By fortuitous chance, a friend I help out building his trikes had a box of spares from a Suzuki GS 550, amongst which was a bank of carbs. Already 'severely annoyed' by the performance of the standard fit carbs on the Kat, I offered up the 550's carbs to the, once again, removed carb. inlet stubs. The spacing is exactly the same, this eliminates 90% of the difficulty of fitting alternative carburettors.
Once fitted and with the required modifications, I have found that the hard starting and flooding have been eliminated. Left standing for months (over winter) it started easily, even I had my doubts that it would.....
I will detail the steps needed for replacement after this passage.
The question must be why these carbs were chosen in the first place. All carburettor suppliers were well aware of forthcoming legislations restricting CO2 emissions. 2 strokes were to all intents a dead end, fuel injection the way forward. Mikuni's last offerings were the BDST model in various choke sizes. On inspection, and comparison, these are definitely built down to a price in quality, plastic parts, thin metals etc. This, in itself, is no barrier to an efficient instrument. When working correctly they perform well. My conclusions here are observational coupled with a fair amount of investigation into carburettor performance.
Some things I think are worth noting. The first is that the engine in the Kat is a close relative of the GSX-RR 400 'supersport'. This model was closely engaged in a war within the big 4 for bragging rights, and sales in the hotly contested 400cc marketplace. This was an extremely important market segment in Japan due to licensing restrictions. Peak power was the goal.
To attain this, large carburettor choke sizes were used, along with heady RPM levels. This does, indeed, result in high BHP, but only at these high RPM. That there is any usable power/torque at lower revs is an acknowledgment of some pretty nifty porting and cam timing.
Now consider. At high RPM there is a considerable amount of air being sucked into the inlet. At low, start up revs.... not so much, in fact, the equivalent of a light breeze compared to the gale force at high RPM.
There are many carburettor ideal choke size calculus programs available. All presume a conventional round choke slide carb. The maximum ideal choke size for a 100cc cylinder is between 20 – 22mm. Now, CV type carbs are less efficient due to the obstructions in the airflow, the convention is to add 2 – 4 mm to the ideal choke size. Even with this adjustment it can be seen that the 400's are very over choked for ideal running conditions. To get a mixture suitable for igniting with these constraints is something of a hit and miss affair, as the many instances of starting problems can testify. So, why are the BS type carbs are far easier to start, even though they are the same choke size? My thoughts are that the BDST choke circuit is too complex. It wends its way through a jet in the float hanger plastics before making its way up the long, thin tubes feeding the choke control valves. The BS choke path is much simpler, just a direct feed into the float bowl.
Also, why are the BS type far less prone to flooding? That is easier to answer. The float bowls are larger and deeper that the BDST's, more fluid pressure on the floats to keep them properly closed. It may be that the greater volume of fuel in the BS float chambers assists the choke circuit too..... Its my theory anyway.
Pic 1 demonstrates the deep floatbowl on the BS and the short choke mixture path compared to the BDST
(tapered needle for demonstration, not from BDST but similar taper compared to straight needle from BS)
Fitment.
The BS32's are more robust than the BDSTs, the outlets are thicker, so take more effort to seat into the inlet rubbers. Some grease and a hot air gun to soften the inlet rubbers allows them to be seated easily, make sure the securing bands are fully loosened.
The really awkward part comes with airbox to carb rubbers. These are a pain with the standard carbs, the BS32's are slightly longer, making this part of the operation even more difficult.
I found that removing the airbox mounting bolts at each side allows some movement rearwards. You need every millimetre you can get!
I removed the intake rubbers from the airbox when fitting the carbs as much to make any adjustments easier, but this gives you the opportunity to mod the airbox rubbers. You will see a double lip where the rubbers seat to the airbox case. I chopped off the internal rubber ring, (Using a Stanley knife). This allows the rubbers to be squeezed past the carbs and seat the rubbers in the airbox. You need plenty of heat and a solid, longish flat head screwdriver to 'ease' things into place.... In other words, it is a struggle, but not that much worse than with the standard carbs.
The other advantages of the BS type are that, one, they only have a pull throttle cable, and the actuating mech. is very easy to locate and fit and, two, the pilot airscrews are on the top of the carb. bodies, making fine tuning sooooo much easier. There are differences in the various BS models. The ones I have modified from the GS650 have vacuum takeoffs on the body for carb. balancing. The GS550 carbs. on mine do not, needing a vacuum take off spigot mounting on the inlet rubbers. These are available on Ebay for a few pounds.
I have noticed differences in the choke bar actuation, too. These GS 650 carbs have a rod operated by a pull knob at the side of the carb body, the gs550 have a central lever action operation, this type marries with the cable operation on the Kat. Not a particular game changer for either type.
This conversion is an easy afternoon job, only the airbox rubbers will cause any swearing, but they do with the standard carbs....
