Thank you Mx5 for the vote of confidence. I will try my best.
Quote:
Originally Posted by mysteriouslyraptured
So what is the difference between jetting and shimming?
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To best understand each, you'll need to understand what is going on inside the carb.
Simply put, the carbs used on bike are really several different fuel squirtguns blended in to one unit. Each part has its job in the throttle range. The trick is to get the flow of fuel vapor optimized for all throttle positions and all rev ranges. The bike comes from the factory with all these things carefully calculated out, taking in to account airflow through the intake, engine specs (displacement, valve timing, valve opening, compression, exhaust relief) and ehaust velocity through the muffler system.
If you look at the usage of each part of the carb, you start to see where each component does its job. The pilot system is only for idle up to about 1/4 throttle. Changing the pilot jet, pilot air juet or adjusting the pilot screw will alter the mix there.
From 1/4 to about 1/2 throttle the throttle valve (butteryfly, plate) does the job of metering mixing from the pilot system to the needle.
from 1/2 to 3/4 throttle, the needle and its nozzle do the mixing. The taper of the needle determines the mix as does the nozzle.
from 3/4 to WFO (wide full open) the main jet is what is metering fuel.
Changing one or a combination will affect the mixture in those ranges.
To fully tune the carb is to carefully calculate out what is needed in each part and make sure the transitions are smooth.
This can be done by trial and error but it is time consuming and can get costly.
Shimming the needle is done instead of replaceing the needle with a different taper. Replacement is better but not all carbs have the parts available to fully tune for modifications so raising the needle in the nozzle a bit thereby having the taper move sooner can get that part of the tuning close.
Changing a pilot jet will tune that porting of throttle response.
Changing a main will affect the wide open running.
Quote:
Originally Posted by mysteriouslyraptured
What is the difference between their purposes?
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With all the stuff I threw at you in the previous treatise, basically each jet, the throttle plate, and the needle/nozzle have their portion of the throttle response to take care of.
Quote:
Originally Posted by mysteriouslyraptured
When should they be added in a mod, or what are the tell tale signs that it is needed?
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The signs of improper jetting come in by either seeing too rich a mixture or too lean a mixture.
When to do this if if you have significantly changed either intake airflow, exhaust velocity or altered the cams/valvles/compression.
It is particularly evident when multiple alterations have been made.
With the Rebel, major engine mods aren't that possible (no hot cams, big bore kits, or oversize valves) since there just isn't a call for those aftermarket parts.
The most likely mods will be dropping the airbox and a higher velocity exhaust.
That will lean things out and then carn work would be in order.
Before you travel down that road, consider that there is little to gain in usable power by changing your airbox and exhaust. Really nothing that will make a difference. Jetting(carb tuning) will be hit or miss, with some trial and error. Engine smoothness, fuel economy and longevity will be affected with smoothness and economy being the most notable.
Here are some diagrams that sort of show the first part I talked about.
The cutaway drawing is of a first generation Hitachi carb used on early Viragos but the princples are the same.
Yeah, what he said above, da stick in da hole meters the go go juice to the cam thingy to make da wheels go faster, by chain drive, I think.
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