Needle Shimming - How much equals up one jet size?

[drdano]

Hello XS Gurus. First my appologies if this has been covered, I can't seem to find searching the forum.

Is there a general "rule of thumb" regarding how much a needle is shimmed before you're effectively at the next size pilot jet? Obviously the thickness of the washers would play into this, but in general is there something like "X millimeters shimmed in height = the next size up jet." ? Or, where does one stop shimming and move to the next size pilot jet?

Thanks all! -Dano

 

[jd750ace]

They overlap. Shimming the needle is going to make it richer at the top end on the pilot jet's range, as the needle is influencing that range as well, but it will still be lean at the bottom of the pilot jet's range, where it overlaps with the mixture screws. Still gonna leave a flat spot in the middle, otherwise, Mikuni would have left out he pilot circuit altogether. You can't effectively work around it. You are begging for balky throttle response at small throttle openings (where you spend a good deal of time) if you try to shim away a problem that is actually a pilot jet problem.

Read this for a better understanding of the overlap.
http://www.factorypro.com/tech/carbtune,CV,high_rpm_engines.html

There are many more guides on tuning CV carbs, but this one gives you a little bit of an idea of what RPM range is messed with by each circuit. Drop everything by about 20% as far as RPM reference, and you'll get it.

 

[5twins]

It hasn't been covered because it has nothing to do with jet sizes, pilots in particular. The pilots are in your idle circuit, the needle setting tunes the midrange. Mikuni needle slots are 1mm apart. The washers we use are about a half MM thick. This allows you to achieve half settings on your needle, like 2 1/2 or 3 1/2. Sometimes, this is needed for fine tuning. Going a full step on the needle may be too much and this is a way to set it in between steps.

 

[drdano]

Thanks to both of you for the info, I had the relationship between the needle and pilot wrong. I've read through that link a few times as well and there is some good info there. The link for the pilot jet tuning is dead on that page, here it is in case others want to read it: http://www.factorypro.com/tech_tuni...t jet, affect of size on idle and cruise.html Thanks again.