While tuning the Mikuni TM flatslides on my T140 there are about 12 different needles and as many needle jets. The needles and jets also fit the round slide VM carb..Of course only a few needles and needle jets are close to correct..The taper on all the needles starts in a different location, but I noticed none of the tapers start before the slide has opened beyond the idle postion...
While there are a page of needles and needle jets if you sort through them there are actually only 3 or 4 that work for a given model and throat size. AND these cover both two and four stroke models limiting choices even farther. There is not the "vast" choice of needles and needle jets which some have come to believe. Actually the false impression of those vast choices draw people away from the few combinations that will actually work.
I did notice that the thick needle diameter and or needle jet orfice does have a small effect on idle mixture..
This is not going to be the case for round slide AMAL
or Mikuni models. For one thing both of these rely on a two transfer port idle system. There is one hole at the bottom of the venturi on the back side of the slide and a second just under the back of the slide. At idle the vacuum is high on the back of the slide, and near atmospheric under the slide. In this position of the slide, the transfer port under the slide draws in air because of the differential in pressure. As the slide is lifted the pressure balances and this causes fuel to be drawn out of the hole under the slide that was delivering air at idle. It acts like a little accelerator pump. Because the straight portion of the needle, both the AMAL
and Mikuni, doesn't clear the needle jet orifice until approx. 1/3rd throttle AND with the needle-needle jet orifice not changing diameter at idle, it will not have any effect upon idle air mixture.
However I can not say what works on a Mikuni flatslide works exactly the same as round slide...
The flat slide is a different animal. It doesn't have the "Tea cup" area under the slide. With a Monobloc
, Mikuni VM, GP, TT, etc there is a delay in throttle response off idle. It takes time for the air flow to draw done the vacuum under the slide and begin to draw fuel out of the needle jet orifice. Obviously it is worse with GP's, TT's and Monobloc
type slides with their deep "cup". But when you experience a flat slide you realize that this is common to the Mikuni round slide and AMAL Concentric
and MKII with shallower "cups". There is no draw down time with the flat slide. The throttle response is instant. Now, before you run out an buy a flat slide it makes sense to understand if wheel spin is going to be a problem. If I was heading to the Salt Flats, or dirt circle racing I would think twice before using a flat slide. Road racing where you found some real HP, but have little racing experience you might pass.
I bought a well used Supertrapp mufller on eBay, massaged out the dents and fitted it to the Triumph using 12 diffussr discs...Holy crap, what an inprovement, it not only cleared the power dip but the engine seems to run smoother at all rpm's..I don't believe it's the so called "you need back pressure" , but its more got to do with complicated matters...
So...keep the exhaust system in mind when tuning carbs...
I discovered SuperTrap's when all of the racing organizations started to clamp down on noise. I never looked back! I found HP I never knew I had.
I read it on the internet, it must be true!