Modifying a Keihin PWK 28 Carburettor to fit a trials bike.
We now have the original Keihin PWK 28 available that are setup for most trials bikes (left idle, 125 main, 45 pilot, 3.5 slide, JJH needle, swivle top, no vacuum, horiztanal overflow)
Please click on the product to the left.
The below information might be of help to people looking at modifying other PWK's to fit their bikes.
The standard Carburettor as it comes from the manufacturer.
The first thing that needs changing is to move the idle adjuster to the left side i.e. suitable for GasGas, Sherco and Montesa.
The picture (left) shows idle speed adjuster screw on the right hand side, this is fine if you own a Beta or a Scorpa SY but all the other trials bike will require this to be moved to the other side. To do this the other side needs to be drilled and tapped out to accept the M5 idle adjuster screw.
Aligning the carb casting to the drill in the vice and drilling the hole to the correct size for tapping (4.2mm), use the side of the boss to set the carb up at the correct angle (shown above left) then centre the drill in the start hole on the boss this should ensure the hole is drilled in the correct place.
As the casting is not square to the hole, the face needs to be squared up to make tapping easier, I use a 5mm slot drill to spot face the hole and then tap the hole while maintaining the same position on the machine table as used to drill the hole, this helps ensure the tap is inline with the hole.
This vacuum port is not used on most trials bike so it needs blocking off.
We remove the brass nipple by clamping it in the vice and pulling/twisting the carb body off it, then we seal the hole with a aluminium bung made on the lathe, it needs to be a interference fit to stop it dropping out and to properly seal the hole, the hole is 4mm
The sealed hole.
The carb breather hole shown above is not drilled as standard, on some trials bikes the right hand breather pipe runs very close to the hot exhaust, this can occasionally get hot enough to melt the pipe sealing it off causing unpredictable carburetion problems due to incorrect pressure in the float bowls. If you see this been a problem the left hand breather can be drilled out removing the problem.
The carb comes with the overflow straight out of the bottom of the float bowl, this overflow boss has to be removed to allow fitment to most trials bikes as it fouls on the gearbox casings, to do this it needs to be bunged up and cut off, then the other side over flow coming out of the left of the float bowl needs to be drilled out.
We make up a small bung on the lathe, this needs to be a interference fit into the hole to prevent it dropping out when used, the hole is usually 2mm so we make the bung 2.01mm so it taps in nicely with a small hammer, make sure you measure the hole first though as some of the float bowls recently have had a larger hole.
The bottom overflow needs bunging before the side overflow is drilled out, when drilling the side overflow boss centre the drill in start hole in the casting, be carefull not to drill all the way through, if possible set up a depth stop so the drill can only just reach the other hole.
Machining the overflow boss off for clearance with the cases, this can be just hacksawed off and then cleaned up with a file, the clearance with the cases is very small so the boss has to be cut off right at the base, on some bikes it is even necessary to machine off a small part off the float drain bolt as shown above, be very careful not to put a hole through it and make sure the bolt is tight before starting.
The standard cable adjuster is way to short to take the slack out of the throttle cable it also lacks the swan neck to put the cable at the correct angle.
A swan neck and adjuster are available from Keihin (see above) or the Dellorto PBHL swan necks and adjusters (see below) just happen to use the same thread, if using these a slightly longer throttle cable inner is required.
The Dellorto swan neck and adjuster can be shortened as shown above, this allows the standard length cable to be used but removes some of the adjustment available. Or if you use the longer Keihin cable you don't need to cut them down.
Finally the jets need changing to a good starting point, we find the following seams to work well with most trials bikes.
Main Jet: 125
Pilot Jet: 45
Air Screw: 1 full turn out from full in