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.
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.
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.
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.
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.
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.
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.
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.
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!
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.
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.
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.