Triumph T140 E and Mikuni 34mm

Reader Contribution by Keith Fellenstein
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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

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