feetupfun

Your message was a bit hard to work out, but it sounds like where you ride, the twinshock class rules include a stopping penalty while the modern bike class rules allow stopping. Contrary to what are saying about the rules in the 1970s, events then were almost exclusively run to no-stop rules. If you were allowed to momentarily stop and balance in the 1970s, you were riding under different rules to the convention at the time. There are reasons for using a stopping penalty for twinshock class nowadays. Depending on the nature of the sections, a bike with poor clutch action by nature of it's design may be disadvantaged by allowing stopping. If the organisers are trying to attract riders, it is useful to make the playing field more level by reducing the advantage gained by having good clutch action. To replicate the feel of the events of the time the bikes were made, orgainsers sometimes like to use the same rules as those used at time. Yes it can be tricky and sometimes quite amusing riding to the no-stop rules after years of stopping and balancing. We have one (twinshock and classic bikes only) event a year that runs to no-stop rules and that is one of the reasons it is such fun to ride. And yes lots of us ride twinshock bikes in modern bike classes (with great satisfaction too)

I have no experience with the Castrol 747 but do know that if you use Castor oil (Castrol R40) as the pre-mix oil in your two-stroke trials bike, the carbon formed on the piston crown and in the head will form very quickly and be extremely hard. To remove it will require emery tape/grinder rather than the usual wiping with a 3M scouring pad that removes the carbon formed when using modern oils. Another consideration is that switching from one type of two-stroke premix oil to another without washing out the motor internals can cause accellerated wear rates until all the old oil has worked its way out of the engine. The concentration of Castor oil used as an upper cylinder lube or odour agent in the fuel for a 4 stroke should be kept much lower than what you would use in two stroke premix - mainly to avoid causing carbon buildup in the exhaust ports and valve/s.

I thought this might be the right time to show a photo illustrating how popular Cota 348s are in our local trials club. Photo taken before a club trial October 2007.

Those fork caps are standard on some model Cota 348s. I have 3 sets of 348 forks and thay all have those caps but I have also seen 348s with plain fork caps. It was a popular thing to do at the time. It provides another way to adjust the front suspension. There is a drawback though that increasing the pressure inside also increases fork seal drag.

Here's the other side of DabDab's 348 brochure. Sorry if its upside down or sideways - remember it was uploaded in the Southern hemisphere!!

These are scans of a 348 brochure I am posting on behalf of DabDab. They are in pdf format so I'm not sure they will display properly.

1 The brand/size that you personally find easiest to ride. For me it would be a toss up between Scorpa SY250 and Beta Rev3 250. 2 It wouldn't be a factor because the all the brands are well supported by their distributors here.

Stripping paint from fibreglass tanks is easy if you use an electric heat gun to heat the paint and then scrape it off with a paint scraper. I've successfully done fibreglass Spanish trials bike fuel tanks this way. The latest was an OSSA MAR tank that had three layers of paint and lots of spray putty. Warnings: Make sure you have removed all the fuel fumes before you use the heat gun Don't overheat the fibreglass (or it may catch on fire). Just heat the paint surface to about 50 degrees C - about as hot as you can stand to touch with bare hands.

The head on the bike in the photo from the France Trial Classic website looks like a DT175 head.

Clark Maunfacturing in the USA make high quality plastic tanks for old MX and mini MX bikes. Some of them would fit the KT but the look is nowhere near as sleek as a steel KT tank.

I like the way you show what you are doing to the bikes as you go. I'm usually too keen to finish to stop and take photos along the way but your show and tell webpage has motivated me so a current resto (M198 Sherpa T) is getting the progress photo treatment. When you put the front guard back on the TY, please put it on the other way round even if it means ending up with holes where the mudflap was. Yes you should use tank stickers that look right - even if they are for a different model TY250. You have invested lots of effort already in the paint job and it would be wasted if the tank ended up looking strange. If those stickers in your "goal" photo are too expensive, the yellow and white TY250B paint scheme looks great and the correct stickers for that would only be about 5 pounds and the pinstripe less than 1 pound.

I've tried running a Rev 3 200 Beta on AVGAS which is high lead petrol made for piston engined aircraft. Once I adjusted the float height for the big difference in fuel density (AVGAS is very light), it ran beautifully, and was probably a bit less powerful overall than when run on you guys across the Pacific would call 98 octane "pump gas". No problem with lead deposits on the sparkplug either. The only problem here is that for competition use we must use (unleaded) "pump gas" or homologated unleaded ELF racing fuel so unless you never compete, here it pays to stay away from the user-friendly leaded fuels. About the ethanol content, I have never heard it promoted here as a performance improver in fact the fuel suppliers have to keep the price about 4 cents/litre below the cheapest non-ethanol "pump gas" to sell it (we have a choice of buying "pump" fuel with either 0% or 10% ethanol). "Pump gas" of all types is still very cheap in Australia (approx $1.15US to $1.25US per litre). Most trials riders use the highest quality 98 octane non-ethanol "pump gas" they can get.

Knobby tyres are terrible on anything that has hard surfaces like rocks and logs.

I would expect that if you have a standard pinky it will have needle roller bearings with rollers that will stay in position within the bearings until you slide the bearing inner bushes out ie you should be able to slide the swingarm axle out and remove the swingarm from the frame without having rollers fall out.

