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There appears to be a lot of confusion over quite simple matters to do with OKO/Keihin carburetion.  Removal of the float bowl, and  fuel leaks from the float vent tube, seemingly causing particular concern. This being the case we hope this page may be of help to anyone who has had any problems in these areas.  

We were contacted by a customer recently who asked about the modification pictured above. This involves blanking off the float chamber vent on an OKO carb.......possibly to prevent excess fuel flowing from the vent tube? Altering any carb in this way, can in some circumstances mean that if the bike to which the carb is fitted is parked at an acute angle with the fuel switched on, and the carb needle valve is not sealing properly, that it is quite possible that the crankcases will fill with petrol.

However the Keihin/OKO do indeed sometimes suffer from excess fuel flowing from the float chamber overflow, so we can see the reasoning behind this type of modification, but would certainly not recommend carrying out alterations such as this, as engine damage can result in some circumstance! If there is a problem with lack of clearance for float vent outlet tube, then it is possible to carefully modify the outlet for more clearance, but it must not be blocked off entirely though!

The first thing to check in relation to any carb which has fuel flowing from float bowl vents when it shouldn't be, is the condition of the needle valve, and its seating. You will need a magnifying glass to properly examine these 2 parts, as very often damage will be so slight its going to be very difficult to see with the naked eye alone. If there is any sign of damage or wear, the affected parts will need to be replaced. Classictrial provides full spares and technical back up to any customers who have purchased carbs from ourselves.

Another not so immediately obvious cause of excess fuel running from float bowl vents, is if the float bowl has been removed forcefully by someone who is not familiar with the OKO/Keihin carbs. In most cases this results in the float tangs being distorted/damaged, and that's  something that might not be noticed, without actually looking for this particular problem specifically.

OKO/Keihin carb set up

Modified” OKO float bowl

Click to enlarge

ClassicTrial OKO carb ready to check fuel level

OKO carb ready to check fuel level

Click to enlarge

After the float tangs have been distorted, the floats themselves often foul on internal parts of the carb itself, and in some cases this  will prevent the floats rising up far enough to properly contact the needle  valve, and stop fuel flowing into the float chamber. Its very very easy to remove the float bowl from any Keihin/OKO PWK type carb, without causing any damage at all.  

To remove the bowl, simply invert the carb (float bowl facing upwards), take out the 2 fixing screws, and lift the side of the float bowl adjacent to the fuel intake fitting first, so that there is 5-6mm gap between the face of the bowl and the carb itself. Then simply lift the bowl off, making sure the angle attained by lifting the bowl on one side first, is maintained as the bowl is removed. The bowl is refitted in the same way, and at the same angle as it was removed.

Taking the float bowl off in the way outlined above prevents damage to the float tangs, and will preclude any fuel leakage caused by floats not being able to move through their full travel due to distortion. This particular problem is very difficult to diagnose unless you are aware of the possibility of it occurring, and the information provided here may be of use to anyone suffering from mysterious leaking carbs!

In regard to the float bowls themselves it is worth bearing in mind that fitting a clear OKO bowl to a Keihin carb may well be problematic, as the OKO clear bowls have 4 fixing screws, and the Keihin carbs have only 2. It may be possible to properly seal an OKO clear bowl fitted to a Keihin carb, but this is not something we would recommend. However the clear bowls are fine fitted to OKO carbs, but its worth mentioning that these do not have any means of draining the float bowl, which is obviously not ideal for trials applications.

It is also important to check that the fuel level is set properly on these carbs: All Keihin PWK and OKO type carbs commonly used on trials applications run best using the stock 19mm float height, and lowering the fuel level can result in weak mixtures, especially so on bikes which are being used in extreme conditions, and overheating and or engine damage can result.

* 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 “Modified” OKO float bowl

Fuel level is easily checked by removing the float bowl as outlined above, inverting carb to a 45 degree angle, and checking that  a measurement between the carb float bowl gasket face, and the top of the float itself is 19mm. In general if the moulding lines on the faces of the floats themselves are in line with the gasket face when the carb is inverted at 45 degrees, the float level will be very close to right.

Having the correct jetting and set up is crucially important on these particular carbs, and while bikes will often run reasonably well on settings used by others, it is worth bearing in mind that what might work extremely well at a altitude of 3000ft above sea level in the US for instance, would be a long way from what is required at 50ft above sea level in the UK! It is also worth bearing in mind that the cost of tuning parts required to properly set up carbs from Ebay sellers, may mean a carb that costs more than buying something properly set up the first place.

However for anyone who is primarily interested in the cheapest possible price, and wanting carburetion that works well, we would suggest looking very carefully at information provided by manufacturers related to jetting and set up, as this provides a very useful basis to help with getting everything spot on, which is really the only way alternative carburetion is going to  provide the maximum possible improvement in performance.

In regard to carburetion for machines that we are not familiar with, unless the bike can be brought to us to be set up correctly, we are unable to assist with supplying a carb, as this generally leads to endless emails and telephone calls, and we simply cannot help if the bike in question is not actually in our workshops. We are able to set up carburetion on more popular machines we are not familiar with FOC, but bikes brought to us for set up must be in good mechanical condition, and 2T bikes need to have had exhausts serviced recently.

Finally it is well worth altering the position of the pilot air screw, to achieve best possible running on the day you are taking part in a competition. Atmospheric conditions do vary day to day, and the setting of pilot air screws must ideally be altered to accommodate these changes. Thumb wheel type pilot air adjusters are available for OKO/PWK type carbs, and these are excellent for trials applications.