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RC Giant Scale: How To Rebuild a Walbro Carburetor

RC Giant Scale: How To Rebuild a Walbro Carburetor

One of the great features that make giant scale RC airplanes so popular, is the bulletproof user-friendly nature of the Walbro carburetors used with most of the gas engines powering them. Once a gas engine is properly adjusted, its carburetor usually won’t need to be tweaked again for most, if not the entire flying season. As an example, my Hangar 9 quarter-scale Piper J-3 Cub PNP is powered by a Zenoah G-20 gas engine, and in its four years of operation, I have never needed to adjust the  high- or low-end needle-valves. But, to maintain optimum engine operation, you should do some basic engine maintenance and cleaning.

There are a few things that can affect the performance of your Walbro carburetor and these include, dirty un-filtered fuel, debris ingested at the flying field, and possibly water which can cause internal corrosion. No matter what the cause, whenever you notice obvious dirt or mud in your carburetor or if you notice a distinct decline in your engine’s performance, the first thing you should do is open up the carburetor and take a look inside.  This takes a minimum of tools and time and this article shows how I keep my Zenoah engine and Walbro carburetor happy. You can make any RC airplane more reliable by keeping its Walbro carburetor clean.

What you need

 To get inside the Walbro carburetor the required tools are a common head and Phillips head screwdrivers, Allen wrench or hex driver, a mild spray solvent like WD-40, some bamboo BBQ skewers and Q-tips. Don’t use a high pressure air gun to clean the carburetor as this can drive dirt deeper into the fuel passages not to mention scattering a bunch of the smaller rebuild parts and pieces.  A soft cotton rag or towel to cover your work surface is a good idea too.

RC Giant Scale: How To Rebuild a Walbro Carburetor

1.

Start by draining the fuel from your model then remove the engine cowl so you can disconnect the throttle linkage and fuel line. You can do this job with the engine attached to the model or you can remove the engine and work on it that way. It’s up to you. If you remove the engine, you’ll have to disconnect the spark plug lead, the ignition timing sensor lead and the engine attachment bolts. You might as well remove the muffler also so you can give the entire engine a good cleaning before reinstalling it on your model.

RC Giant Scale: How To Rebuild a Walbro Carburetor

2

Here’s the engine removed from the Piper Cub. I prefer to do this job apart from the model to give me plenty of elbow-room to work in. With a very dirty engine, place it on top of a paper plate or a disposable foil tray so you can keep your work are as clean as possible.

3.

Remove the carburetor from the engine. Simply remove the two attachment bolts and the spacers from either side of the intake. Be careful not to damage the gasket. Unless your carburetor has suffered a major, muddy ground strike or it has suffered a severe fuel blockage, all you’ll need to is give the carburetor a good cleaning and possibly rinsing out the main filter screen.

RC Giant Scale: How To Rebuild a Walbro Carburetor

RC Giant Scale: How To Rebuild a Walbro Carburetor

4.

On the inlet side of the carburetor body, the removal of the single center screw gives you access to the fuel pump diaphragm, gasket and the unit’s fuel filter screen. Remove the screw and side cover then inspect the thin diaphragm and the flapper tabs to make sure there is no deterioration. Clean out any obvious debris using the BBQ skewer. Don’t use a sharp tool or a hobby knife as these can damage the surface of the passages.

RC Giant Scale: How To Rebuild a Walbro Carburetor

5.

Should you find that the filter screen is blocked with gunk or has trapped some dirt, clean it out with a quick spray of WD-40. If that doesn’t do the trick, a replacement screen is included in most Walbro rebuild kits. These kits are available from gas engine manufacturers as well as at local small engine shops. This one is from Enforcer at Warehouse Hobbies. Some hot shot pilots have suggested you can discard the filter screen all together then use clean filtered fuel. I don’t recommend this as the increase in fuel flow is minimal.

RC Giant Scale: How To Rebuild a Walbro Carburetor  RC Giant Scale: How To Rebuild a Walbro Carburetor

 6.

In most cases, a good cleaning of the inlet side and possibly the replacement of the gaskets is all that’s required to restore proper engine operation. If however that does not help, you can check the fuel metering section by removing the other side cover that’s held in place by four corner screws. Again, check for any obvious debris and give a light spray of WD-40.

RC Giant Scale: How To Rebuild a Walbro Carburetor   RC Giant Scale: How To Rebuild a Walbro Carburetor

7.

Held in place with a small screw, the inner metering needle, spring and lever arm can wear over an extended period. These parts automatically adjust the fuel flow to maintain proper fuel flow relative to outside atmospheric pressure. If these parts show wear, carefully replace them with the rebuild kit parts. Use care and don’t force anything in place.

RC Giant Scale: How To Rebuild a Walbro Carburetor

8.

Again, it important to limit your cleaning only to soft non-metal tools to avoid damage to the carburetor components. Gasket scrapers, razor blades and any other sharp utensil can seriously harm the relatively-soft aluminum the carburetor body and cover plates are made of. Using them can lead to air leaks which are not a good thing!

