ClassicTrial on line logo.
07761 971756 or 02380 600850

WillBrown Internet - Ref:WP10*CT-Main-120517q15_cookies -  Updated 27/5/12 © 2005-11.

ClassicTrial keyword search:
* This site may use cookies, find out more.

Carburetion is something that many people seem to find rather daunting, and there are often questions on Trials related chat forums, concerning what jetting to use, and sometimes also even asking what settings should be used for air/fuel primary mixture screws! Baseline settings when jetting a carb from scratch are very useful, but final settings can and do vary from bike to bike, so accurate set up must ideally be arrived at on an individual basis.

However on older twinshock machines, which may be in dire need of mechanical work, these are areas which must be properly dealt with before looking at anything remotely to do with the carburetion………. in other words fitting a new carb is simply not going to cure other problems! Furthermore it is worth remembering that the carb is itself a working part, which wears out in the same way as anything else, and that spending money on rebuilding badly worn OEM carbs, is generally a waste of time and money, as carb body itself will often be US.

For riders wanting the very best possible performance from a twinshock machine, then fitting a new carb is as important as good cylinder compression or a properly working ignition system. New OEM type carbs are most often still easily available, but it is worth bearing in mind that when bikes were made, the manufacturers always being keen to save a few pennies on unit costs, would very often fit the cheapest carbs around at the time, which may not have been the best choice for the intended application. In common with more modern ignition systems, more up to date carburetion results in better running, and if properly jetted, very often more power.

Invariably modern flat slide carbs will work a great deal better than OE round slide designs, and the Keihin PWK flat slide carb has for some while been used by various works supported riders, and is currently being used by Beta on most of their 09 Evo bikes. The main reason the flat slide is superior to the round slide for trials applications, has to do with the enhanced signal strength provided by the more efficient  slide design, which results in cleaner low RPM running, and very often noticeable increases in top end power. We are able to supply properly set up OKO Keihin PWK copy carbs, which work very well indeed, and are less costly than the genuine Keihin carbs. However it is worth bearing in mind that even though the OKO costs less than the Keihin, a wrongly jetted OKO can easily end up costing substantially more than the purchase price if changes to jets, needle, and or slide are required.


Classictrial OKO carb kit on RTL Honda

Click to enlarge

ClassicTrial OKO carb convertion on a Pre 65 machine

Classictrial OKO carb on Pre65

Click to enlarge

The Honda 4 stroke Trials machines respond particularly well to having an OKO flat slide carb fitted, in place of the OEM 22 or 24mm Keihin round slide, which is often badly worn, and the root cause of various problems, often attributed to other things by those unfamiliar with these motors. If mechanical condition of motor is otherwise good, and exhaust system fitted is not particularly restrictive, then the OKO carbs will provide smoother low rpm power, reduce tendency to stall, and allow bikes to rev more freely, resulting in more top end power.

Piston port and reed valve Fantic 212 and 249cc motors work very well on 26mm OKO flat slides, with improved low speed performance, and added top end power, but will make a little more top end using a 28mm carb. Again to benefit most from changing the carb on these bikes, mechanical condition must be good, and exhaust systems should be clean, and free from unnecessary restrictions.  Going on what we have found from using these carbs on Fantic's, it is very likely that similar improvements can be expected when fitted to other 2 stroke trials bikes, and we do have jetting and set up information available for several other common applications.

We have also successfully tried OKO carbs on several P65 machines, with very good results, so unless a rider is comfortable with the poor performing and rapidly wearing OE type carbs, its well worth contacting us to see if we can help.

Finally once more, please remember that fitting a new carb will not improve running if an engine is badly worn, or is suffering from other problems not directly related to poor carburetion, and in the case of older twinshock machines, even though the OE carbs fitted will almost certainly be completely worn out, that general wear and clogged exhaust systems are very common, and the full benefits of a new carb will not be realised until these problem areas have been properly dealt with. If you have any difficulties with carburetion then please email us at and we will do our best to assist.

Our OKO carb kit page

Setting up a OKO carburettor

* 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. *

ClassicTrial OKO carb convertion on a Honda TLR.