Needles. How they do?

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.

Re: Needles. How they do?

To make things more fun, I tried to also do some reading. Reading comprehension is something I’ve always struggled with when it comes to motors and engineering things I’m a very hands on learner. And I found this which perplexed me


The w7 has a 3.5 degree angle and according to that a smaller angle will lead to a leaner mid range. I went to the w24 which has only a 1.7 degree angle and it somehow got much richer everywhere.

¯\_(ツ)_/¯ mopeds is fun. Even if I have to keep just guessing on stuff and dropping in whatever needles I have at the shop to experiment, I’m ganna keep doin mopeds. I just like learning and understanding better.

Re: Needles. How they do?

Awh man just did more reading and I think I’m starting to get it. A longer taper means less straight section. The straight section is really what is ganna help keep my low throttle position mixture lean. It blocks THE MOST of the hole possible. Longer taper is giving my bike gas sooner since the taper takes up more of the needle. Hmmmm learning in real time. Soooo maybe shortest taper with a thinner tip? Yeah? Crap they don’t have that. Seems the 1mm tip is only available in 22mm taper and longer. So w8 may be my man. 1.4 tip same as the w7 but 2mm shorter taper (edited)

Re: Needles. How they do?

Yeah the other thing that I learned way too embarrassingly late was to think of it like a reduction to flow rather than adding flow. It sounds kind of stupid/obvious but the needle is really removing flow at part throttle when you have high vacuum and are cruising it doesn't add flow at low speed high load.

The secondary air circuit or pilot jet circuit is making up for situations where you have large throttle opening and low speed AKA high load but it would deliver too much fuel with the throttle closed at higher speed and low load so you need to add fatness to restrict the fuel. If you think of it as adding fatness rather than adding fuel it will make a lot more sense when you are riding and feeling how the bike behaves.

Re: Needles. How they do?

I think that sums it up pretty well, thinking of the needle strictly as a restrictor that’s shoved in that main jet atomizer. It restricts your flow until your taper kicks in.

It’s like you said high speed low load when you’re basically in a flat, and sitting half throttle and just letting the engine go the comfortable speed it likes too.

Re: Needles. How they do?

Yeah basically think 'where do I want this needle to take fuel away' as your riding, then feather the throttle to feel for the rich spot, then make the needle more fatter there.

For years I was like 'where do I want the needle to give me more fuel' and that always frustrated the hell out of me. It sounds dumb because it should be the same thing, but in my brain I can feel the needle taking away fuel better?

Re: Needles. How they do?

Honestly my derbi is the only bike that I've had needle trouble. W9, 2nd from the top is wayy too lean at 3/4 and one more down is so rich I can't reach the main. Great write-up

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