RC Giant Scale: How To Rebuild a Walbro Carburetor

9.

The last parts to clean are the high- and low-end needle-valves. Give them a good external cleaning before removing them from the threaded mixture ports. Before removing them completely, gently screw them in (clockwise) and count the number of turns it takes before they bottom out. When replacing them, the high-end needle should be screwed in completely then adjusted out 1 ½ turns out. The low end needle should be close to 1 1/8 turns out. Make sure the needles and threads are clean, give them a light shot of WD-40 and reinstall.

RC Giant Scale: How To Rebuild a Walbro Carburetor

10.

With your carburetor properly cleaned and flushed of any debris, reinstall the components using either the undamaged original parts, or the needed replacement parts from your rebuild kit. When you reattach the carburetor to the engine, replace the intake gasket between the carburetor and the engine. It the gasket leaks air it will cause the engine to run lean. Check the alignment of the carburetor’s pressure holes and make sure they line up properly with the holes in the attachment block. They allow crankcase pressure into the carburetor to activate the fuel pumping diaphragms. If they do not line up, the engine won’t start.

RC Giant Scale: How To Rebuild a Walbro Carburetor

RC Giant Scale: How To Rebuild a Walbro Carburetor

>Conclusion

Being able to remove, inspect, clean and rebuild your Walbro carburetor will ensure a smooth running engine and will save you a few bucks from not having to send your engine in for service. Glitches with Walbro carburetors are far and few in between, but should something come up you’re now ready to fix it yourself. Happy flying!

>Quick Field Tips

  • Always filter your gasoline. Use a filter in your fuel pump plumbing and inspect your fuel container to make sure nothing gets in the fuel.
  • Add a fuel filter to the fuel inlet line for your model.
  • Snug down the carburetor cover screws snuggly but do not use thread locker. This can damage the aluminum threads in the carb body.
  • If you are going to store your model away for an extended period of time, drain the fuel tank and then run the engine to burn up the fuel left in the fuel lines and inside the carburetor. Use a spritz of WD-40 into the air inlet and flip the prop over a several times to coat and protect the inner fuel passages.

RC Giant Scale: How To Rebuild a Walbro Carburetor

Updated: July 16, 2015 — 4:06 PM
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9 Comments

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  1. Hi Gerry, Very good timely article for me, but I am having problems getting info on which side goes up on the gaskets and diapragms etc. Also , how high should the arm for the inlet needle be set? I bought a used engine that was missing all the carb gaskets. Any info would be greatly appreciated. I could not get the link to the video blog to work.
    Thanks.

  2. Con, do a search on google for Youtube videos. There are tons of them out there.

  3. The last time I purchased a rebuild kit from a dealer , I specifically asked for the alcahol tolerant rebuild kit. At that time, older and newer( alchol tolerant ) were available.
    ,

  4. Gerry, thanks a million for this article!! I run strictly gas engine because I fell in love with their reliability, low maintenance, etc., but I know absolutely very little about the Walbro carbs which I have on all of them. Again, thanks a million. This article is very helpful and RIGHT ON TIME!!!

  5. Pay particular attention to the placement of the gaskets when you remove the covers, they MUST go back on the SAME way.
    Example; the gasket on the needle valve metering side goes against the carb base, then the diaphragm, then the cover. The diaphragm center post (longer protrusion) goes against the needle valve lever.
    The pump side diaphragm, with the two flappers goes against the carb base then the gasket, then the cover.
    I keep the cover and gaskets together until just before I put it back together so I don’t forget the order. Do you want to know why I do it that way? Or take photos when you take it apart so you can refer back to where the gaskets go.
    The needle valve lever should be VERY (maybe 1/64th”) slightly above the immediate carb base surrounding the needle valve. It is very sensitive to adjustment (to high= to rich, to low=to lean). Check the lever height of yours before taking it out and make sure it is the same when putting a new needle in.
    There are web sites that can give more details on rebuilding a Walbro carb. Also, a small engine shop should be able to rebuild for you if you don’t feel comfortable doing it yourself.

  6. where to get rebuild kit

  7. From many years of taking care of these engines I would caution everyone NOT to use WD40 on any carb parts as described here. Wd 40 is a very light oil that if not flushed out within a few days by running the engine, will evaporate and leave a guey or solid film on your parts. This will plug the fine hoes in the carb and you will have to but another. WD 40 should also never be used for long term storage of any motor. Use Marvel Mystery oil or Hopps lubricating oil available from gun shops.

  8. Working with the needle, seat, lever, and spring is a bear of a job and having a carburetor parts breakdown illustration is a great help and should be a must have if you really get into a rebuild.

    Bill Doran AMA 572612

  9. When I notice decreased engine performance, the FIRST think I check it the Spark Plug.
    More likely fouled, burnt or cracked. Easier to check and no need to re-adjust anything if it only needs to be replaced.
    Wouldn’t want to pull the engine, carb and tear everything down only to find it still runs crappy after and wonder if you did something to make it worse when it was really just a fouled plug.

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