Shimming Needle

[woodlark]

Over on the Baron Forum, the administrator has stated that the engine output of the Baron and Wussi 250's can be improved by raising the needle valve by putting shims under it. I have a couple of questions:

1. What is the negative side to doing this (every silver lining has a cloud!)?

2. How much shimming is good versus too much?

3. Where do you get the shims?

[toxicutor]

Take apart the carb and needle jet and look at the size of the needle at the top. Any motorcycle shop will have shims for a few cents, or a hardware store if you can find washers small enough.

Just try one shim at a time, as you raise the needle you make the mixture run richer (more gas).

There shouldn't be any negative, do a plug reading at moderate throttle loads. If you get too rich, simply back off on raising the needle. If you get hesitation, backfire, less power, or a dark colored plug you are runnng too rich. If you suspect the scoot is running pretty lean from the factory, you might want to check the main jet and the pilot jet.

Heck, if you are going to go to all that trouble you might as well install a unifilter and then rejet the whole thing.

[scooterollie]

woodlark;
Saw that post also. Last night I took the top off my carb and pulled the needle valve. No shims on mine and decided I don't really want to play with it. Couple of recommendations I have seen indicate just one thin shim is needed. Obviously the mod will allow more fuel to flow but not being overly familiar with this type of carb, don't know the difference between doing that and installing a larger jet.

[Jacine]

The shim is for mid range throttle approximately 1/4 to 1/2 (3/4). You can correct bogging down in the midrange rpms around 4000 (3500-4500) on a 7000 rpm bike. Some needles have clip positions 5 or 6 that you can use to adjust. The chamfer of the needle may be retooled by the manufacturer to change the fuel flow in the mid range. Chamfer changes were made to the needle on the Rukus from 05 to 06 to improve performance.

It does not superchage the bike, and in fact if it's not required, just burns more fuel.

[woodlark]

Thanks, Jacine. Since my scoot appears to be running ok as it is, I'll just leave well enough alone.

[scooterollie]

Jacine;
Thanks for the clarification. I have had a couple of carbs apart in the past that had the adjustable needle. The one on my Roketa 250 Keihin carb is fixed. As with Woodlark, mine seems fine so I will leave it alone. This winter, think I'll check my variator roller weight and replace the rollers with slightly lighter "Dr. Pulley" sliders and a new Kevlar belt.