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Problem with Aftermarket Jet Needles

You might consider this item: Randakk's GL1000 Carb Rebuild Video

Recently, I've discovered some improperly sized jet needles that were provided with aftermarket carb rebuild kits. The needles looked identical to the original but did in fact have a significantly narrower cross section at the mid-point of the tapered section. Effectively, this made the needles much richer. The brand in question is NAPCO and the kits were for a '76 which happens to be one of my bikes. The result of these improper needles was the bike in question ran fine near sea level (although a plug check showed it was running slightly rich in the mid-range and the fuel mileage was terrible). The real problem showed up at altitude...I spent 3 days riding on the Blue Ridge Parkway in NC at elevations ranging from 2000-6000 feet. The bike was fine below 4000 feet elevation, but above this elevation it would foul all 4 plugs in about 20 minutes of riding (at sustained 4000 rpms!). I know from prior trips that this particular bike always ran fine at these altitudes. Suspecting the recently installed aftermarket needles, I measured them when I got home and sure enough they were significantly undersized...thereby flowing too much fuel at a given midrange throttle position.

Curiously, I investigated my current inventory of NAPCO kits that I have on hand at the moment and discovered that all the '75 and '77 NAPCO needles were correctly sized, but all the '76 needles were incorrect...way too rich. I'm not sure if this is a temporary problem with this brand of kits or not.

Needles are hard to measure accurately along the taper without special tools. I use a micrometer and measure at several points along the taper. A crude, but effective quick test is to use holes in a box designed to hold your drill bits. Pick a size that approximates the diameter of the midpoint of the taper. Insert a known good stock jet needle and note how far the needle goes before it bottoms out in the drill bit holder. Then do the same with the aftermarket needles. You should get the same amount of "insertion." Any observable variance is enough to result in poor results.

I generally re-use the OEM stock jet needles when doing an overhaul unless they show visible wear or in cases where the owner reported over-rich midrange problems not explained by other causes. I recommend that the original jet needles be re-used whenever possible to eliminate potential problems with aftermarket quality control issues.

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