mranga

My Feedback: (1)

I set up my gas glow engine conversion :

Homelite 25
10/10 glow mixed with gas in 1/3 to 2/3 ratio( as recommended )
Walboro carb
OS F plug
Bennett muffler

Fired it right up and it worked great after just a little tweaking (max RPM was 7500 for a 16x8 prop) until Ithought the idle was too high at 3500 rpm and went to set the idle. Alas! In doing so Imessed up the needle valve settings. Now, Ican get it to idle at under 2000 RPM but it dies when I transition to high speed. After some tweaking Ican get the high speed all the way to 6500 but no higher.

Can somebody please help me with some guidance on how Ican go about tuning this engine back to good health. What proceedure does one follow to set the needles on a walboro with this setup?

Thank you for any guidance in advance


Regards

Ranga

av8tor1977

My Feedback: (6)

Sounds like you got things totally out of whack, which happens sometimes. You need to start over from scratch. Gently close both needles, and then open them 1.5 to 2 turns. Then fire it up, and set the high speed mixture first. Then work on the idle. Then check the high speed once again, and you should be good to go. Make no more than 1/4 turn adjustments at a time, then when you get close, less than 1/8 turn adjustments. It will need to have the idle a touch richer than optimum for a good throttle transition.

AV8TOR

mranga

My Feedback: (1)

I found that Icould not open up the throttle (i.e. it went up to 3/4 throttle) and then it would falter. What setting is out of whack here (low or high needle ?).

Thanks.

av8tor1977

My Feedback: (6)

Well it is hard for me to tell if it is going rich or lean, though I strongly suspect lean on the high speed needle. How many turns out is the high speed needle adjustment? If it is in the four turn or more area, you probably need to raise the lever in the regulator about .015". That will help the carb deliver more fuel, and get you rich enough on the top end. It is fairly common to have to do this on Gas/Glow setups.

I think, and this is just a guess with the limited info I have here, that your low speed was rich, and helping to make the high speed rich enough. When you got the low end dialed in, that leaned out the high speed until it doesn't have enough fuel to run full throttle. Unlike glow engine carbs, the low speed needle setting affects the high speed mixture on these Walbro and Zama carbs.

If you raise the lever in the regulator, you will have to start over with your needle/mixture tuning.

AV8TOR

mranga

My Feedback: (1)

Forgive me for being clueless but what is the "regulator" in this context? A picture or diagram would be most helpful.

I*guessed* it might be the projection that limits the throttle but Iam not sure.

Thanks

av8tor1977

My Feedback: (6)

Here ya go; everything you always wanted to know about a Walbro carb, but were afraid to ask....

http://helifreak.com/showthread.php?t=326599 The "metering lever" is the one you want to raise about .015". Careful, a little movement here makes a pretty big change.

AV8TOR

mranga

My Feedback: (1)

I think I raised the lever too much.  The low end is now so rich that it will not idle except when I keep the throttle open beyond what the set screw will allow me to do. However, it transitions and the high end is around 7200 RPM (steady).  I guess I need to push the lever down a bit.

The engine is inverted.  I notice that it runs better with the glow plug igniter ON at idle. Are there issues (similar to glow) when running the engine inverted or is this because of my tuning being off ? Does gas glow need an on board glow plug igniter when it is mounted inverted?

Thank you for your help.

Ranga

av8tor1977

My Feedback: (6)

I have no problems running mine inverted. Your problem is probably because the idle is too rich now. Onboard glow should not be needed.

When an engine is run cylinder down and too rich, the fuel accumulates in the glow plug/spark plug area and creates running problems.

Some of mine increase in rpm a bit at idle when glow power is applied, but they idle smooth and low without glow power nonetheless.

AV8TOR

mranga

My Feedback: (1)

I fixed it as follows:

- Replaced carb pump membrane and gasket.
- Raise metering lever to be level with the top surface of the carb. Place a straight edge across carb and raise it so it just touches.
- Tune by setting low end first so it idles at around 2500 RPM and then work on transition to high end by opening up high needle and low end needle. Then come back and adjust low end needle again. I could achieve around 7200 RPM reliably. The low-high transition issue was resolved by opening up the high end needle.

So now it looks good. I still have more testing to do but so far so good.

Question: Do I need a "velocity stack" for this?

Thanks in advance for your answer

Ranga

av8tor1977

My Feedback: (6)

A velocity stack is almost always useful if you have room for one. It helps contain and draw back into the carb the reversion, or "spit back" a two stroke engine has.

If you fit one to your already tuned engine, you will notice it runs rich and will need tuned again.

AV8TOR

mranga

My Feedback: (1)

Thanks for replying.

Is there a need to solder a tube to the vent hole on top of the carb fuel pump so as to reduce the effect of pressure variations in the cowl area as shown here :

http://tech.flygsw.org/walbro_carbvent.htm


I am not sure how exactly this helps (given that the inside of the plane is essentially at the same pressure as the outside of the plane).

av8tor1977

My Feedback: (6)

Well, if yours is a cowled installation, you have two choices. One, fly it and see what happens. Sometimes it is not an issue. Two, go ahead and install the vent as a precaution. I usually just go ahead and install a remote vent myself.

What happens, is that, depending on the installation, higher pressure develops in the cowl in flight. This gives the carb a false sensing of the ambient pressure, and the engine goes rich. So if the engine runs great on the ground, but goes rich on you in flight, that is most likely the problem.

The line is generally run inside the fuselage, where hopefully there is no ram air pressure effect happening. The same thing could be accomplished by running a static pressure line outside the cowl. As long as the carb isn't sensing a false high pressure due to pressure build up in the cowl, you're good to go....

AV8TOR