Drilling Jets

I decided to get myself setup to drill out the main jet on the Mikuni mounted on Salty. As it happens, I also have been dialing in the tuning on the same model of carb mounted on the 140cc chinese engine I have in my Trail 55 rat bike project.

I came across this fantastic page, which saved me the time and effort of converting the numbers and matching up the drill sizes:

http://www.affordablegokarts.com/Drilling%20Main%20Jets.php

As luck would have it, I had picked up a cheap drill bit set from Harbor Freight last month, just to fill in some gaps I had with other drill bit sets.

Salty's carb had a 95 main jet, and is acting as if it too lean. I've had to move the needle clip to the bottom most position to get it to pull at part throttle, yet it totally falls flat at wide open throttle. My spark plug is also too light in color after I did a few spark cuts.

Ratty's carb came with a 97.5 main jet, and has always acted very lean at part throttle application. Almost like a lean surge you would feel with a weber carb on a car when it was too lean.

So I made up a little aluminum block fixture to mount the main jet in, which fits nicely in my bench vise. I made a one jet size increase at a time (or as close as I can get) so I used a #60 drill bit for Salty, and a #59 for Ratty. The process was very easy and quick. I can remove the carb, take off the bottom half and have the main jet in my fixture in less than 5 minutes.

Unfortunately, I've had no daylight hours to run either bike since I drilled the jets. And the weather has started to turn a bit rainy this evening. Hopefully I can get out on the road this weekend and try to dial things in.

C'