Needle taper 101?

When I got the Keyster rebuild kit for the '76 - '79 750 twins, I notice the supplied needle does not match the stock. It does have the five slots for adjustment, but it is about 1/8" shorter and the end has less taper (I assume it's the closest they could find). The stock needle tapers down over a longer distance and ends up at a smaller diameter at the bottom. Looks sort of like if you snipped a bit off the bottom of the stock needle, they would be similar.

What is the difference in the longer taper? I believe it would mostly affect light throttle opening where the slide is just starting to rise.

How would the performance change using a fatter needle set to the same height position?

The reason I want to understand: I put the Mac 2-1 pipes on and have been screwing with the needles. The stock needlse and stock position ran WAY too lean and the engine was very weak opening up from closed throttle at lower RPM.

I went to the richest slot on the stock needle and it runs pretty well, still s bit weak at lower RPM. I like having good pull in the 2000 - 3000 range because that's mostly where you cruise these old twins.

So, I tried the fatter, shorter needle because by chance, the slots were cut lower on the needle and I could get a richer height. I got fantastic low end hit, but it was running too rich (sooty plugs).

I am trying to figure out if there is a way to get the good low end hit and not run too rich at cruising RPMs.

I'm still no expert, but I did find some good references on needle tuning:



www.justkdx.dirtrider.net/jetneedles.html



yhst-17379619078614.stores.yahoo.net/carbtuning.html



www.carbparts.com/keihin/needles_tuning/jet_needle_changes.htm

The jet needles that you bought came from a company in Japan. About 90 percent of what they manufacture is accurate as they are fairly careful to reproduce the specs of the original item. The other 10 percent of the time, they seem to drop the ball. I personally have sent carb assemblies to them so that carb kits could be made up for specific models and they do a decent job. In this case, it sounds like the ball was dropped. There are two possibilities... the jet needles they cloned when they came up with the KZ750B carb kits were the wrong ones in the donor carbs they used OR you had the wrong jet needles in your carbs but I suspect that you read the badge label and compared it to the spec in your manual. The badge should have said: 4JN19 or 4HL12... Both are correct for the various assmeblies from 76-79... (don't recall which year you have). The numbers and letters on the aftermarket jet needle will be different... probably Yxx as that is the system that the Japanese company usually uses. They SOMETIMES will stamp the actual Mikuni badge number but MANY of the jet needles they make will use their proprietary label.

The 4 in your jet needle number is an overall length range. The other numbers and letters don't mean diddley... you have to look them up and when you see the chart, it will show an overall length for a particular jet needle and a diameter at every 10mm point from the sharp end. The taper effects the air/fuel mix. The skinny taper allows a richer mix and the fatter towards the top is lean. Obviously most jet needles are very lean when the slide is down all the way in the needle jet and progressively get richer the farther the slide raises. The diameter at the 10mm points determines the relative rich/lean nature of the mix at any given throttle point. Most GOOD jet needles have a relatively linear taper and are neither too rich or lean at any point. Perhaps the best examples in the older carbs are found in the KZ900 variation of the VM26... the 5DL31 has an excellent taper and power delivery is real smooth. The short stubby and very tapered jet needles used in the 78 VM26s are much harder to tune 5CN7 and 5CN18 come to mind. This is one of the reasons that most Dynajet jet needles are not real good... they are super skinny and provide too much gas over most of the range and that is why they generally recommend starting out by clipping to the 2nd slot (2 of 6)... Some of the DJ jet needles are actually fine; especially some of the constant velocity carbs as they are fairly similar to the stock jet needle and seem to be decently matched to the needle jet size. There are places where you can get custom made jet needles if you know enough to give them the right specs but I have never attempted this as I am not sure I could improve on the OEM jet needles in most cases. The Mikuni company put out a VM carb tuning book which has a bunch of jet needle specs but unfortunately, I have never seen almost any of the ones that are documented and the ones that are common in Suz and Kaw VM carbs don't seem to be there for some reason.

Mine is the correct needle which is 4HL12-3. The Keyster needle is a bit shorter, has more taper, and the slots are offste a bit. Putting the clip in the top slot of the Keyster needle gets the needle closest aligned to the stock needle with clip in middle slot. It looks like they have a stable of needles they choose from and this one was closest. The kit is for the KZ-750B for '76 to '79 and there were needle variations.

The straight sections are the same length and exactly the same diameter (.1175" )

My problem is, ever since I added the Mac pipes the low end hit has been very lean. I am up one already on the pilot jet to 50 and that is the best running one (I have tried the whole range).

I can improve the low end hit by raising the needle all the way, but then the upper mid RPM range is too rich. I am thinking of taking the Keyster needles and lathing the straight section down a couple of thousandths to rich up the low end a bit so I can run the needle at the stock height and still have good low end pull. Since I will never need the extra needles (I have two stock sets) I might as well play with them a bit.