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Here in OZ the Mitas competition trials tyres are also far cheaper than Michelin, Dunlop and Pirelli but due mainly to their low profile shape (shorter contact patch) they perform quite poorly in trials competition compared with the others. However one thing that Mitas trials tyres are good for is if you want road registration for your trials bike they have US DOT approval so can be used on the road here in Australia and are far better than any other trials type tyre that has DOT approval. Mitas trials tyres also sell like hot cakes for flat track (sliders) racing on decomposed granite tracks where previously the rear Michelin XII trials tyre was the favourite (because they are far cheaper but perform on that surface just as well). By the way the standard retail price for a rear Michelin tubeless XII here is AU$180 which is virtually the same as 60 UK quid. David Lahey Queensland Australia

Jesus with his knowledge of Spanish is spot on here. The late 1960's OSSA trials bike made famous by Mick Andrews was called an OSSA plonker in the USA and the name has eventually caught on elsewhere. When sold new elsewhere it was generally called the OSSA MAR or OSSA Mick Andrews Replica. Plonker was simply a generic term for any trials bike at the time. When I started looking at TODO Trial website I used the machine translation into Australian (it's a bit like English) so I could read about the history of some of the Spanish bikes and was at first stumped because the machine translation of OSSA MAR is OSSA SEA. I eventually worked out that the translation software read MAR as Mare, which is probably Spanish for ocean or sea. There are some other weird translations from Spanish like the Wild Level (Montesa Cota) and some other weird thing I don't remember which really means Honda TLR200. After you read a (fair) bit of the machine translations it almost makes sense (sort of). Plonker certainly came from the characteristics of early trials bike engines as does "bogwheeler" from people increasing the flywheel mass of their old 4 stroke trials bikes using "bog" (lead). David Lahey Australia

179/180 pounds sounds a bit on the light side for a TY175 that is ready to ride and has a sidestand. My TY175B has had the oil system and all the lighting and its wiring removed and weighs 186 pounds ready to ride with a near empty fuel tank. Maybe the fibreglass seat/tank unit makes a big difference? Mine still has standard steel tank and seat base. Maybe gravity pulls harder here in the southern hemisphere? Don't be tricked by the weight quoted in the Haynes manual. Modern tyres weigh a fair bit more than the crap Japanese Dunlop tyres on brand new TY175s so don't forget to add a bit for those modern Pirelli tyres. The TY175 certainly feels a lot lighter to ride than other popular twinshock bikes of the era which generally weighed somewhere around 205 to 210 pounds ready for competition. for mcman56 Yes early 1990s trials bikes such as Gas Gas do weigh about the same as a TY175. David Lahey Queensland Australia

Left side shifting on Bultacos worked well from the beginning in 1975. The shifter shaft being extended through to the left side is a good design. The shift action is best if you use a rigid steel shifter as sold by Sammy Miller. The ultra lightweight aluminium shifter which came with my LH shift Alpina was a bit springy for my liking. As far as Sherpa models to watch out for, I seem to remember a mid 1970s model Sherpa that uses the old style rear hub with brake and sprocket both on the left and a bowden type cable coming from the RH brake pedal. This is a pretty dodgy arrangement because it reduces sensitivity for the rear brake. Alpinas, Fronteras and Pursangs from 1975 had rear hubs with brake drum and sprocket on opposite sides so the RH brake had a simple rod back to the brake arm. I don

Steve The new Talon sprocket without keyway works fine on my 348 Cota provided the shaft is clean, undamaged and the nut is tightened correctly. I don't know if the 330 is the same arrangement but my 348 also had an old keyed sprocket on a taper which I recently replaced with the unkeyed Talon sprocket. Options for removing the old sprocket are to make a special puller or cut the sprocket off with an angle grinder. I have heard that the sprocket will loosen by riding the bike with the nut loose but would not recommend it in case the key shears and damages the shaft taper surface. I have had great success using an 100mm angle grinder with the chain removed and the gearbox in gear. Take care not to grind too far. If you can see where the keyway in the sprocket is, grind there to reduce the risk to the shaft. It took about 2 minutes to get it off and no damage to anything. David Lahey Australia

Rockhopper You have good taste in bikes. 348 Cota timing is 1.8mm BTDC or 17.5 degrees BTDC Ignition points gap 0.4 mm Sparkplug gap 0.5 to 0.6 mm Sparkplug NGK BP5ES or Champion N12Y Cota 348 comes standard with MKII AMAL concentric (square body round slide AMAL same as what came on Bultacos of the same era) AMAL model 2626/402 Some people fitted a Dell Orto carby to replace the AMAL on the 348. My 348 goes great with the standard AMAL carby with standard jetting. AMALs tend to wear out needle jets and needles quite quickly but parts are usually readily available. New AMAL and MIKUNI carbies are available for the 348. David Lahey Australia

Ignition points gap 0.3 to 0.4mm Spark plug gap 0.5 to 0.6mm Ignition timing 1.8mm BTDC

