Gerry Farell notes
Contents
1. In order to follow these instructions for cleaning amp tune up it is essential to first learn the names of the carburetor s parts and if possible their function first recommend that a pilot understand the general principles on how the Walbro metering system works Walbro Service Manual PDF 5 8MB http www southwestairsports com PPGTechInfo MiniPlane HRServiceNotes Wa lbroWG8 WalbroSM pdf This article is divided on two basic sections The first is Cleaning amp Tuning and the second is the Modification for Smooth and Slow Idle Since you have to take the WG8 carb off your engine let s start with a cleaning and calibration There are two different Walbro repair kits available The K12 WG has almost all the parts including needle fuel strainer screens etc Note that it does not contain a Metering Spring it is sold separately as Walbro part 98 320 7 OEM 503 28 12 18 The D12 WG kit contains only the gaskets and diaphragms Place the carb on a clean surface and remove both end caps With the metering diaphragm removed you can push on the lever in the center of the carburetor body to see how things work The lever is connected to a main needle valve and this is what meters the desired fuel pressure to the high and low speed jets The rivet in the center of the diaphragm pushes down on the lever and this in turn lifts the main needle valve out of its seat Fuel fills the cavity and the venturi effect in the carb s throat2. deposit at the base of the porcelain down inside the spark plug where the porcelain insulator and outer steel spark plug casing meet A ring of dark brown at the base of the porcelain denotes ideal fuel mixture light brown is lean and a ring of black is over rich This is the only area of the spark plug that accurately indicates fuel mixture Furthermore this reading only indicates full throttle fuel mixture No part of the spark plug can indicate Low Speed or mid range fuel mixture The upper part of the spark plug porcelain by the electrodes is often very light or white in color however this coloring is mostly affected by additives in the gasoline and oil The coloring of the end of the porcelain in no way indicates appropriate fuel mixtures of any throttle range Low Speed Needle Adjustment The initial setting for the WG 8 should be about 1 5 turns HR With the engine completely warmed up apply about 35 throttle or as fast as 4500 5000 RPM Turn the low speed mixture screw in or out in 1 8 turn increments As you get closer to the ideal setting the engine RPM will increase At the ideal mixture setting turn in a richer or leaner direction will cause a rough idle and cause the engine to die To confirm your perfect low speed mixture setting touch the kill button during idling and then restart the engine about ten seconds later The engine should restart instantly and idle steadily without touching the throttle Avoid running
3. idle pick up hole the lowest hole in the carb throat For testing purposes purchased a few throttle plates and started out by cutting a notch of about 0 2mm deep and gradually increasing its depth up to 4mm and after several static tests settled for a 2 0 mm deep notch for the Radne Raket 120 with either a WG or WB series carb Alternatively you can also grind a somewhat shallower flat straight cut instead of a curved notch HR For the Polini 100 the maximum depth is 1 8mm and the maximum width is 4 7mm Exceeding these values will make the idle speed too fast made the mods at 4 000 MSL so the values have given here may have to be adjusted according to your altitude above sea level Throttle plates modified for correct idle air requirements of a Raket 120 Left ight WG8 The next two pictures are of a WB 32 modified by Mr Scott Travers for use on a Solo 210 engine These two photos were taken from both ends of the same carb WB32 carb modified for use on a Solo 210 engine Note The Solo s need for more air at idle as compared with the Raket 120 using a similar carburetor Photo by Rick Grim IMPORTANT Wherever the idle pickup holes are in your carb throat make the plate notch in front or aligned to them and make sure you do not grind any more than 2mm If you grind away too much the engine will simply have a fast uncontrolled idle that cannot be adjusted with the idle stop screw or the Low speed
4. pulls fuel into the airstream Note where the parts go and ensure it is s squeaky clean inside Dirt is the most common cause of carburetor problems Using a carburetor cleaning solution be sure to clean the Inlet Needle s a k a main needle valve needle metering valve metering needle etc rubber tip amp seat as well as the internal wire mesh fuel filter Compressed air is the best method to clean all orifices without leaving fibers from commonly used items such as a cotton swabs Pop Off Pressure and Inlet Valve Reseat Pressure The Metering System is very simple It consists of a pivoting lever working like a seesaw with a spring on one side and an Inlet Needle with a red rubber tip valve on the other If you don t have the pressure testing tool don t worry just make sure you clean the carb blow it dry and use new diaphragms amp gaskets and set the Metering Lever height The resulting pop off and reseat pressure will be functional as long as you don t damage or modify the Metering Spring 1 Don t deform or cut the small Metering Spring If in doubt replace it with Walbro part Walbro part 98 320 7 OEM 503 28 12 18 WG8 carburetor 2 Set the Metering Lever at the correct height as explained below Setting the Metering Lever height Photo by Richard Cobb The height of the metering lever on any diaphragm carburetor is critical for controlling when and how far the inlet needle pops open thus configuring the
