Gas engine tuning for newbies....

[PTS]

TTT! Lots of good info!

[electricpaps]

	Quote: Originally Posted by mfuess
	Thanks Kent. Nothing sounds better than a well tuned engine.

Hi everyone.
I’ve read just about all of this thread and have learned a great deal. Many thanks to all contributors.
I’m a newbie to gassers but have been messing around with model aircraft, on and off, since 5 years old (so that’s almost 50 years). I’ve pretty good knowledge of model diesel and glow engines and for the past couple of years I’ve been building electric powered aircraft.

Then I decided to build a glider tug. A 2.5m wingspan pushed me to purchase my first gas engine. Mark will be hopping mad I’m afraid : I got it from United Hobbies, a FTL 45cc with CDI electronic ignition. Nicely finished engine for only $200.
The apparent weak points are minor, the worst as far as I can see could be the Teflon adapter between the carb and the engine ( may leak a little under small deformations, I don’t know).

I’m getting to the point and my question. In all of the thread here just about everything has come onto the table except the problem I’m experiencing : How do I get a steady idle at around 1500 rpm ?

I can olny get the engine to idle at an acceptably low rpm if I enrich the low end excessively (3 or 4 turns out from closed). Then the engine runs very rich in flight from mid rpm down. When I lean out the low end, rpm increases and then it dies, before rpm drops back down to an acceptable idle.

Here is some basic info about my setup.
- engine mounted inverted
- Carb : walbro WT 717
- carb sticking out of cowl with static pressure vent forwards in airflow from prop.
- Bottom half of cowl cut away (excellent airflow around cylinder head
- timing advance (measured using a graduated disc and plug removed) exactly 30°
- runs fine at top end, 6400rpm with wooden 22 x 10” prop. I haven’t tuned it to MAX rpm yet with rev counter due to “pressure” from guys at the field. From what Mark says, I should.
- I plan to solder a brass tube to the static vent and lead back to the fuselage, maybe this will help.
- I lifted the metering lever about 1mm in an attempt to help, but it just made the engine flood during starting, so I’ll bend it back to the original position (which I carefully noted beforehand).
- Fuel : run in on about 5 litres of mineral oil at 3% with 20x10” wood prop. Now using 3% synthetic. Sure is sticky stuff this synthetic oil!
- Once the tug has flown, the idle is sometimes faster. Even with the butterfly valve completely closed the engine sometimes idles (fast). Imagine the landings with a friend running behind to catch it by the tail!!

So I still have the following things to try:

- Solder brass tube to static vent
- Tune rigorously according to Mark’s method, but normally the high end needle doesn’t affect idle does it?
- Bend metering lever back down.

I like these gassers, they smell good and sound good. I’m pretty impatient to get my FTL 45 tuned properly.
Well that’s it for now, any suggestions or comments will be greatly appreciated.

Happy flying,
Paul

[mfuess]

It sounds as though the engine is not broken-in yet...

[electricpaps]

Hi Mark, I guess you're right.

Anyway, I've ordered the GF 45i carb adaptor (steel, so should'nt deform and leak). After posting previously I thought about the effects of an air leak and as far as I can work out that would increase the minimum "lean" idle speed obtainable. Would you go along with that?

[mfuess]

It certainly could.

[lennyk]

I used this method to tune my ZDZ80 today. The engine is about 4 years old, never really did much fine tuning on it apart from high needle peaking.

Anyways I recently pulled it out of storage and overhauled it, cleaned out the cylinder and ring etc.

I peaked the high first as stated and then brought down the throttle and leaned the idle/low to get rid of the 4 stroking/burbling. This engine always had the mid range burbling which I lived with.
However after getting rid of almost all the mid to low burbling, I checked the low needle and it was about 1/3 turn! whilst is used to be more like 1 1/4.

Anyone else with ZDZ80(Walbro WJ38 carb) have similar needle setting ?
Is safe to leave it so ?

[mfuess]

	Quote: Originally Posted by lennyk
	I used this method to tune my ZDZ80 today. The engine is about 4 years old, never really did much fine tuning on it apart from high needle peaking. Anyways I recently pulled it out of storage and overhauled it, cleaned out the cylinder and ring etc. I peaked the high first as stated and then brought down the throttle and leaned the idle/low to get rid of the 4 stroking/burbling. This engine always had the mid range burbling which I lived with. However after getting rid of almost all the mid to low burbling, I checked the low needle and it was about 1/3 turn! whilst is used to be more like 1 1/4. Anyone else with ZDZ80(Walbro WJ38 carb) have similar needle setting ? Is safe to leave it so ?

If your engine is running properly, leave it alone. Otherwise, open up your lowend.

[Billa]

Ok here is a new one for the DA 50 guys and the master ofcourse. Engine has been tuned using the described method and works flawlessly when it runs. Sometimes, after the choke is used with ignition on and it fires, it floods up with the choke turned off. The procedure of opening up the plug to check and clean has to be repeated a number of times before the engine comes to life and then it is fine. Is this a diaphragm issue ? Haven't come across this before.

TIA,
Aamer

[electricpaps]

	Quote: Originally Posted by Billa
	Ok here is a new one for the DA 50 guys and the master ofcourse. Engine has been tuned using the described method and works flawlessly when it runs. Sometimes, after the choke is used with ignition on and it fires, it floods up with the choke turned off. The procedure of opening up the plug to check and clean has to be repeated a number of times before the engine comes to life and then it is fine. Is this a diaphragm issue ? Haven't come across this before. TIA, Aamer

Could it be the metering needle letting too much gas into the chamber. I had a similar problem yesterday (plus a bad idle) and I fixed it by lewering the metering lever which results in the needle being shut more often so a leaner fuel supply.

[BorderLine]

Hi Guys...Good info.. On my DA 100 I have good transition from Idle to full power, but when I taxi out to take off,throttle up. It seems to be loading up and not giving me fullpower. Once it gets flying its OK. When I finish my flight and land my Idle has also increased. The bottom of my plane has quite a bit of burnt oil on it, after a day of flying. What can I do???

JR

[Dangerous Dan]

	Quote: Originally Posted by Jerry Ruscheinski
	Hi Guys...Good info.. On my DA 100 I have good transition from Idle to full power, but when I taxi out to take off,throttle up. It seems to be loading up and not giving me fullpower. Once it gets flying its OK. When I finish my flight and land my Idle has also increased. The bottom of my plane has quite a bit of burnt oil on it, after a day of flying. What can I do??? JR

I wouldn't mess with it, you are slightly rich when you take off it loads up then clears its throat(leans) in the air. Make a throttle to throttle mix and put it on a slider, use this to give you a landing and flying idle.

[mfuess]

Having a flying & landing idle works good, but I prefer a toggle instead of a slider.