16mm carb vs 18mm carb      Back to Carb Section

velocity-1.jpg (15750 bytes) Here you can just see the blue Takegawa velocity stack installed on the carb. It's about 2" long and has a screened intake cover. So far testing seems to show improved high rpm power. Cost is about $45.

 
carb18mm-1.jpg (31967 bytes)    On the left is the 18mm Kehin carb I will be using for the 2000 race season. I feel I need something larger than the 16mm. The 16mm on the right was used by Honda on the early 1970s 72cc bikes in the US, since my motor has a ported (flowed) cylinderhead and a larger camshaft and oversize intake valve I felt it could use a larger carb. The 18mm is from a 1973 Honda CB175 twin, which used two of these, each one supplying a 87.5 cc cylinder, so it should be just right for a tuned 72cc motor. In the 16mm carb I was using a size 68 main jet, and a 40 intermediate jet. In the 18mm I will start with a size 90 main jet and a size 40 intermediate jet but may have to change the main jet size after testing. 

As you can see the 18mm carb is quite a bit larger than the 16mm, I'm not sure how they really determine the size of the carbs but it sure seems more than 2mm larger. The actual body of the carbs are the same size, so you could use the same air filter. However the floats, fuel inlets etc. are in different locations.
carb18mm-2.jpg (25957 bytes)  

  Here you can see how much larger the intake port is on the 18mm carb (left). With my new 18-20mm intake manifold the intake system from the carb to the cylinderhead should be a much better match and flow much better.

Of course I will report on the success of this modification as the racing season progresses. I do expect substantial power increases from this change as I believe the 16mm carb was quite a bit too small for road racing.