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Classictrial carburetion

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OKO carb for 300 Fantic.

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If you are looking for authenticity then there are several suppliers who can help with OE carburetion, which is fine if your machine is collectable and you need to retain its originality, or perhaps if there is some need to comply with the curious “nostalgia” rules relating to P65 machinery. However if you are serious about obtaining the best possible performance from your bike, then upgrading inefficient and generally badly worn OE carbs really is a must do!


Here at Classictrial we believe that most riders taking part in serious competition are looking to be as competitive as they can, and the improved responsiveness and increased power provided by an appropriate carburetion upgrade, does mean a big advantage over retaining old fashioned and often badly worn OE carbs. Very few people continued to use the old 4 ply trials tyres following the introduction of the far better 2 ply ones, and anyone needlessly choosing to retain an old OE carb is compromising performance in much the same way!

The 2008 British twinshock championship was won by Phil Wiffen, who was using a Classictrial OKO carb on his 300 Fantic. The 2009 championship is currently (30/3/09) being led by Ian Baker riding a relatively standard TLR200, which uses a very special  Classictrial OKO carb conversion, which provides more power, increases torque, and gives smoother low RPM running (not available as a retail item currently).  

However the OKO’s do require very careful setting up and some minor modifications to obtain the best possible performance from a trials machine. All carbs supplied by us have been properly set up, and are jetted correctly with the required specific modifications carefully carried out. In addition to receiving a carb that’s spot on, and is going to work perfectly on your particular bike, we also supply intake adaptors where required so stock air cleaner hoses can be retained (see pic of RTL carb). See also OKO carb kits.

As well as OKO we can also supply Keihin and Mikuni carburetion, again specifically set up for a particular application. This means there is no need to contend with the hassle related to Ebay “bargains”, which may be anything but, bearing in mind the possible cost of the parts needed to properly set up a carb for a specific application! We have decided to offer Keihin and Mikuni as well as OKO, as this provides customers a choice between the very best trials carburetion options currently available.

Please don’t take our word for it that a carburetion upgrade will greatly improve your twinshock or P65 machine, just talk to anyone at a trial who is riding a bike fitted with more modern carburetion, and ask them if running has improved since the carb was fitted! There is much confusion over jetting issues though, so it’s worth remembering that in some cases performance increases may not be quite as great as could be expected if carb settings were more appropriate.

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Mikuni VM series carb fitted to BSA.

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We would suggest that while fitting a modern type carb will always improve performance, and make an older machine far more responsive and easier to ride, it will not compensate for the effects of other engine problems. So if you are fitting a new carb to cure a specific running issue, it would be helpful if you could let us know, as in many cases the carb may not be the main cause of your problems, and fitting a replacement might not be the best answer.

Classictrial are able to provide full parts back up and technical support in relation to every carb supplied by us. All carbs provided by us come with an instruction sheet, providing some hints and tips relating to the particular carb, and a general tuning guide, that should be helpful in relation to any type of carb.  However setting up a carb from scratch, is not something that can be recommended to someone not familiar with this process, and can be quite costly if there is any need to change several internal parts to obtain proper running.

In regard to carb eligibility for P65 machines, the Yorkshire Classic club has already had the good sense to allow modern carbs on Tiger Cubs being used in its events, as it’s sometimes very difficult to get these bikes working properly using OE type carburetion. The ACU “Traditional” series of events also allows the use of modern carbs on P65 machines, and hopefully as time goes on other sensible eligibility rules such as these will be introduced more widely.

Classictrial OKO 24mm powerjet carb fitted to RTL250.

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In relation to rebuilding original carbs with new parts, we would suggest caution here, as in many cases the carb body itself is likely to be badly worn, and this is certainly going to be the case if there is any sign of wear on the throttle slide, which strongly indicates the carb body itself has worn oval, and needs to be replaced. For anyone wanting to retain originality a brand new OE carb is by far the best option, and will work far better than something that’s worn out, and has merely been fitted with a few expensive new parts.

Finally please contact us direct for help and advice on upgrading carburetion on your twinshock or P65 machine, but please remember that we do not have the essential set up data for bikes that we have not fitted carbs to previously. However for anyone able to bring their bike along to us, very often we will be able to assist with setting up an appropriate carb to work perfectly, and in some cases this work will be carried out FOC, other than  the cost of the carb. Please do mention if you are wanting to replace carb to resolve poor running, as in some cases we will be able to provide advice, which might save the cost of replacing your carb needlessly.

Our OKO carb kit page

Setting up a OKO carburettor

ClassicTrial OKO carb convertion on a Honda TLR.

* Finally please remember that all of the above information is provided on the strict proviso that any modifications or changes not carried out specifically by Classictrial, are not guaranteed or warranted in any way, and are entirely the responsibility of the individual carrying them out, and that Classictrial are not responsible in any circumstances for  damage or injury, that may occur as a result of poor workmanship. *