This thread lacks the same thing most discussions of carburetor tuning on this forum: An understanding of how to tune a carburetor and how to relate problems so we all understand what the carb is doing.
So some thoughts:
1. As the needle is out of, and clear of, the orifice that creates what we call the needle jet, above 3/4 throttle to full throttle it is the size, or flow in cc's, of the main jet that control the fuel.
2. Now if you have done any reading about tuning a carburetor you will come across the process called "plug chop."
This is where you run the bike at a specific throttle slide position, turn the motor off while the throttle is still in this position, and roll the bike to a stop. While this is thought to be used for checking the main jet at Full throttle, it can also be used for other jetting circuits.
But I mention this because of your statement, "I cannot raise the needle because the plugs look a litte rich already."
It is very possible to have a main jet that is so lean as to leave no carbon on the plug; and the mid-range needle position be so rich as to compeletly cover the plug with carbon; and have a needle jet diamter/pilot jet so lean as to cause the bike to seize.
Any running of the engine, at other than the jetting stage you are working on, can/will mask the plug and give you an irrevelent reading. THIS INCLUDES IDLING!
If you are going to learn how to jet a carburetor you must learn:
a. The stages the carburetor delivers fuel.
b. The parts of the carburator that are involved in that stage.
c. And how to read the symptoms, whether they be lean or rich. To do this you need all of your senses, not only the spark plug!
Because it is possible to have a main jet that is too small to pass enough fuel for the bike to run properly at mid-throttle it is mandatory that you set the main jet first!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
All of the gas required by the needle jet/needle comes through the main jet. if it is too small you can waste a lot of time and effort working with the other stages.
The Workshop manual
, or AMAL
basic jetting for the Carburetor type and size is always a good place to start. If there is ANY question start with a main jet a bit bigger than recommended.
After you get the main jet selected adjust your pilot air screw and idle speed (which I like to leave a bit high when tuning a carb 1200rpm instead of 900-1000 rpm).
Now this is not well understood, but from idle to about 1/3rd throttle the diameter of the needle jet is in control of the fuel mixture.
If you cut the carbuterot in half and could see the needle as it is raised out of the needle jet, you would see the straight part of the needle does not clear the needle jet's orifice until about 1/3rd throttle (varies with which position the needle clip is in).
Thus small changes to the needle jet makes a big difference in how the bike will run. It is important to grasp that nearly all 4 stroke engines use one of three needle jets. .105", .106" or .107."
A bike that runs perfectly well with a .106" needle can run rich enough to 8 stroke with a .107" and can run lean enough to seize with a .105." Because the nedle runs up-and-down inside this brass jet it is subject to wear.
Now without getting into details, there have been a lot of needle jets avaialble that are not the size marked on the hex!!!!!!!!!!!!! I am sorry to say they must ALL be checked before using.
While there are well documented adjustments, such as raising and lowering the needle, there are quite a few used by professional carburetor technicians to fine tune an instrument.
As well as raising and lowering the needle one can shim or cut a portion of the hex away from the needle jet to raise or lower the jet. This way you can split the difference between needle clip positions.
The length of the needle jet's orifice can also be shortened as well as enlarging the orifice a few tenths of a thousandths at a time.
Also there is the crossed drilled .035" hole on the 4 stroke jet that can be enlarged, further reducing the manifold vacuum signal on the jet (be careful as a couple of thousandths of an inch makes a big difference. People suffering "megaphonitis" should see some improvement with the hole at .039."
This works while the straight part of the needle is still in the jet. A similar effect can be had by changing the air bleed visible at the mouth of the carburetor. AMAL
MKII's actually have a jet available for this air passage.
So get some tape, mark your throttle so you know which jetting circuit or stage you are running the engine in, and use all of your senses to determine if the bike is lean or rich.
Take the main jet out of the carb. You will quickly learn two things. The main jet has no effect below 1/2 throttle and what 8 stroke sounds, feels, look smells like as the carb starts to make the transition from the needle/needle jet to the main jet.
It is expensive way to do things, but ALWAYS go rich first. The worse thing you will do is waste a little money and foul a plug.
Remember to do EVERYTHING RF mentioned above first.
YOU DO NOT NEED TO RIDE THE BIKE TO ADJUST THE SLIDE!!!! It can be done while the bike is still on the stand. http://www.tioc.org/tech_article.htm