There are many possible causes. Float height is maintained by a little, adjustable tab on the float arm. It doesn't take much for that tab to be moved, which immediately changes the level of fuel in the float bowl. That tab can be relocated when disassembling the carb, or with rough handling of the float or float arm.

Walbro carbs aren't too difficult to tune up if you know what you'redoing. First of all, you need to know how the carb works and how the settings interactwith each other. About 95% of all the gas airplanes I've seen at the field are somewhatout of tune. How can I tell this?

Simple, at some point the engines 'four cycle'in flight. Two Cycle engines are not supposed to 'four cycle' PERIOD.This is caused by a rich mixture that is forcing the sparkplug to intermittantly missmaking it sound like a four stroke. This is not good. HOWEVER the good news is;gasoline two stroke engines are very very tollerant of richsettings (most of the time) and will run fine. You'll just consume a little more gasolinethan necessary, and create a little more oil mess on your plane. You may eventually foulyour spark plug as well. So why do so many people leave their engines tuned like this?Simple answer, the engine will start much easier when it's cold AND there's littleor no warm up time needed prior to flying.

Those are pretty good reasons! But the factis.

The engine is not running like it's supposed to.LET'S BEGIN:All Walbro carbs have their own fuel pump, a needle & seat controlled by a floatdiaphragm, a highend and lowend needle set, and some have chokes, some don't. There are somany variations of Walbro carbs, it's ridiculous. I will restrict this editorial to themore common carbs found on airplane gasoline engines. This will include the WA, WB, WG,WT, WL, WS, WTL and a few others. While there's a substancial difference in these carbs,they share a common tune-up procedure.FIRST and foremost, you need to know how the carb works and how the settings affect therunning. Then you also need to know how to make the adjustments. You also need to knowwhat is a correct tune, and what isn't.

After that, it's free sailing!Before you make any adjustments, you need to consider what's really going on and whatpart of the carb is being used at that time. There are a lot of functions going on withinthe carb and one or more functions can make the carb act strange. What could sound like arich lowend needle setting could actually, and easily be a float setting on the needle& seat! A stiff diaphragm will make it run rich too. It could also make it run lean.The highend and lowend needles interact throughout the entire throttle range. So if youadjust one needle, you will likely need to adjust the other.Let's begin with looking at the components of the carb:Startingfrom the gas tank, the fuel is pumped to the carb through the inlet. The fuel works itsway through the fuel pump through a little diaphragm pump which is controlled by 2one way valves (little flaps).

The fuel then passes through a needle & seat that iscontrolled by the 'float' diaphragm. The 'float' diaphragm manages howmuch fuel is available for the idle, low-speed, and high-speed throttle positions, byopening & closing a passage utilizing a small lever attached to the needle. The fuelis then 'standing by' in the float cavity area waiting for a vacuum signal atthe various jets. The lever setting is very critical since it controls the available fuelto the jets. If the lever is too low, the engine will run lean, if the lever is too high,the engine will run very rich and will likely flood out atidle.The fuel starts its journey through the pump assembly first.Then the fuel is regulated by the float diaphragm that controls theneedle & seat.All of these parts reside in the float cavity area as well as the fuelready to be fed through the jets as needed. The amout of fuel available in the cavity isregulated by the lever and its relationship to the float diaphragm.

So it's critical thatthe lever be set properly.Within the cavity, there are distribution holes that are managedby the lowend and highend needles. Plus the idle circuit, which is a fixed size.SPECIAL NOTES:(1) The lowend needle is ALWAYS the one closest to the engine, the highendneedle is the closest one to the intake/choke.(2) Both lowend AND highend needles feed the topend fuel supply.Let's tune up a Walbro!Set the lowend & highend needles to about 1 3/4 to 2 turns each.

Chokethe carb or prime it, until the carb is wet. Fire up the engine and let it warm up.

Let'sset the topend first since it's the easier of the two. Go to full throttle. Adjust thetopend needle for peak RPM. Leave it wide open for about a minute to see if it changesany.

Should the engine go lean, open the lowend needle slightly, if this dosen't work.you will have to adjust the needle valve inside the carb.( I will explain this later) Ifthe topend runs OK, then slowly pull the throttle down until the engine begins to'four cycle' hold the throttle there. Adjust the lowend needle until the'four cycling' barely stops.

Now lower the throttle more until it 'fourcycles' again, and adjust the lowend again. Keep doing this until you reach fullidle. Now, from full idle begin to throttle up until the engine starts to bog or hesitate.Open up the lowend needle just enough to eliminate the bog or hesitation.When this is done right, you will be able to set the throttle in anyposition and it won't four cycle, plus you will be able to transition from idle to fullpower without any hesitation at all. Now, for easy starting it's best to have the lowend alittle rich and it will four-stroke a little.TYPICAL PROBLEMS encountered in Walbro carbs:The float diaphragm will go bad occasionally and reap havoc on tuning,especially the lowend and idle.The internal needle valve WILL leak when they get old or worn out. If younotice gas dripping from your carb, or if the idle gets unreliable, replace the needlevalve and adjust the lever even with (or slightly below) the carb housing if you don'thave a gauge.The fuel pump membrane gets stretched and/or sometimes hardens from thegas.