Dear kcj Here in OZ we call it a pneumatic impact driver or "rattle gun" and with one of those driving the socket head screw at the bottom end of the slider and with the fork leg assembled, the screw should loosen easily. If you need more force on the damper rod to hold it still, assemble the forks and wheel into the bike and compress the forks. A bike trailer and tie downs works well for this. Once you have done it once, you will probably be able to change fork seals in about 20 minutes next time. PS It is good in theory to make a special tool for next time but it is usually not necessary. David Lahey Australia

DG Shannon with his 247 Cota said exactly what I also think about twinshock trials bikes. Where I live, it is legal to compete in Twinshock class on SWMs, Fantic 240s and OSSA TR80s and they are certainly easier to win a trial on but there is no way I'm even slightly interested because I have no connection with trials bikes made after 1978. They look way too modern for my taste. Due to impressions cast deeply on my psyche back in the 1970s, I get wobbly knees when walking past my 1975 TY250B, 1976 Cota 348, 1976 KT250 and 1974 250 MAR OSSA. If I wanted a bike that felt modern to ride, I would ride my modern bike. Thankyou DGShannon. Great to hear your message and your Cota 247 is certainly a lovely bike. Vive la difference David Lahey Queensland, Australia

You can successfully use pistons intended for many Yamaha 175cc bikes singles of the same era in the TY175. Some include 1973 CT3, 1974 and 1975 DT175 and all TY175. Wiseco pistons for these bikes are fine also. All these pistons provide the same compression ratio. Don't worry about the shape or size or even complete absence of the holes in the back of the piston. More information needed about the whiskering please. First what do you mean by whiskering? We need to clarify this because you seem to be saying that the whiskering is "destroying" the plugs. If one of my plugs whiskered I would just flick the whisker off and put it back in again for more use. Spark plug whiskers can be composed of steel or cast iron or carbon. If you are getting steel whiskers it could be that the engine work wasn't very sanitary or the port edges weren't chamfered or there could be something rubbing metal to metal where it should not. Check a whisker with a magnet. If it sticks it is steel or cast iron. If the whiskers are carbon and the engine has just been rebuilt, then there is something pretty amazing going on in there because carbon whiskers are usually (1) old carbon being loosened up by a change in engine operation ie lots of heat in an engine used previously for lots of gentle slow speed work or (2) a change in fuel type or premix oil type which loosens old carbon. If your engine has just been rebuilt, it shouldn't have any carbon deposits anyway. I suggest you use an NGK B7ES or B6ES sparkplug, high quality fuel, high quality premix oil at between 25 and 35 to 1 ratio and when you get another whisker, see if you can work out what it is made of. I just thought of something else. I remember a friend once added graphite to his premix on the advice of a graphite salesman and he managed to ride for about 20 minutes before the first carbon whisker formed on the sparkplug of his otherwise perfectly tuned bike. He cleaned the fuel system out and then spent the next few weeks replacing sparkplugs as the graphite (carbon) worked its way out of the engine!! If you want more help, please provide more information about the way you ride, the fuel you use, the ignition timing, the carburettor type and size and jetting and how long in time the bike runs between whiskers. David Lahey Australia

Dear Daywen Dating TY twinshock Yamahas is not easy because not all models were sold in all countries. If you post details of the appearance of your bike and where it was originally sold from, you will probably get some useful feedback about your bike. Here in sunny Oztraylia we had four 250cc twinshock models. They were called TY250 1974, TY250B 1975, TY250C 1976 and TY250D 1977-1979. Our TY250 and TY250B both were prefix 434 but were substantially different to each other. Appearance wise, the TY250 was metallic white and yellow. The TY250B was solid white and yellow. The TY250C and D were substantially different to both the TY250 and TY250B but were the same as each other mechanically, differing only in colour and graphics. The C was white and dark blue while the D was pale blue. The TY250C and D share the prefix 493. I could go on all night describing the differences but if you have a bit of a surf around all will be revealed about these and a dfiierent model TY250 twinshock that was sold in Europe with a white tank and a red stripe. Yamaha sold three model TY175s here. The TY175B 1975 and TY175C 1976 were virtually the same apart from the colours and graphics and the TY175JC 1976-1978 was a fully road registerable model that was based on the TY175B/C but with mods to allow it to pass the strict Australian motor vehicle design standards. David Lahey Australia

A good way to start a head moving is to put the piston up near TDC, fill the combustion chamber with oil, refit the spark plug then rock the bike back and forth in top gear. Once it is moving but it is still tight, suspend the bike off the ground a bit by the head and gently bump the bike downwards. Gravity will do the hard work for you.

