Bonneville America Forums Forums Technical Forums Performance Mods Neddle Shims (I dont get it...yet)

OK...I went to Dinquas site and read up a bit. I did some searches on this site but didnt find anything that was satisfying. Since Im getting ready to do some work Id kinda like to know what the dealio is with needle shims.
What do they do?
How do you know when and if you need them?
Where do I get them? I read one post that I can use washers from a hardware store...was it 5mm? Are they really the same thickness or just a good enough?

Im curious here and basically looking for clarification.

The needles meter the fuel allowed into the fuel circuit by the main jet. All needles have a taper to them (first pic). You can adjust how rich/lean the main circuit is by changing either the needles, jets, or shimming the needles. Shimming/changing the needles is the easiest way to richen/lean the mains because they are easiest to get to.




For grins, here's a couple of pics showing idle/low & wide open throttle positions:





Here's a graph showing the interaction (clip = shim, sorta):

Love the purple and green "bubbles".

so shimming isnt necessarily needed. Its just another way of making it balance?
From what I can tell if you shim your basically keeping the needle further out from the main jet.
So if I increase the size of my main jet then I dont need to worry about shimming unless I need to make an adjustment that is smaller than I can make by replacing mains?

Idle to 1/4 throttle the main doesnt come into play..its all on the needle. 1/4 to 1/2 and the needle is far enough away from the main its now flowing more fuel but the taper is still managing flow. etc...

Am I right or way off base here?
IF I am correct then how do I know I need shims instead of a different size main jet?

if you only change the jet to be bigger, yes its richer thruout, but what if its too rich at WOT? So you go back on the main, and shim the needle to make the midrange richer, but keep WOT where it is. Capice?
G

Lonzo...that was perfect!
Its now printed and ready for my stuff next week.

Thank you all very much .

Im kida leary about fixxin something that ain't broke...mine just turned over 19,000 miles, and runs great. The dealer removed the AI for me, and I have removed the snorkel from under the seat, and cut out 2 stages from my stock exhaust, but have'nt touched the carbs. She does "pop" quite a bit on deceleration, but I kinda like that. My pipes have a distinctive "blue" tint, which, to me, adds character. My brother, who is a SUPER bike mechanic, says all these mods and adjustments are great for overall performance, but will greatly decrease the reliability of the machine down the road, in general, and advises me, that if it aint broke, don't fix it. Gonna keep her as stock as possible for as long as possible to increase overall life of the bike...just my opinion...

b717doc, you can cut down on those explosions in your pipes during deceleration by adjust the mix a bit richer and that actually will help the longevity. An air cooled engine should always run a bit richer since it aids in cooling the motor and a cooler motors last longer. Just my two cents...

I was under the impression that the "pop" was caused by an excessive "rich" mixture, producing un-burned gasses to enter the exhaust system and exploding, producing the "pop" experienced on deceleration...it's all too confusing to me. I actually taught carburetion classes in A&P school (aircraft), but it's been like 12 years ago...

If I remember right it is the unburned fuel from a lean misfire that propagates in the exhaust causing the popping. someone correct me if I'm full of it

mike

hi just wondering do you have info on useing 98 tunderbird needles and the fitting is it easy on keihin cvk 36 carburettors I have re-jetted to 135 mains and I read on this site that these needles work well hope you can help

George, there are tons of threads in the tech vault and maybe Dinqua's site to concerning that topic., I also suggest starting a new thread in the perfomance mods section. you'll get a lot more replies.

mike