How To Clean & Rebuild A Walbro Carburetor

After sitting for a few months, gas engine carburetors might need a cleaning to get them in flying condition.  Here’s a good article from Model Airplane News that will show you how.

http://www.modelairplanenews.com/blog/2010/07/27/how-to-clean-and-rebuild-your-walbro-carbureto

How To Clean & Rebuild your Walbro Carbureto

By Gerry Yarrish

One of the great features that make giant scale RC airplanes so popular, is the bulletproof user-friendly nature of the Walbro carburetors use with most of the gas engines powering them. Once a gas engine is adjusted, it’s carburetor won’t need to be tweaked further 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.

1.

Start by draining the fuel from you 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.

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.

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.

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.

  

 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.

   

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.

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!

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.

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.

>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.

Gerry YAbout the author

The Future of R/C Flying: Robot Pliots

Could this be the first test pilot, controlled via transmitter? The KT-X SUPERBOT

Take a 30% with a 50cc, build a cockpit with full flight controls; aileron / elevator gimbal for the right hand, throttle stick for the left, and rudder pedals for the feet.

KT-X SUPERBOT SPECS

HEIGHT: 13.5 in.

WIDTH: 6 in.

DEPTH: 2.5 in.

WEIGHT: ~ 3lb.

PRICE: $1,395

http://find.botmag.com/print/385

Some pilots can be a bit of a showoff:

Ashlok Connectors

Wings can be a pain installing when you have lots of servos to connect.  Besides having a rats nest of wires stuffed into the fuse, it’s important to be sure you have the connections correct.  Adding to the pain is finding out after you installed the wing you missed plugging in a servo.

The Ashlok connector solves this problem by combining all those servo connections into a single connection.  Ashlok has connectors for up to four servos.  If your wing has more than four servos, you group them using a “Y” harness.

I found a couple of sources for these connectors.  Thunderbolt R/C in Canada has the best information about the connectors and a source in the U.S. is Shulman Aviation.

Spektrum DSMX

http://www.youtube.com/watch?v=QaoXipp_288&feature=player_embedded

http://www.spektrumrc.com/DSMX/

The DSMX take away is “Frequency Agile”.

DSM2 uses two frequencies across the spectrum to hop on. DSMX uses 23 frequencies to hop on and uses a random frequency selection method.

I’d say that this approach is similar to the Futaba FASST system.  One difference being, if I’m not mistaken, is FASST hops on frequencies from one end of the spectrum to the other. DSMX randomly hops frequencies across the spectrum.

This reduces the chance of frequency collision with other 2.4 transmitters.  Not a big deal at a local club but becomes an issue with hundreds of pilots at large events.

DSMX has been in the field for over a year.  It’s claimed to have been thoroughly tested.

DSM2 transmitters can use new DSMX receivers and new DSMX transmitters can use DSM2 receivers.

DSM2 transmitters can be upgraded to DSMX.  The DX8 transmitter can be upgraded through the internet.  Other Spektrum and JR DSM2 transmitters can be sent to Horizon and for a fee upgraded to DSMX.

How to Tune a Gas R/C Engine with a Walbro Carburetor

Found this information on tuning a gas engine with a Walbro carb very helpful:

It is important to keep in mind that the low (L) speed needle is always active, and that the high (H) speed needle is only active above 1/4 throttle when there is sufficient low pressure in the venturi for the high speed needle to start drawing fuel.

Here is the drill to start from scratch:

Low Speed Idle mixture
Open both needles L~1.5 to 2.0 turns, H ~2 turns. This will ensure a rich setting.
Close the choke valve and switch on ignition.
Flip prop until the engine pops or runs for a short time.
Open the choke valve.
Using a slightly high idle throttle setting, start the engine and let it warm up a bit.
Adjust the low speed idle needle (closest to the engine) for best rpm and then open it 1/4 to 1/2 turn. This will be a safe rich setting to start tuning, and from here the high speed needle can be set.

High Speed mixture:
With the idle needle set about right, slowly apply full throttle. If the engine stays too rich, close the high needle a bit so the rich condition is cured, but no more than that.
This is the basic high speed needle setting, at which the engine will run without damage due to lean mixtures.
Keep this setting while running in the engine.
Let the engine run for about five minutes, then the spark plug can be checked. It should have a desert sand tan, or slightly darker. Not black, because that is an indication of an overly rich mixture, nor pale-white, because then the engine is too lean. This causes damage!

The next step is to get the mid-range just right. This can only be done after the engine is run in. If done before that, settings will change, and the process has to be repeated.

So far, you have been running the engine with a rich idle, and quite rich main needle settings. This causes four-stroking in the mid-range.
Let the engine warm up, and apply full throttle, until the rpm has stabilized.
Now slowly reduce throttle, until the engine starts to four stroke. Cure that condition by leaning the idle needle.
Go back to full throttle, and adjust for max rpm, then open the needle 1/16 turn extra, and throttle back again. This time, the four stroking transition will be at a lower throttle setting.
Repeat the above steps, until the engine runs well at all, but slightly high idle.
check for crisp throttle response. If the engine lags, then open the idle needle until the condition is cured.
Let the engine idle for a prolonged time, and apply throttle. All should be well now, and throttle should be accepted quickly and clean, with maybe a very slight initial four-stroking to clean the lungs.

It is a fact of life, that two stroke engines do not fire every stroke when the rpm approach idle speeds. That means, that below half throttle, the engine may break into an uneven pace, so do not worry too much if the above mentioned method does not provide clean running all the way down to idle.
If after these adjustments the engine becomes harder to start, the idle needle is too lean, and if easy starting is needed, the needle should be opened up again and the high needle readjusted (leaned). Clean mid-range will suffer though. Sometime you can, but most of the time you can’t have the cake and eat it.

To recap:
Because at full throttle both needles contribute to the mixture, you can run the engine with lean idle and richer main needle for good mid-range, but starting will suffer.
You can also run the engine with rich idle, and leaned out main needle, but mid-range will suffer and burble.
A larger prop will run cleaner at mid-range.
Too large a prop will make a good tuning very hard to do, and it will be almost impossible to get full throttle just right.

Aeroworks Extra 260 DLE 20cc Gas Engine Conversion

Stopped by the Aeroworks booth during the recent AMA Expo and saw the .60 .90 Extra 300 with the DLE20cc conversion.  I picked up the Extra 260 conversion kit and ordered the engine, fuel tank, fuel line installation kit and motor mount.

As soon as it all arrives I’ll start the conversion and add it to the original build log.

Voltage Isolator

Had a conversation at the field about the JR 791 High Torque Servo.

http://www.horizonhobby.com/Products/Default.aspx?ProdID=JRPS791#quickSpecs.

Turns out that the three of us had burned these servos up because we weren’t aware that they can only handle 4.8v. Now, you’d think that a more expensive higher torque servo would handle the same voltage as the less expensive lower torque version http://www.horizonhobby.com/Products/Default.aspx?ProdID=JSP20080#quickSpecs
but that’s not the case.

So if you have this servo or any other 4.8v servo and need to use it in your 6 volt application you’re going to burn it out unless you use a device I just happened to come across.

It’s a Voltage Isolator from Dionysus Design and cost $17.99.

http://www.dionysusdesign.com/product_info.php/cPath/44/products_id/171

The Voltage Isolator is designed to allow your model to use higher voltages on certain servos and other electronics that should not be powered by 6 volts. The Voltage Isolator provides a barrier between the 4.8v electronics and the 6v electronics.

Can also be used with BEC’s for electric planes.