When I first started working on the clone motors, especially the carbs, there seemed to be a limit on what you can change for tuning. Primary air jet setting is absolutely necessary for idle and low speed. Even a drop of oil in the primary air jet will ruin your day, and it happens. I use a .0225 primary jet for my modifies.
With the stock clone carb we are only given jets and emulsion tubes correct? Different jet sizes .038 thru .043 are used for both the smaller gx200 and the gx270 carb. All the tubes and jets are interchangeable. The real tuning is in the emulsion tubes. Most commonly, the 140 emulsion tube is offered by most companies selling parts. What’s the big difference between the 140 and stock? So far I have found 6 different tubes. Different size holes and at different locations on the tube. The measured volumes of all these holes varies very little. The big differences is in the inside diameter (id) of the tubes. They range from .063 thru .087. The 140 tube is .068, only the second largest. So if you solder holes and make the tubes close to the same except for the id, you can tune your head temp by ten degrees. Add spark plugs and a little air density information and your good to go.
Spark plugs can allow your engine to run at different temperatures and are also good for tuning. We all live in different climates so your tuning may differ from my tuning out here in the middle of California.
So far I haven’t seen a carb, Tillotson or stock, that couldn’t be helped with enlarging the mid-range fuel delivery. Good for 3 tenths or a second off your laptime with a stock carb.
I enjoy getting the most of what is available before I buy what everyone else has and I like using my mind and experiences rather than sticking on a Mikuni or Tillotson.
What are your experiences with stock carb tuning? Share your thoughts in the comments box below.
This post was written by Ken Hill, a karting engine builder with over 25 years in the industry.