5. Adaptation of Walbro Diaphragm Carburetors for Slow and Smooth Idle and Progressive Midrange RPM Edited and adapted by Gerry Farell Additional notes in red by Had Robinson From http Awebspace webring com people bf flphg idle_adaptation html For the most part a carburetor that comes out of the box unless it was designed and built for a one of a kind engine will need some changes made to fit the needs of the engine These changes are also needed due to the type of fuel amp oil that will be used From there it goes on up the ladder in stages to fit various needs and levels of performance These changes for high performance are called Blueprinting Although stock WG8 carbs are being used on powered paraglider and hang glider harnesses they were not specifically designed and fabricated for the Radne Raket 120 or many other engines for that matter The fact is that the balance of air to fuel ratio is not proportionate under certain circumstances so it has to be made proportionate depending on the engine it is mounted on Don t think of this so much a modification or full blueprinting but just an adaptation of the Walbro carb to your particular engine needs A lot of pilots don t really understand carburetors but the Walbro is a very simple carb and to get it to perform properly it s just getting a balance and combination of needle jets metering lever height re seat pressure and for full diagnostic purposes pop off pressure
6. You ll eventually find a narrow adjustment range where peak rpm operation seems unchanged However within this range there should be a noticeable difference in mid to high range acceleration Finding the setting within this range that gives the best middle through high range acceleration usually requires patience to train your hearing Once you are satisfied with the settings make a full throttle run of no more than one minute and then take a spark plug reading Remember that a spark plug reading is a must Repeat the High speed circuit adjustment until your top end is smooth and plug reading is light tan in color Spark Plug Reading HR This is generally not a good way as it is often inaccurate install a cylinder head temperature gauge instead Radne switched the recommended spark plug from Nipponderoso to the NGK BPH7A with the electrode gap set at 0 6 mm Determining proper fuel mixture by inspecting the color and condition of the spark plug can be very helpful in situations where the engine is being operated constantly at full RPM under full load Using a NEW and calibrated spark plug is required At the end of the full throttle running the throttle should be chopped and the kill button pushed simultaneously If the engine is run at partial throttle for even 5 seconds after the full throttle run the plug reading will be invalid After the full throttle running a combination flashlight magnifying glass must be used to view the carbon
7. an over rich Low Speed mixture screw setting in an effort to cure a mid range hesitation lean condition Idle Adjustment Once the engine is operating at normal temperature you can now adjust the idle stop screw till you achieve a smooth idle What did all this achieve You now allowed enough airflow for the engine to run properly at idle You allowed for a smooth controlled idle since the Low speed mixture is finally balanced By increasing Low Speed mixture volume you also decrease the midrange carry over lag because the Low Speed jet progressively transfers control of the mixing action over to the Mid and High speed mixture circuit Your Raket 120 never had it so good so it will allow you to power down to idle and taste the thermal you just found and even lock the propeller without the inconvenience of stalling the engine and suffering a tough restart A point to consider is that if you can get your engine running at an iffy idle at about 2000 2100 RPM then it will be at Nirvana idling at 2500 2600 RPM without engaging the centrifugal clutch How does notching the butterfly allowing more air to pass differ functionally from limiting the butterfly s closure with the idle stop screw Balance of air to fuel ratio is not proportionate within the mid range or even at idle on most Walbro carbs It has to be made proportionate depending on the engine it is mounted on You increase airflow across the butterfly and increase Low jet flow
8. at idle thus you achieve a smoother mid range and lower idle speeds You cannot compensate for this with merely opening the butterfly Your objective is to increase Low speed jet action along with more air In other words the flow through the notch when the butterfly is closed is directly across the jet holes rather than around the entire periphery of an almost closed butterfly ad Edited and adapted by Gerry Farell Full credit and appreciation to Scott Travers for sharing this technique that was part of Mr Travers engine maintenance clinic For carburetor tuning details it helps to learn the names of the carburetor s parts and their function highly recommend to start by understanding how the Walbro metering system works Walbro Service Manual PDF 5 8MB http www wind drifter com technical WalbroServiceManual pdf Walbro Carburetors Troubleshooting Chart http www wing drifter com technical walbro_chart jpg Alex Varv s articles http Awww aerocorsair com id29 htm are clear and very useful but never cut the metering spring
