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The Pilot circuit (which the PMS screw controls) is responsible for primary fuel delivery at idle and throttle positions below about 1/4 turn (see Bulldog's article for more complete explanation).
All circuits in the stock carb are functional at all RPMs (except the accelerator pump) and will need to be balanced for best performance. Primary responsibility for fuel delivery will change with RPM, but they all provide fuel at all settings. For example the PMS circuit never turns off and is always delivering the maximum that it can contribute at any RPM. The main and needle circuits are essentially off at idle RPM levels, but will always leak a little fuel regardless of RPM. This procedure outlined in this article will not make up for being too rich, too lean, wrong main sizes or wrong needle clip positions, but will provide best drivability that can be achieved with these other circuits.
It can be a long process, and some folks have chosen to do it over time, making adjustments between rides. Others have packed their bags with whatever tools they needed, and gone for an extended tuning ride to do it all at once.
On initital setup of your carburetor, set your PMS to a safe and slightly rich setting based on the recomendations that have come with your jet kit or the recommendations of someone who may be helping you to set it up.
Then proceed as follows. Adjustments should be made one at a time, then ride tested to evaluate whether another adjustment should be made. Each time you ride to evaluate, be sure the bike gets completely warmed up before deciding on a next step:
- If you are getting backfire on decelleration (test this with hard decels - not braking but complete release from fairly open throttle positions), turn the PMS out 1/2 turn at a time until it stops.
- Next, turn the PMS in in 1/4 to 1/3 turn increments until you are able to induce backfire on decel. At this point we have a good reference point for where the circuit is too lean and can adjust to balance out the circuits.
- Balance out the circuit by turning the PMS back out in 1/8 turn increments until the backfire on decel quits. This will be the point at which the circuits acheive best balance for the operating temperature and elevation where you did your testing.
- If you have slight hesitation at initial throttle, turn the PMS out one more 1/8th turn.
- If you are unable to completely eliminate the backfire on decel, you may need to move up to the next size pilot jet. You may also need to consider the settings in the other circuits of your carb, ie: main jet size and needle clip position. If you are already using a larger pilot jet, you may be set too lean on either of these two other circuits.
- Having acheived this best balance, take the time to turn the PMS in all the way and record the number of turns as a reference (I like to use 1/8 turn increments, ie: 2 1/8 turns). Then turn it back out that number of turns to where your best setting is.
You now have a reference for where your PMS will need to be set to achieve best balance with the other circuits in the carb. If you know you are going into a hotter environment than you normally ride in, you can adjust it slightly to compensate. Just keep track of the turn increments and turn back when you return. If you get lost in the process, turn it all the way back in and then back out to the position that you got your best performance from.
MaxMix users should halve the turn increments that I am suggesting in this article. The MaxMix PMS screw is much smaller at the tip, and will be considerable more sensitive to these adjustments.
Anytime you make a change to your bike that could potentially change the jetting requirements, you need to dial in the PMS again. For example changing the air kit, pipes (even removing baffles), or porting the manifold. Naturally any change to the main jet or needle clip position will require this to be done as well.
Good luck, and please leave feedback (link below) if you have any tips or commentary that others would benefit from.
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