The chain will slowly wear the swingarm away unless there is a protector. The protector is sacrificial and wears out slowly. It is no big deal in the short term to have the chain touching the swingarm. Pinkys have steel swingarms so the wear rate will be slow. Yes the swingarm has to come out. I expect it would take a motorcycle mechanic about 30 minutes to replace a protector on a clean bike. Sounds like you haven't done much work on bikes yet so it would probably take you a fair bit longer. You will need tools to fit the swingarm pivot axle and both shock mount bolts. Make sure you clean the bike well before you start or you will get dirt in the swingarm pivot bearings. I would suggest buying a new protector and fitting it when convenient ie when you have to take the wheel out next to clean the brakes or replace the tyre.

Ah the fun of buying a bike in pieces! A standard TY175 head has only one plug hole and there is no place to fit a second plug or decompressor. Your TY175 probably has a head from some other model Yamaha. There are lots of interchangable parts between Yamaha bikes of the same era as the TY175. Having a different head may not be a problem. You may have a few other non-TY175 components so if you want to know, I suggest you start off by checking a few things; Have a look at the number cast into the barrel between the bottom two fins on the right hand side. It should be 52500 if it is TY175. Have a look at the engine number stamped into the top of the right side engine casing. It should start with 525 or 1N4 if it is TY175. Have a good look at a brochure photos of TY175s and post a photo on the forum of anything you are not sure about.

It may be something as simple as a ring stuck (with gum) in it's groove.

Maybe it was just the execution of the idea in the case of the Rotax jetski motors that was crap. The most successful 250cc road race GP bikes in recent years (Aprilia) have rotary valve induction. They are noticably faster than their reed valve competitors (Honda and KTM). In the case of bikes for Joe Public, yes they are a bit more complex than reed valves but do produce the goods. In twinshock trials the SWM with the rotary valve Rotax motor is a beaut and the SWM enduro bikes of the late 1970s with similar Rotax rotary valve motors were at the top of the performance tree at the time.

Regarding the lack of throttle position input to the oil pump on your Jap import TY mono, the amount of oil sent the carby does not have to match the quantity of oil required on such a short time scale as you are imagining. The intention is to match demand and supply over a timescale of a few seconds rather than a few milliseconds. For this reason it is not essential that throttle position is a control input on an off-road motor. Yes it is a compromise and would probably put the motor at risk if you used the bike for a application where the speed and load did not vary for long periods ie road riding.Yes it is not as technically sophisticated as the system on the TY twinshock bikes but there is also less to go wrong. I have lost count of the number of friend's bikes that have had the engine ruined when the autolube system stopped autolubing without warning. If you want oil metering with the highest reliability and the rate well matched to the demand, I recommend using pre-mix fuel.

I couldn't resist having a bit of a giggle reading this string about the merits of different fork springs for a 35 year old motorcycle. For many years I wanted new fork springs for some of my TY250s and could find no suppliers for them anywhere. In about 2001, I finally found a supplier and was very happy to be able to buy something somewhere (B&J Racing Products - Tenessee USA). Now a few years later again we have people arguing about whether German made TY250 fork springs are better than UK made TY250 fork springs!! Things are looking great for supply of Twinshock trials bike parts when we see arguments like this cropping up. The debates about which are the best shockies for twinshock bikes are a similarly good sign. By the way, for quite a few years (not sure if it is still the case now) if you bought a new BMW car in Australia, the suspension springs on it were made in Australia - something to do with the German springs being too soft for the spirited driving BMWs are subjected to here in OZ.

When you say owner's manual, do you mean the parts/service manual? The one I mean starts with drawings of all the special tools and then has specs including all the usual things like oil types and quantities, fastener torques, crank runout and piston clearance then good quality exploded view drawings.

Yes I remember having to go searching for those springs for one of my Cota 348s but it was so long ago I can't remember the details of what I tried but did end up finding enough of those strange montesa springs on another bike. I seem to remember that TY175 shoes are the right diameter and will fit the 1970s Cota trials front brake backing plate but can't remember if the TY175 brake springs have clearance to the outside of the bearing housing. If you don't get enough useful info on this from someone else, I will do a test fitup of Ty175 shoes (in a Cota 348 hub) to see how it goes. David

You may not have a tyre slip problem. The tube can get pulled around without the tyre slipping on the rim. I was obsessed with rimlocks for a while till I painted marks on the rim and tyre to prove to myself that the tyre wasn't actually moving relative to the rim. Don't ask me how to prevent the tube moving though. Powder on the tube works OK until water gets in between the tyre and tube. I've found that the tube will always move one way as far as it can in the stem hole and will then generally stop the tube moving further without getting the stem pulled out. I use Thai, Malaysian and Taiwanese tubes of a smallish tube size (3.50/4.00 x 18) in Michelin and IRC tyres.

I suggest you get the 16. My 6 year old 17kg (38 lb) and small-for-age son is well sized for his 16. He would be cramped on the 12.5.

If you use Barnett brand friction material clutch plates, the clutch spring preload can be backed off quite a bit further than with the all steel plates before slipping. You may even need to machine the spring nuts shorter so they don't rub on the casing.