Yes I have a 4GG TY250Z here in Sunny Queensland, Australia. They were available here in OZ from 1993 to 1995 but very few were sold due to the incredibly high price compared with other contemporary trials bikes. Production was discontinued because Yamaha sold so few of them. The TY250Z has many hand made and imported (into Japan) components which could not be sourced from the Yamaha parts bin so the production cost must have been quite high. There are at least two other TY250Zs still ridden in competition her in Queensland and I know of three others that have been for sale her in Queensland in the past few years. Yes the motor is the same as the Scorpa SY250 except when the change was made to hydraulic clutch on the SY250. This involved modifying the clutch actuating arm so it was a push rather than a pull to suit a slave cylinder. I have seen TY250Zs advertised recently in the UK but not in the USA. They were popular in Japan. As far as I know there is only one of them in Indonesia. Mine was imported second hand from Japan in 1998. It is a 1993 model and I recently took the head off for the first time for a decoke. It didn't even need rings. The head had never been off before. The clutch requires servicing annually to roughen up the steel plates. Spark plugs last more than a year. The TY250Z is great to ride for its age and fantastically reliable compared with modern European Trials bikes or Italian bikes from the same era. I would say it compares with a late model Beta Techno for ease of riding but has better suspension than the Techno. Mine (which is absolutely standard) weighs 83kg which is the same weight as my TY175. The TY250Z feels a bit heavier to ride than the latest crop of trials bikes. If I was to buy a modern bike it would have to be the SY250 because of it having the same motor. If you want more info please email privately. davidlahey at hotkey.net.au

If you are going to the trouble of relining your fibreglass fuel tank, I suggest that you use low viscosity epoxy resin as the liner and buy it from an aircraft or boat repair supplier as previously suggested. The reason for this is that the automobile fuel or "gas" we buy will eventually contain ethanol if not already and polyester resin (which is what fibreglass fuel tanks are normally made of) is affected by ethanol. I'm not actually advising relining with anything unless it is absolutely necessary because it adds weight and can be a bit tricky to get evenly spread inside. Almost any fibreglass tank damage can be fixed by external repair if you take the advice already given on this string. David Lahey Australia

If you are asking about a twin shock Yamaha TY250 clutch, beware of reducing the clamping force because they don't have much margin to begin with. If you are serious about getting a light action, fit aftermarket kevlar (Barnett) plates if available so that you can safely reduce the clamping force by a worthwhile amount without causing slippage. This works wonders on Bultaco Sherpas. I reckon the TY250 twinshock clutch action is quite light if set up and lubricated properly so I haven't bothered modifying any of mine. David Lahey Australia

Falcon Classic shocks are terrific for twinshock trials. I have used them on two of my twinshocks and have never found anything better. David Lahey Queensland Australia

Dear Paul Mac Q1 Is the blue polypropylene tank from a M198 the same as the tank for a M199B? Q2 Where is Bungonia NSW? David Lahey Gladstone QLD Australia

Useful mods for the TY175 are: Use Sammy Miller footpegs. They are a tiny bit lower and further back but the best thing is that they are much larger, providing a much more comfortable ride and better leverage for turns. I will repeat what the previous respondent suggested. Falcon Classic shocks transform wonderfully the way both the TY175 and TY250 handle. Mine have 40 lb springs which are a tad soft for the 83kg rider. Don't make the swingarm longer unless you want to go desert racing. Again retain the standard fork tube location unless desert racing. About 20mm of tube extending through the top clamp is ideal for trials and trail riding. The front end from the TY250 twinshock (larger diameter fork tubes and clamps) fits straight in the TY175 headstock if the rider is exceptionally heavy or picky about fork flex. I find the standard forks to be a bit soft for very heavy landings even with 15wt oil. Increasing the fork spring preload upsets the steering a bit in floaty turns and doesn't have much effect on bottoming anyway. I don't know of alternate springs that are any good. I have tried single rate springs from a TM125 Suzuki set up to provide the same static sag but with a higher spring rate but the standard TY springs were so much better in turns I went back to standard springs. There are aftermarket springs available for the TY250 from B&J Racing in the USA. These work very well in the TY250 forks compared with the standard Yamaha TY250 springs. If you are serious about improving the steering, buy a Mini Majesty frame for the TY175 running gear from Craig Mawlam in the UK. They were made in 1982 and have superior chassis geometry. If you steepen the steering on a Yamaha frame, you may have to redo the exhaust to keep clearance for the front mudguard. The motor can be increased in capacity for more grunt to 205cc with a larger cylinder sleeve. Breathing may be compromised by this ie it may not rev as high as the standard Ty175. Increasing the compression ratio slightly from standard improves pulling power also but requires careful fuel selection and meticulous spark timing maintenance. David Lahey (TY175B fan) Australia

Gaff Yes it is the oil pump cable at the front end of the RH case. The clutch cable is about 1/2 way along the RH case. It is piston port ie no reeds no rotary valve. They run great and are as reliable as an axe. Yes there is a website and forum and there is even someone (in New Zealand) making first rate new otherwise unobtainable parts for these bikes. There are two models KT250A1 and KT250A2. A1 1975. A2 1976. They are the same mechanically but have different paint schemes on the tank and sidecovers. Have a look at the engine and frame numbers if it is important which model it is. The site is: http://members.tripod.com/kawasakiKT/index-4.html David Lahey Australia