2T trials tuning basics
Fundamentally a 2T trials motor is little more than an air pump with a means of introducing fuel into the air, compressing it and then igniting the compressed air/fuel mixture with an electric spark, finally following the ignition process exiting the burnt gases through an exhaust duct of some sort.
Looking at various alterations carried out over the years on twin-shock and ACM (air cooled mono) machines, it seems that effective intake and exhaust systems have been largely overlooked completely and increasing capacity is something that was commonly carried out?
For increased power/torque, updating the intake and exhaust systems in conjunction with changes to the porting are the specific areas which will provide the most effective performance gains and will not adversely affect reliability in any way.
Effectively improving a 2T trials motor is not particularly easy, as it’s not simply a matter of over-boring and fitting a larger piston and numerous motors have been badly compromised over the years through this particular rather short-sighted approach!
Some things though such as replacing points with a modern day electronic ignition are relatively easy and in combination with changes to the intake system will improve running noticeably. If you are thinking about modifying your motor at any stage though make sure that any electronic ignition you might be considering is adjustable, as timing requirements generally differ on modified motors.
On most older machines air box volume is too small for best possible performance and this situation is often made even worse by the very restrictive intake hoses fitted, which significantly reduce the ability of the duct to flow air, even if a new free flow competition or modified stock air-box is connected to the stock intake hose.
Its worth bearing in mind though that while an improved intake system alone, isn’t going to increase performance as much as uprated intake, exhaust and porting changes all being carried out at the same time, it’s something that is feasible in the home workshop and need not be that costly for anyone who can cope with very basic GRP work. NOTE: This is an upgrade which works particularly well on bikes which have stock very restrictive induction systems (TY175 being a good example).
However, to work well a properly jetted, good quality carb MUST be fitted on all uprated intake systems! Accurate fuel metering at all engine speeds is obviously very important on a trials motor and achieving this may be difficult with Chinese made clone carbs, whose quality control seems questionable? Essentially if you choose to go to the trouble of making or buying a full flow competition air-box its probably best to avoid cheaply made low quality carbs. OKO Taiwan carbs are far better than Chinese, but unfortunately quality control issues are also not unknown with these!
On bikes such as the TY175 and TY Mono if the intake system has been improved, its also worth taking a careful look at the stock reed block. On the 175 the stock reed cage doesn’t flow particularly well (see pic) and fitting a RD250 reed block with dual stage reeds will improve things quite a bit, but the intake cavity must be enlarged for these to fit. On the Mono the stock reed block has overly large petal area for the capacity of the motor and this tends to dull throttle response and will worsen low rpm performance, especially if your motor has been modified. Fantic 305 reed blocks fit the Mono perfectly and are worthwhile even if your motor is stock.
In relation to exhausts using a shorter front pipe will help to increase power/torque, through improved scavenging. Back in the day long convoluted front pipes were very popular as these provide very soft gentle power delivery, which was ideal for entry level riders and helped bikes to grip better using the rather poor tyres around back then. For anyone who can weld reasonably well, making a competition front pipe at home is possible, using exhaust bends and tube obtainable from Ebay. However, if mid box and back box are clogged with carbon/oil the gains from a more effective front pipe will be significantly reduced.
Changes to the porting are something which in conjunction with enhanced intake and exhaust systems, provide the most noticeable performance related improvement to many older machines. Its worth bearing in mind though that the porting changes which work most effectively on a trials motor differ greatly from those commonly carried out on MX or road racing motors. We have seen TY Mono cylinders which have pretty much been destroyed by modifications which seem to have been carried out with a view to entering the TT, rather than competing in trials!
Port changes on trials motors are not really something that can be carried out at home, but it’s certainly possible to tidy the ports up a bit but this doesn’t often seem to make that much difference to be honest! However, on the TY Mono in particular a properly carried out “Hamilton-Yamaha” port job and changes to the intake system at the same time will transform the bike, making it far more competitive and much more fun to ride! Porting on machines such as Fantic is already very good in standard form and the only improvements here involve adding an extra port to improve mid-range power/torque, which was carried out on factory machines (240).
The above is only a thumbnail sketch of very fundamental matters related to improving a 2T trials motor and while there are plenty of books on 2T tuning, the content is directed mainly at obtaining power increases at high RPM and following the advice given in these books, may well result in an un-rideable bike and the need for a replacement cylinder, so tread carefully if you choose to have a go yourself!
If you do choose to try porting yourself then getting hold of the proper tools and equipment really is a must. Most of what’s required can be obtained from UK suppliers, but its probably likely to be a lot less costly to get a 2T porting kit from CC Specialities in the USA who can supply all you need in the form of ready to use kits. Cheap Chinese made air porting tools are available from Ebay, but the durability of these is questionable and the high speed, low torque operation makes getting good results rather difficult.
We use Foredom and Faro pendant motor flex drive systems with foot pedal controls and Foredom as well as German and Japanese made handpieces. Add to this a good selection of carbide burrs, abrasive rolls, measuring and marking out tools and you will have spent over £1500, so a relatively good set up is pretty costly!
Mono reed blocks