Shimming the needle - TwoWheelForum: Motorcycle and Sportbike forums
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post #1 of 5 (permalink) Old 07-08-2005, 09:34 PM Thread Starter
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Shimming the needle

While cleaning my carbs last weekend, I removed one needle from the piston just to see how it was held in and put it back together. The bike accelerates like new, but there is a light surge at steady throttle between 3-5K. I can accelerate out of it, but the surge is right at my cruising range around town.

Has anyone shimmed their needles? There's one washer under the needle and it would be easy to put another one there. Unfortunately, I didn't measure the stock washer for thickness, or inner or outer diameter. Putting another washer, or a thicker washer would raise the needle, but keep the piston at the same relative opening. Can washers be purchased from a hardware store? If the needle was shimmed with a thicker washer, would it cure the surging?
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post #2 of 5 (permalink) Old 07-08-2005, 10:05 PM
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It depends on if your surging is being caused by a rich or lean condition. Shimming the needles will allow more fuel to flow for a given throttle position. This is a good thing if you are running slightly lean, not so good if you are already running rich. The shims are not necessarily a specific size but you should be able to get the right ones at any shop.

Give it a try, if it runs worse...take them back out.


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post #3 of 5 (permalink) Old 07-09-2005, 08:37 AM
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What kind of bike is it? On my Bandit, the shims are a Radio Shack washer thats like 8 cents a piece. If your bike is stock I would guess it is lean but thats a guess. Hope that helps.
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post #4 of 5 (permalink) Old 07-23-2005, 12:40 PM Thread Starter
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I followed Bandid's advice and went to Radio Shack. The washers were only 2 cents each, but I had to buy a bag of 100. Five different diameters, of incredibly small thickness, so one had to fit. I propped up the gas tank and took off the top of one carb. I could see trouble.

The piston is plastic. Looking down inside the piston, the nut holding the retaining plate is plastic. Slipped a 7mm socket on the nut and stripped the head. My speciality is stripping phillips head screws and breaking bolts, but this is the first time I stripped a nut. Tried a 1/4, but it was too small. Tried a 9/32 (How many have 9/32 socket?), but it also stripped the nut. Frustrated, I put it back together.

I checked the little rubber caps that cover the carb-synchronizing vacuum tubes, hoping one was missing. A missing cap would make the bike run lean. Nope, they were all there. I hadn't synchronized the carbs when I cleaned the pistons, so I got out the old Carb Stix.

The synch was way off! No. 1 cylinder was two feet up the stick, No. 2 cylinder was about 18 inches up the stick, No. 3 cylinder was about a foot up the stick, and No. 4 cylinder was not moving the mercury. I first thought that No. 4 cylinder had a hole in the piston, but as I adjusted the screw for No. 4 it came back up. Here's how it looked when I finished. And it ran great after the synch.
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post #5 of 5 (permalink) Old 07-23-2005, 12:44 PM
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I love it when these little problems solve them selves...Great info too...I am sure this will happen to someone else!!!
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