9. bove below the edge of the carburetor body no gasket but does not restrict the gage from moving across it If you go higher in the lever it will give a lot more fuel and it can become a little hard to start because the inlet valve will not be able to close in time When reassembling first place the thick Metering gasket on the carb body followed by the thin Metering Diaphragm Inlet Valve Reseat Pressure Calibration HR if you must change the value add shims under the spring Contact us if you have questions The normal position of the inlet valve is closed the metering spring keeps it closed The Walbro manual states that calibrating its reseat closing pressure is far more important than pop off pressure When the Inlet Valve gets unseated pops open under fuel demand it must then reseat close at 10 12 PSI You ll know by watching the pressure drop on your pressure gage After pop off happens the pressure drop should end and remain steady at 10 PSI or above It should never drop below 10 PSI This will ensure that fuel pump pressure does not override push open the Inlet Needle Valve since the fuel pump produces 5 7 PSI To increase the inlet valve reseat pressure gently stretch the Metering Spring this increases the force on the inlet needle seat shuts tighter To decrease the reseat pressure carefully compress the metering spring careful not to lose it NEVER CUT IT If in doubt replace the Metering Spring Walbro par
10. difficult part Here s how to make a Walbro equipped engine start hot or cold with a smooth and progressive midrange Look at the throttle butterfly a k a throttle valve throttle plate throttle flap in the carb throat and you will see there a notch on the butterfly s outer edge some flaps have the small indentation and some do not It should be on the adjustment needle side of the carb close to the lowest pick up hole idle Almost appears as though someone accidentally nicked the butterfly Here s the problem That little nick in the butterfly plate accomplishes the entire airflow for the idle circuits This is why your idle stop screw and spring are mashed all the way in while your Low speed screw is opened a mere one turn out Your engine is starving for air while you ve cut its idle fuel supply to nothing Your High speed needle in the WB carb series is open at 1 to 3 4 turn for top end and your Low speed needle is open 1 8 turn That is a major expanse of territory with no midrange coverage The Low jet is too low to cover midrange while the High jet is too high to do the same This is an old technique used in go karting that can be applied to slide and butterfly carbs alike for specific tuning needs Note the exact position of the throttle butterfly plate and lower jet apertures remove the plate and use a 6mm diameter Dremel grinding stone 932 or rat tail file to cut away a small half moon so that the notch is aligned with the
11. engine s acceleration characteristics If the metering Lever is set too high relative to carburetor surface it may create a rich condition while a metering lever set too low may create a lean condition The distance from the outer frame of the carburetor body to the top of the Metering Lever must be 0 75 mm or 0 067 WG carbs and 1 45 mm or 0 057 WB carbs Walbro sells a calibrated brass plate gage to be used as depth meter Walbro item 500 13 1 available through most power tool shops recommend you also take a look at Wind Drifter s photos on this http www wind drifter com technical wg8walbro php HR The rebuild kits distributed by Walbro for their WG carbs have had diaphragms without a tang on the button for many years now Also the metering lever in these kits is preset for a height of about 1 7mm WG 8 carburetors off new Miniplane Top 80 engines have a metering lever ML value of 1 7mm However the Top 80 service manual specifies a distance of 0 5mm 0 7mm from the top of the metering lever to the top edge of the carburetor body As Gerry notes too great a value can lean out the engine at high loads If the 1 7mm value is used the total travel of the ML valve is about 1 0mm from closed to full open Testing has shown that this value is too low especially if the throttle is opened suddenly That is the valve will not open enough to allow enough fuel in to power the engine when it is suddenly accelerated As a resul
12. may be too rich or too lean For these reasons it best to stick with the official Miniplane ML value New engines need to have the carburetor checked to be sure the ML value is correct While the Top 80 will run with the 1 7mm value it will run much better if the range is opened and adjusted to the factory specification If you get your rebuild kits from Miniplane the diaphragm has the tang on it Note that if the metering lever value is too low the engine will not start because it will always flood If too high the engine will go lean at high loads and possibly burn up It is best to error on the side of too low as it won t hurt anything If you have a kit with a tanged button be sure to engage the tang in the metering lever when reassembling the carburetor As Gerry notes below the pop off pressure is one of the two most important factors that changes the air fuel mixture ratio the other value is the size of the main jet If you do change the ML height bend the short end not the long If you bend the long end you may affect the lever s smooth functioning on the pivot Remember that the ML moves up and down at over 9 000X minute Pop off pressure calibration HR Warning high pop off pressures will lean out the engine and possibly burn it up Pop off is the pressure at which the Inlet Valve pops open on demand and allows fuel to travel toward the needle jets It is important to understand the difference between the measured
