5ES and BP6ES. Yes 0.021" is a good gap if you still have the standard ignition system.

Can't tell from your photo if yours has been resleeved. Be aware when you resleeve a motor the sleeve usually has a shoulder at the top to stop the sleeve moving downwards through the cylinder, so if you are going to measure the OD of the sleeve, measure it at the bottom end.

I found it. This is taken from a 2014 post by ksmith Yes this 250cc OW10 engine was from the 1973 cantilever bike, but it is worth pointing out that all other Yamaha Works Trials bikes ( what ever the model ) had there engines numbered OW10, they are all are very different, ( O= prototype W= works and the 10 denotes TY). note the clutch case and clutch cable on Mick Andrews OW10 cantilever bike below. About ten (250cc) engines were made for the cantilever bike but only a few have survived, at a glance they look like TY twinshock engine`s but none of the components are interchangeable they are one off`s. The barrel is the Majesty S type only used on the Majesty Works bike`s it is a TY barrel bored to 74 mm and re`sleeved with new port timing, the finished bore is 70 mm ID with a stroke of 63 mm. Please see the photos below. Many thanks to Mr Clay Seltzer (vintageworksbikes.com) and Mr Torsten Hallman (four times World MX Champion) for explaining the meaning of Yamaha works numbers.

OK now that you have said that there is an increase in power when it happens it's more likely to be the throttle slide sticking. This is a common problem with AMAL carbies mainly because they suffer from rapid slide and body wear but also because the slide and body are made of metals that seems to become sticky where it wears. Water inside the slide bore can make any stickiness a lot worse.

The remaining two bearings on the drive end of the crankshaft can be removed like this. Ease the outer one away from the inner one using wedges between them, then when there is enough gap, use a bearing puller. Ease the inner bearing away from the crankwheel using wedges between it and the crankwheel.then when the gap is big enough, use a bearing puller. For both of them, heat up the bearing to make it move easier. As for the magneto side bearing having been moving relative to the shaft, that is commonly seen with these motors. Even with the C3 bearings there is not enough side freedom for the crankshaft expansion during heating/cooling cycles of the motor and whichever bearing fit is the loosest will see movement which gradually further loosens the fit. As long as that bearing remains free-spinning and there is at least enough interference to spin the bearing, the tightness of that fit is not a problem. On some of these motors the outer race is the fit that moves which can cause enough wear to require that the casing hole be sleeved back to size.

Where are you seeing that reference? It doesn't sound right

From your description it sounds like it is free revving without much power meaning an air leak. On that bike the two likely places for this would be the rubber connector between the carby and the cylinder and the igniton side crank seal.

If you look where the sleeve meets the aluminium casting at the transfer ports you may be able to see if the sleeve has ever been replaced. This is because on Yamahas the aluminium part is cast around the sleeve but when it gets resleeved the old sleeve is completely machined away and the new sleeve is inserted. What is it about the cylinder that makes you think that it once had a sleeve for an 80mm piston?

Apart from changing the way the 250 Sherpa T motor performed, Bultaco made multiple internal and external changes along the way to your M198A, for reasons including styling, frame design change and mechanical durability. One of the reasons I love Bultacos so much is that by working on their bikes you can learn what they changed when and try to understand why they changed it. A recent quest on this has been understanding the sequence of changes made to the external engine casings around the time that the Sherpa T frame design under the motor changed from a single tube to two side tubes during the 3 years of the M49 production. Just when I thought I had worked it all out, I find there is a third type of M49 primary drive cover that must have only been produced for a very brief interval. One thing that surprised me about my M198, which is the latest model Bultaco I have, is finding a copper head gasket much like what is in Japanese engines of the era. I've yet to work out the reason for why they changed this part of the design when the previous gasket-less design worked so well.

Pursang Mk5 and Mk6 325cc, Mk7 and Mk8 360cc, Mk9 and Mk10 and Mk11 370cc The engine that changed capacity for tax purposes was the Sherpa T 250 and it went to 238cc

Another thing that might clear up some confusion is that the number stamped on a Bultaco cylinder is usually the Bultaco model number that the cylinder first appeared on. For example the first 5-speed 250 Sherpa T was the M27. The M27 cylinder is stamped 27. The following Sherpa T models (49, 80 and 91) also have 27 stamped on their cylinders. This may explain why your M213 Alpina cylinder is stamped 116. M116 is an earlier model 350 Alpina than the M213.

I see you have M166 and M213 cylinders. These are 350 Alpina cylinders and will be for a longer stroke bottom end than the Sherpa T cylinders you mentioned. The first "350" Alpina (M99) was actually 325cc and had a 60mm stroke. The "350" Alpina models after the M99 were actually 350cc and this was from having the longer stroke. Because of this, the M166 and M213 cylinders will be longer than your Sherpa T cylinders.

When I said that the early 250 Sherpa T would not pull to as low an RPM as the later Sherpa T 250, I'm actually talking about low RPM as used in trials riding, not the 3000rpm that is the lowest RPM on those power curves. 3000 RPM is mid-range RPM on a 250 trials bike. When you get your 198A going, find someone with a round barrel Sherpa T that runs well and have a ride comparison. They are very different motors and in my opinion the numbers on the curves reflect reality. The power curve for the late model Alpina 250 looks like the late model Sherpa T 250 curve because apart from the gearbox ratios, the motors are the same design.

Which engine are you asking about?

The later 238cc 250 has different porting and compression ratio to the early 244cc 250. The later "250" motor has very different characteristics to the early "250" motor. While the later motor in standard trim does feel less powerful, it will keep producing useful power to lower RPM before dying than the early motor. The later 250 motor also will rev out better than the early 250 motor. Neither of the Bultaco "250" motors is what you could call powerful compared with many other "250" twinshock trials motors. The early 250 is strong in the mid-range but signs off early while the late 250 is fairly weak everywhere but has a useful RPM range that is about as good as you can get. Spanish power numbers of the early 1970s are usually derived from using an engine dyno. While the early 250 Sherpa T is declared as being 20 HP, a TY250A (which is declared at 16 HP) will leave a standard 250 Bultaco Sherpa T way behind in a drag race. How they work in trials sections though is the most important thing and is also why the 325 is only 21 HP. As for the 325 motors, again there are two different motors. They share the same bore x stroke but in the mid 1970s the motor was redesigned and became a much more refined thing to ride than the early 325 and is very highly regarded for competition use.

Hmmmm porky according to the specs. I reckon this calls for an official weigh-off of TL320 vs Jumbo