Quote:
Originally Posted by funhunter63
WOT test:
Starting @ 2000 RPM in 2nd gear-then WOT. It stutters or bogs for an instant until about 2500-3000 RPM, then it takes off....yippee...until I hit the 6500 RPM pipe restriction spot that only slows it down a little. Doesn't appear to make much difference whether it is hot or cold for this test. But today was a cool day here...didn't even get to 60. This is not something I would do in normal riding anyway. I am almost always over 3000 RPM.
Future thoughts and questions:
If I put another shim or two under the stock needle I think it would help the accelleration @ 70MPH in high gear @ 4000 RPM??? If I put Thruxton needles in, would I need to shim them??? What RPM range do the shim affect? What RPM range does the difference in the tapers between the Thruxton needle and the stock needle affect?
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The needles/needle jets supply a small amount of fuel at low rpms/small thottle from the straight section of the needles. The difference between needle straight diameter and needle jet inside diameter forms an
annulus which is essentially a small orifice -- like a pilot or main jet. The velocity of the airflow determines how much fuel is 'drawn' into the airstream.
As the slide rises and the needle lifts, the tapered diameter of the needle starts interacting with the needle jet inside diameter and increases the area of the annulus which allows more fuel to be 'drawn' into the airstream. More lift on the slide means a decrease in needle diameter due to the taper and more fuel flows.
So.... If the slide isn't lifting enough or the needle taper starts too late (straight section too long) then you're going to experience a lean WOT mixture at low rpms.
At part throttle the lean operation can be masked (swamped) by oversize pilot jets and large mixture screw openings, but at WOT those options are essentially bypassed (due to low airflow velocity) and the needle taper effects become dominant.
The Thruxton needles are virtually identical in configuration to the FP needles with the clip on the bottom notch. In this configuration the needle head to taper distance is around 2mm shorter than the stock needles, so the tapers start acting with 2mm less slide lift than the stock needles -- much sooner.
I've found that it's the needle taper distance (from the head to the start of taper) that is critical to WOT operation in the 3000-4000 rpm range and that the effect is most pronounced at 4000 rpm with a modified air intake. Longer cams exacerbate the problem because the increased duration of the cam generates a lower velocity through the carb and doesn't open the slide as far -- spreading the cylinder fill over a greater time period means the cylinder doesn't have to fill as fast.
Shimming the needle effectively shortens the distance from the needle head to the start of the taper, but stacking 2mm of shims under the stock needle is not a good idea. The needle tends to 'wobble' more and will contact the needle jet and that may cause it to jam and bounce. Stuck or bouncing needles don't run well...
The Thruxton needles are less expensive than the FP needles, so that makes them worth a try, but be sure to check the pricing on both. With the value of the dollar down against the Euro, the FP needles might be a good value in terms of adjustability and price differential.
Shimming the Thruxton needles should be unnecessary. I tried a 0.5mm shim with the FPs and it was way too rich at low rpm to the point of misfire and bucking. When that didn't work I opened up the slide vacuum port from 2.5mm to 3.0mm. I don't recommend that except as a last resort with the bigger cams, but it is reversable with a judicious application of expoxy and re-drilling.
Does all this make sense?
Jim
