1. Set the low & high end needled to about 1 - 1 1/2 turns each.
2. Choke the carb and prime it until it's wet. 3. Fire up the engine and let it warm completely up for several minutes. Then let's set the top end first as it is the easiest of the two. Go to full throttle. Adjust the top end needle for peak RPM. Leave it wide open for about a minute to see if there is any change. Should the engine go lean, open the low end needle slightly. If this does not work, you will have to adjust the needle valve inside the carb (I will explain this later). If the top end runs OK, then slowly pull the throttle down until the engine begins to "four cycle" and hold the throttle there. Adjust the low end needle until the "four cycling" stops. Now lower the needle until it "four cycles" again and adjust the low end again. Continue doing this until you reach full idle. Now, from full idle begin to throttle up until the engine begins to bog or hesitate. Open up the top end needle just enough to stop the bogging or hesitation. When this is done right, you will be able to set the throttle in any position and it won't "four cycle". Plus you will be able to transition from idle to full throttle without any hesitation at all.

 Ok this is for people who need a little insight on how a Walbro carb works:
                       
                                       
 Starting from the gas tank, the fuel is pumped from the tank and enters the carb thru the inlet. The fuel works it's way through the fuel pump through a little  diaphragm  pump that's controlled by 2 one way valves (little flaps)  then the fuel passes  through a  needle and seat that is controlled by the "float" diaphragm. This "float" manages how much fuel is available for the idle, low speed and high speed circuits or throttle positions. The "float" diaphragm opes and closes the needle through a small lever attached to the needle. Then the fuel is regulated by the "float" diaphragm that controls the needle & seat.
 All of these parts reside in the float cavity area as well as the fuel ready to be fed the jets as needed. The amount of fuel available in the cavity is regulated by the lever and it's relationship to the float diaphragm. So it is critical that the lever be set properly. Within the cavity there are distribution holes that are managed by the low end and high end needles & the idle circuit, which is a fixed size.
 NOTE: All walbro carbs will run in ANY position but they tune best as a side draft carb. The downdraft tends to run a little rich & the updraft tends to run a bit lean at idle. No big deal though, it's easily tuned none the less.
 Now let's talk about the pulse signal for the fuel pump.
YOUR engine will determine which pulse jet type you need! If the engine "carb base" has a hole going to the crankcase, you will use the STANDARD pulse port and the optional pulse port (in the unlikely event you have one) will need to be blocked off.  If the intake manifold does not have a hole going through to the crankcase (again, unlikely) you will find a fitting somewhere on the crankcase. Use a piece of fuel line to connect the fitting to the optional pulse port on the carb.
The carb MUST get a pulse signal from the engine! This pulse pushes and pulls on pump diaphragm that feeds the carb fuel.
 

 Now let's check out the "float needle and seat" setting.
This is the single most important setting on a Walbro carb! Walbro offers a "setting gauge" to properly set the height of the lever for your particular carb. If you don't have one the setting will be a "trial & error" adjustment and a real pain in the butt since you have to open up the carb every time you make an adjustment. For general purposes this lever will be almost perfectly parallel to the carb base. This will get you close . If the lever is too high your engine will  tend to run a bit erratic at idle. If the lever is too low your idle will be OK but it will tend to run lean from the mid to high end range. It may also run the float cavity dry at full throttle and die regardless of your high and low speed needle settings.
The needle valve is pressed into the carb base and you should NEVER remove it without the correct setting gauges and specialty tools. DON'T DO IT!!


  Troubleshooting:
1. One of the most common problems is the engine stalls when accelerated. This could be cause by the high end needle being WAY too lean or the lowend needle being slightly lean.
2. Engine goes lean in flight: High end needle slightly lean AND lowend needle is rich. Float lever may be set too low.
3. Engine goes rich in flight: Low end needle too rich. Float lever may be set too high.
4. Engine runs good but no idle: There's crap in the idle jets and the carb will have to be removed, disassembled and cleaned. May also be caused by an air leak at he base of the carb caused by a bad gasket or worn throttle plate shaft or damaged/ worn throttle plate (butterfly).
5. Carb leaks fuel when engine not running: Float needle is bad, theres crap stuck in the needle seat, lever setting is too high or the diaphram is bad.
6. Engine "four cycles" momentarily when backing off the throttle then runs OK: This is perfectly normal with engines NOT equipped with a "check valve" high speed jet. If it does have a check valve then the float valve setting is a bit too high or the float needle is leaking a little.
7. Fuel leaks back into the tank when engine is not running: Bad fuel pump membrane or a hole in the fuel line.
NOTE: A very common problem with open air & cowled in engines is air pressure in flight changes the "natural" or ground level pressure on the float diaphragm. These carbs were designed to be used at ground level and not at high speed through the air, both of which has an effect on how much fuel the diaphragm "thinks" the engine needs. This causes the engine to run rich in flight but there are a few possible ways to fix this. Most of the time you can simply tune your engine for flight by trial and error.  You can try moving the diaphragm cover around to different positions (sometimes helps). You can run a duct to blow prop wash air directly at the carb on cowled engines but the best way to fix them ALL is to solder on a small brass tube with a 90 degree bend away from the airstream or better yet, run a piece of fuel line from the bent tube all the way into the fuse through the firewall, then your "ground tune" won't change change in flight anymore!
    

NOTE 2: Carbs Equipped with a high speed check valve are greatly superior for flying aerobatics or flying whereby the throttle will be used extensively. All the check valve does is prevent the jet dripping when you back off the throttle.
Straight through (non check valve) jets always drip a bit when you back off the throttle causing a normal, momentary "four stoking" until the jet stabilizes to the new airflow rate. A couple of things that will help this is running a velocity stack on the carb and directing prop blast to the carb opening so as to "normalize" the airspeed the carb senses.