13. needle buy a 0 79 cent spare butterfly plate just in case Throttle Valve Walbro 34 305 For Walbro distributors in your country or state click HERE When you place the throttle flap back ensure it closes completely before tightening it to its shaft and don t push too hard while tightening it or you will bend the delicate brass shaft Add one small drop of Loctite to its central screw Mount the carb on the engine and make sure the throttle cable permits the complete throttle assembly to open and return all the way against the stop With this done you will now open the Low speed needle 1 turn WG8 WB series Open the Low speed needle 1 turn and open the High speed needle 1 2 turn to start out and back out that mashed idle stop screw to where it s supposed to be about half way out Again check that the complete throttle assembly works correctly and the throttle flap closes completely and freely Also test the choke mechanism for free movement Start your engine and allow it to warm up at low mid RPM High Speed Needle Adjustment If you have a WB series carb next make a high speed run and adjust your High speed needle at high RPM The ideal high speed mixture is the richest setting that still permits strong acceleration up to peak RPM With this in mind it is always wise to start out slightly over rich and slowly lean the mixture out If the high speed mixture is too lean you ll experience weak or lazy mid range acceleration
14. pop off pressure during a bench test and the actual functional pop off pressure of a running engine In a running engine the pop off pressure will happen on demand and it depends on various dynamic factors not present during a simple bench test Factors such as fuel demand and atmospheric pressure determine the actual pop off pressure in a running engine The one thing we must do regarding setting the pop off pressure is setting the correct and critical Metering Lever height Since we are pressure testing the carb while subjected to an abnormal non running situation the measured pop off pressure will not be the same as ina running engine but this test can be used for diagnostic purposes For example if the pop off pressure during the bench test happens at 30 PSI or more it is too high and it is indicative of sticky diaphragm flap valves or a sticky Inlet Valve seat If you wish to actually pressure test a Walbro carburetor prime it as usual and then simply remove the fuel supply line and attach the Walbro pressure tester onto its fuel inlet fitting Walbro s pressure tester part 57 11 57 42 USD Pump air into the system and watch the gage needle drop when the Inlet Needle pops off its seat opens You want a pop off pressure between 14 and 25 PSI To adjust the pop off pressure run a depth gage across the Metering Lever calibrate the gap to 0 75 mm WG carbs and 1 45 mm WB carbs HR Please see the discussion on this a
15. t 98 320 7 OEM 503 28 12 18 WG8 carb The test pressure gage should remain steady at about 10 12 PSI when the Metering needle valve Inlet Valve reseats A leaky carburetor is indicated if the pressure continues to drop According to the Walbro pressure pump manual a continuing pressure drop can indicate Flawed or dirty Inlet Valve seat Worn Inlet Valve rubber tip Internal crack of porosity in body casting or fuel pump body casting Leaking fuel pump gasket or fuel pump diaphragm Important Notes on Carb Re assembly 1 When re assembling you must place the gaskets and diaphragms in this precise order Fuel pump side Pump diaphragm on the carb body followed by the gasket and then the lid Metering side Metering gasket on the carb body followed by the metering membrane and then the lid 2 When re assembling make sure the Metering Lever clips into the Metering Diaphragm or the engine just won t go This does not apply it you are using the metering kit that does not incorporate the lever amp diaphragm clip 3 WB series carbs have an internal Circuit Plate next to the metering system The plate s gasket goes next to the carb body followed by the membrane and last the plate Modification Acknowledgement to Scott Travers for sharing this technique that was part of Mr Travers engine maintenance clinic Once you ve done this basic cleaning and calibration homework you can relax as you ve done the
16. t the engine can stall The huge question is why does Miniplane deliver new engines with the ML distance set to too great a value and thus risk leaning it out The Miniplane service manual clearly states that the distance should be 0 5mm 0 7mm WG 8 s set to this value on the Top 80 run and accelerates better The diaphragm has a total independent travel of about 3 25mm which would mean that the ML would have a range of movement of about 1 2 that value 1 62mm because the distance from the pivot to diaphragm button is about double from the pivot to the ML How much does the ML valve have to be raised in order for it to deliver enough fuel to a fully loaded engine If the ML is set to 1 7mm the ML valve only has an operating range of about 2mm If the ML is set to 0 7mm the ML valve range doubles to 1 0mm Because of the principle of operation of a diaphragm type carburetor if the diaphragm is subjected to a constant pressure the force exerted on the button does not change no matter where the diaphragm is within its fixed range of movement This means that the diaphragm will exert the same force on the ML at the same pressure no matter the ML s relative position to the range of diaphragm movement The tech at Walbro failed to note this As long as the diaphragm and the ML have their full range of movement there is no danger of the fuel mixture being too rich or too lean It is only at the extremes of diaphragm movement that mixture
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