Motorcycle Classics Blogs > Tech Corner

Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

Triumph T140 E and Mikuni 34mm


Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as your subject. 

Mikuni jetting

Q: I own a 1979 Triumph T140E, and I have a pair of Mikuni 34mm VM round-slide carburetors that I would like to use on it. The problem is that I don’t know what size jets and needle to use. The bike is stock with the exception of a Boyer ignition and shorty mufflers. I am from Trinidad and Tobago in the Caribbean, so the temperature is very hot and I’m at sea level. The bike will be used as a daily rider and there are no emission controls here. Any advice would be greatly appreciated. — Brian Amow/via email  

A: I’m still running Amals on my bikes, so I did a little searching and found that the consensus for the T140 using twin Mikunis is 260 for the main jet, between 25 and 35 for the pilot jet, a P4 or P6 needle jet, 6DH3 needle and a 1.0 air jet. That will get you started, then you’ll have to fine-tune it for your temperature, altitude and humidity. The good and the bad about Mikunis is the near infinite amount of adjustments you can make changing main jets, needles, needle jets, air jets, slides and pilot jets — it’s difficult to know when to stop. As always, start by getting the main jet the right size at full throttle, then the idle mixture, and then the midrange. The midrange is where you will find most of the variables. Raising/lowering the needle, selecting a richer or leaner needle jet, and getting a needle with a different taper are all options for setting the midrange mixture. Good luck! MC