It needs to be replaced occasionally. If your pump doesn't pump properly, (1) it willbe hard to start (2) it will tend to run lean, and opening the needles won't helpmuch if any.Erratic idle, or no idle is often traced to a bad internal needle valve,bad float diaphragm, bad pump, and most common. Crap in the carb. You may also have anair leak!Things to know.There are three types of fuel pump membranes available.

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The black one isthe rubberized fiber, the Beige one is fiberglass based, and the blue one is Acetate. Theyall work well, but the black one moves more gas than the others but it wears out thefastest. The Beige one works best if you use methanol fuel and is reasonably durable. TheBlue Acetate is the most durable but pumps the least fuel.All Walbro carbs will run in any position, but they tune 'best'as a sidedraft carb.

The down draft position tends to run a little rich at idle, and theupdraft tends to run a little lean at idle. No big deal though, it's easily tunednone the less.This photo depicts one of Walbro's premier carbs, having a large bore andequipped with a highspeed check valve and external fuel pump pulse inlet.This carb is typically found on 50cc to 65cc engines.Now let's talk about the pulse signal for the fuel pump.Your ENGINE will determine which pulse inlet type you need! If the engine'carb base' has a hole drilled into the crankcase you will use the STANDARDpulse port and the optional (if you have it) port must be closed off.If there's no hole drilled, you will find a fitting located somewhere on your crankcase.Use a piece of fuel line to connect the crankcase fitting to the fitting on the optionalpulse inlet.

There's no need to block off the standard port, as it's already blocked offby the engine mounting.The carb must get a pulse signal from the engine! Thissignal 'pushes and pulls' on the pump diaphragm which feeds the carb fuel.Now let's check out the 'float needle & seat' setting.This is the single most critical setting on a Walbro carb! Walbro offers a'setting gauge' to properly set the height of the lever for your particularcarb. If you don't have one, the setting will be a trial & error adjustment and a realpain in the butt since you have to open up the carb to make the adjustment. For generalpurposes, the lever will be almost perfectly parallel to the carb base. This will get youclose.

If the lever is too high, your engine will tend to run a little erratic at idle. Ifthe lever is too low, your idle will be OK but it will tend to run lean on midrange andhighend.

It may also run the float cavity 'dry' at full throttle and die,regardless of your highspeed /lowspeed needle settings.The needle valve seat is pressed into the carb base, and you should notremove this without having the correct tools and setting gauges. Do not remove it!TypicalPROBLEMSPOSSIBLE solutions. The engine stalls when accelleratedHighend needle way too lean, or lowend needle slightly leanFour strokes as fast idle, mid RPMLowend needle too rich, float diaphragm needle lever slightly too highEngine goes lean in flightHighend needle slightly lean AND lowend needle is rich, floatneedle lever may be set too lowEngine goes rich in flightthis is a special problem with cowled in engines. The float diaphragmcover vent will need work (SEE NOTE 1)Engine runs good, but no idle at allThere's crap in the idle jets, the carb will have to be removed andcleaned. You may also have an air leak at the base of the carb. The throttle butterflycould be damaged or worn out.carb leaks fuel when not runningfloat needle is bad or has crap stuck in it, or the float lever settingmay be too high, or the float diaphragm is bad.My engine four cycles momentarily when I back off the throttle, then runsnormalThis is perfectly normal for carbs NOT equipped with a 'checkvalve' highspeed jet. If you do have the check valve, then your float needle settingis slightly too high, or your float needle is leaking a little.(SEE NOTE 2)the fuel leaks back into the gas tank when it isn't runningBad fuel pump membrane, or an air leak in the fuel line at the carbNOTE 1:A very common problem with cowled in engines is, the air pressure in flight changes the'natural' pressure on the float diaphragm.

This causes the engine to run rich inflight. There are several possible fixes available.

Most of the time you can simply tuneyour engine for flight by trial & error. However, the easiest fix is to open up thecowling around the carb area to lower the air pressure. You may also rotate the cover todifferent positions to see if that works. The 'BEST' fix is to solder a piece ofbrass tubing where the vent is, and route the vent line to a better location. I normallyroute it into the fuse going through the firewall.

It works perfectly every time! Plus,your ground tune doesn't change in flight!NOTE 2Carbs equipped with the highspeed check valve are greatly superior for flying aerobatics,or flying whereby the throttle will be used extensively. The check valve prevents jetdripping when you back off the throttle. That's all it does.Straight through (non-check valved) jets always drip a little fuel while thethrottle is being backed off, and causes a momentary four stroking of the engine until thejet stabilizes to the new air flow rate. This is completly normal.Converting a Walbro carb from gasoline to methanol:Most Walbro carbs will tollerate methanol without modification as long asyou keep the nitro content low, less than 10%. If it doesn't tune properly, somemodifications will be necessary. The fuel flow rate within the carb will have to beincreased The float needle & seat needs work first.

