Needles. How they do?

Nathan Wizard /
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So dellorto phbg needles.

There is a cool chart for tuning


I like to think of the needles as “classes” by the tip width (measurement B) .6mm,, 1.4mm, and 1.8mm. The thicker the tip is, the more of the passage it is blocking at all times, making your bike all around “leaner”. At WOT the tip is normally basically all the way out, and I imagine the tip diameter has almost no effect on WOT because of this, and I know the needle is mostly for your mids and low rpm range. There’s another chart, which shows each throttle position and what jet is mostly controlling that range of pull that also shows this, however it is in the wiki under “phbg tuning” so I’ll save posting that again here.

SO my assumption was that a longer taper (measurement C) meant your mix would stay leaner, longer. I thought this because a longer taper means it takes more pull of the throttle, to get the needle all the way up and out of the way. (A shorter taper needle is covering more of the atomizer hole than a long taper at the same amount of pull) And a shorter taper would give you “full amount of gas” sooner in the pull. Essentially controlling “when” you get your full main jet of gas charge.

However, I recently have been battling high top end temps while keeping my low end from four-stroking with my 48/49 derbi. I figured, if I could keep my low end pull leaner until later in the throttle position (you really have to slow roll in the throttle) that I could continue to go up on the main to help with those high temps and also keep my low end leaner and responsive.

Stock needle on the 15phbg is a w7, 1.4mm tip and 18mm taper. I I figured, well heck, I’ll go one class “leaner” to the 1.8mm which will cover more of the hole and get a longer taper. Since I’m thinking at WOT where I need more gas the tip diameter should not be playing in too much. so I went with the w24 which is the fattest 1.8mm and a whopping 24mm taper. Hella long.

Slapped that bad baby in second from the top, (w7 ran good top and second from the top.) annnnnnnnnnnnd it four stoked so bad all around the rev range I could not get it to variate while on the stand. I happened to have a w10 laying around. Fat class 1.8mm tip with a 16mm taper. Shorter than the w7 but a little leaner. Ran great. Need to ride more and tune to see if it’s right, but now I’m not so sure that’s what I needed. Now here’s my dilemma.

Was I wrong all along?

Does a longer taper do something else entirely? The only thing I could come up with is the idea that maybe the longer taper almost acts as a Venturi and speeds up the gas and longer taper actually makes your mid pull and low end pull richer??? Meaning maybe I actually need a thinner tip and a short ass taper? The thin tip so when it’s pulled all the way out it’s blocking the minimum amount of the atomizer hole (or at least less and that’s also if I’m wrong about there being no relation between WOT and the needle tip diameter) and a short taper to ensure my lower rpm throttle positions stay lean? I love phbgs and have them on basically all of my bikes at this point. Ive had them all tuned great now using my past logic, great WOT all day no over hearing and responsive down low. But this build is being a little more finicky pushing much more out of a 49cc cylinder. Can someone explain if I was right in my thinking or why a wider needle was causing my bike to be richer somehow?

Thanks Hosses. Kiss kiss.

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