RCU Forums - View Single Post - DLE 20cc Gasser! (Data & Links 1st Post)

08-29-2011 | 07:27 AM

#4435

DaleD

Joined: Jul 2009

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From: North Lauderdale, FL

RE: NEW DLE 20cc Gasser! (Data & Links 1st Post)

Quote:

ORIGINAL: microdon2

will185 \ SpinnerRow - I've noticed the same thing with this DLE 20 - I can't seem to kill it with the throttle control alone. And I've also confirmed that the throttle plate is seating correctly. Must be some other air\fuel leak in there? Does anyone know if this carb is designed this way? Seems odd. But, as Spinnerrow states, I use an Opti-kill swtch or a servo-controlled choke to kill it (depending on the plane). Haven't tried to kill my DLE 30 with throttle control yet - will have to test that next time out.

will 185/SpinnerRow/microdon2 -Have a look at the literature below, especially the bottom bit which is highlighted. Hope this helps.

Informatipn found in a post on the DA85 thread right here on RCU.

DaleD

Walbro Carb TUNE UP & Illustrated Guide

Walbro carbs aren't too difficult to tune up if you know what you're doing. First of all, you need to know how the carb works and how the settings interact with each other. About 95% of all the gas airplanes I've seen at the field are somewhat out 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 miss making it sound like a four stroke. This is not good. HOWEVER the good news is; gasoline two stroke engines are very very tollerant of rich settings (most of the time) and will run fine. You'll just consume a little more gasoline than necessary, and create a little more oil mess on your plane. You may eventually foul your 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 little or no warm up time needed prior to flying. Those are pretty good reasons! But the fact is... 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 float diaphragm, a highend and lowend needle set, and some have chokes, some don't. There are so many variations of Walbro carbs, it's ridiculous. I will restrict this editorial to the more 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 the running. Then you also need to know how to make the adjustments. You also need to know what 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 what part of the carb is being used at that time. There are a lot of functions going on within the carb and one or more functions can make the carb act strange. What could sound like a rich 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 you adjust one needle, you will likely need to adjust the other.
Let's begin with looking at the components of the carb:

Typical PROBLEMS POSSIBLE solutions
The engine stalls when accellerated Highend needle way too lean, or lowend needle slightly lean
Four strokes as fast idle, mid RPM Lowend needle too rich, float diaphragm needle lever slightly too high
Engine goes lean in flight Highend needle slightly lean AND lowend needle is rich, float needle lever may be set too low
Engine goes rich in flight this is a special problem with cowled in engines. the float diaphragm cover vent will need work (SEE NOTE 1)
Engine runs good, but no idle at all There's crap in the idle jets, the carb will have to be removed and cleaned. You may also have an air leak at the base of the carb. The throttle butterfly could be damaged or worn out.
carb leaks fuel when not running float needle is bad or has crap stuck in it, or the float lever setting may be too high, or the float diaphragm is bad.
My engine four cycles momentarily when I back off the throttle, then runs normal This is perfectly normal for carbs NOT equipped with a "check valve" highspeed jet. If you do have the check valve, then your float needle setting is 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 running Bad fuel pump membrane, or an air leak in the fuel line at the carb

NOTE 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 in flight. There are several possible fixes available. Most of the time you can simply tune your engine for flight by trial & error. However, the easiest fix is to open up the cowling around the carb area to lower the air pressure. You may also rotate the cover to different positions to see if that works. The "BEST" fix is to solder a piece of brass tubing where the vent is, and route the vent line to a better location. I normally route it into thefusegoing through the firewall. It works perfectly every time! Plus, your ground tune doesn't change in flight!

NOTE 2
Carbs 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 jet dripping when you back off the throttle. That's all it does...
Straight through (non-check valved) jets always drip a little fuel while the throttle is being backed off, and causes a momentary four stroking of the engine until the jet 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 as you keep the nitro content low, less than 10%. If it doesn't tune properly, some modifications will be necessary. The fuel flow rate within the carb will have to be increased The float needle & seat needs work first. Bore out the seat 25% larger than its existing size and raise the float lever up about .040 inch. Try the carb out. If it still doesn't take a steady tune, then bore out the lowend & highend needle orifaces 25% larger than its original size. This should do the job. The flow-rate of methanol is much higher than gasoline.

* * * SPECIAL NOTES * * *
(1) I would like to note at this time, that a properly tuned bottom-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 or vent hole in the butterfly. You can solder the vent closed if you want carb kill capability.
(2) The diaphragm (needle & seat) setting is critical. If you are not familiar with this setting, leave it alone!
(3) I would also like to cover two other possibilities that would make your carb "APPEAR" to be out of tune. One is carb size. If the carb is too big for the prop/engine combination, it will not tune up properly and be erratic and un-reliable. The other is engine timing! IF your timing is too low it will make the carb appear out of tune! This is often over-looked and will cause you to tinker with the carb and you'll never get it running right. I've found that 30 to 32 degrees works best for bigger carbs and or bigger props.
Copyright © 2007 M. B. Fuess

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