Bore out the seat 25% largerthan its existing size and raise the float lever up about.040 inch. Try the carb out.

Ifit still doesn't take a steady tune, then bore out the lowend & highend needleorifaces 25% larger than its original size. This should do the job. The flow-rate ofmethanol is much higher than gasoline. SPECIAL NOTES.(1) I would like to note at this time, that a properly tunedbottom-end will not 'kill' the engine when the carb is pulled to 100% closed.This is typical for Walbro's as most (but not all) Walbro's have an air-bleed notch orvent hole in the butterfly. You can solder the vent closed if you want carb killcapability.(2) The diaphragm (needle & seat) setting is critical. If you are notfamiliar with this setting, leave it alone!(3) I would also like to cover two other possibilities that would makeyour carb 'APPEAR' to be out of tune.

One is carb size. If the carb is too bigfor the prop/engine combination, it will not tune up properlyand be erratic and un-reliable. The other is engine timing! IF your timing is too low itwill make the carb appear out of tune! This is often over-looked and will cause you totinker with the carb and you'll never get it running right. I've found that 30 to 32degrees works best for bigger carbs and or bigger props.Copyright © 2007 M.

How to:Setup and adjust correct and accurate DHLA40, DHLA45, DHLA48 C,D,E,F,G,H,L,M,P,R,S,T emission and non emission fuel levels and float levels.First we need to understand there are 3 types of float.They all look similar! They all need to be catered for in their own setting.Below shows the EARLY 10gram float, used in some, early types of DHLA40, DHLA40E, DHLA40F (early), also some early DHLA45 and DHLA48Below shows the 7 and 8.5gram floats which became the 'norm' for all carbs after about 1972.These 7 and 8.5gram are similar in setting, but the 10gram needs a different setting.I prefer the heavy EARLY 10gram float for some reason, more constant fuel level and running in my opinion.

I always fit these to my carbs set at 17.5mm initial setting or 18mm using SUPERIOR Viton Tipped needle valves.For a given fluid level, a lighter float will naturally ride higher.Either the weight or the float part number should be engraved on the float. 3 types the 7g, 8.5g and 10.0g floats.The basic part number for the floats is 7298- with two final digits for each specific weight:01 = 10g 16.5 - 17.0 mm (float height start setting)02 = 8.5g 15.5 - 16.0 mm03 = 7.0g 14.5 - 15.0 mmAbove and below images show the needle valve has a spring inside.

ALL static adjustments to float heights MUST be done with the lever tab on the float touching the rounded tip on the needle valve shaft, but NOT compressing the spring at all when the measurement is taken.Below shows the distance A, which depending on the float used 7,8.5 or 10gram and will be initially set to the figures quoted above. Fit the new valves and assemble the float and cover (with gasket!).Set your calipers to the figure quote and to work with your float weight, check they are accurate with a ruler or micrometer and proceed to tweak the float, if needed, this is to obtain the correct figure at the floats furthest extremity (A) from the needle valve.As shown below in other images, to adjust the float level tweak the floats.As seen below. It helps to hold the carb top vertically or just off vertical unless you are an octopus. This is so the float tab rests naturally against the needle valves rounded tip.This MUST be done with the top plate gasket in place.Most people just finish here and call that setup. This is just stupid, as it often does not translate into a good fuel level in the carb.

Depending on if metal or viton tipped needle valved are used and also the valve body can be No1 or No2, floats may have been 'butchered' by previous 'engineers'.So this is where my technique starts. Hi.6 tube is for non-emission.You have H model from 1800-2000cc, they should have 58idle, 11 tube, 148main?In that case you just need some 60idle jets.If the carbs are jetted;32choke.148main11 tube210 air corrector38-40pumpSimply obtain some 60 idle jets and some 190 air correctors and you should find the carbs ok.Don't change the idle jet holder to a 7850.? Stay with the unmarked holder.Don't use 6 tubes!You might find no tuning needed at all, mild cam, 1700. It's ok to use them as they are, I suggest some 190 air correctors so you can bring the main-jet down a little if there is any lean miss crusing in the mid range.That easy.They come from 1800cc 8V with good exhaust and mild-yet ok cams. They should be fine, there was an airbox on the Alfa so a larger depression on the jets, so thats why you need a larger idle and smaller air corrector. Maybe get some 144 main jets too.

Hi Dave, thanks for your reply. The were sent to me jetted:Choke 32mmAux Ven.3Emul tubes.11idle 60pump jet 40idle holder without numberneedle vlve 150Air 200main jet 148With this set up it was working fine at low revs but at high revs it was not developing all the power. It will rev 6000, when before with other carbs it revs 8500!At the moment I upgraded to 34mm chokes and came worst than it was as now with 3000 revs, will not take any more gas, like pulling back and some sneezing.ThanksChris.