Hi!

I have a "low wing trainer" with a Jen .37 mounted. I have tuned the engine to run nicely on full throttle and to respond quickly to full throttle at idle.

But this setting doesnt allow me to idle for very long. After about 10-20s on a comfortable idle speed the engine dies. Is this something that i halfto learn to live with? Do i halfto choose between low idle rpm and good throttle respons?

//Alexander

No, you shouldn't have to "learn to live" with the engine dying at idle, nor is that a good thing. Have you flown the plane yet? If not, you'll want the engine to be able to run for extended period of times as you need to chop the throttle for landing approach. If it keeps dying, then you'll be doing a lot of deadsticking. If you can't get the engine to run at low idle with the throttle down, for a quick fix, add some trim to the throttle channel until the engine does run smoothly at idle. However, you do want the engine to idle smoothly but also at a low rpm for landing. If the engine doesn't run smoothly unless the rpms are up, you setting up for disaster on landings. I had to adjust the control rod on the throttle on my NexStar this weekend since it got colder out, the engine isn't idling quite right to I had to adjust accordingly. Thing is, I moved it too far, and the first landing attempt I tried the plane was a bit too hot with the engine at idle, so I had to put some trim into the throttle to slow it down a bit and lower the idle. There definitely is a balance there. If you can't get the engine to idle smoothly at all at low rpm, you might need to take a look at the engine itself.

check the easy, fuel line leaks, leaks on the engine, glow plug. I had an engine that ran good, but was problematic of dying at transition if I let it idle too long. I swapped out the A3 plug I was using, for a thunderbolt with an idle bar. that solved my problem.

Sounds like your low end is set wrong, probably still too rich. The test of how the engine advanced from idle to full throttle is not always enough.

Get/borrow a tachometer. Run the engine at WOT, then drop to idle and watch the RPM at idle. You should be able to hold a constant RPM at idle, for at least 15-20 seconds. If you find that the RPM is steadily dropping, the low end is rich. If the RPM is increasing, the RPM is lean.

is there a general rule on how long an engine should stay at idle without dying or it depends on the engine? I mean, does any engine suppose to be able to idle for let's say 5 minutes? does it depend on the settings or just the model of the engine?

How much time on the engine? Some engines need a gallon of fuel run through them before you can get a reliable idle. Others do well on the first tank. A different plug and the tach tricks are good ideas, but also remember that when the airplane is in the air, the airflow tends to help the engine continue to turn over, so even if it has problems on the ground it may not in the air.
Also, how low an RPM are you trying to run at? What prop and fuel?

Airplane engines idle better as they break in. How new is your engine? Also, the ambient temperature plays a role. If the combustion doesn't produce enough heat to keep the head hot enough, the cylinder pinch will eventually stop the engine, especially if it's fairly new. Using too cold a plug will do the same thing. A well broken in engine with the right plug will idle indefinitely if the mix is set right.

Hi!

Im running the engine on 10% nitro, using 18% syntethic lubricant. The prop is a 10x4. The engine is new, it has only run about 10 tanks. Sadly i havent got any tachometer, so im not really sure about the rpm...

I put in some more effort today and got the engine running realiabily on idle, aswell as responding nicely to throttle.


Tanx for the help!

//Alexander

You will likely see the idle smooth out even more after about 10 more tanks. That should give you another 300-400 rpm drop.

You likely need to lean the low end just a hair. It doesn't take much of a turn.... Just enough to see it move. First thing to do is when your engine is idleing pinch the fuel line and listen to the engine. If it keeps running for about five seconds then revs up and dies, it's rich. It needs to slightly rev up and die a few seconds after you pinch the line. You may need to adjust the high end a little after you get the low end right.

As already mentioned, your low speed needle is set too rich.

One great way of determining the correct Low Speed setting, is by the use of your ignitor.

Plug your ignitor in, start the engine, run it up and then let it settle into an idle with the ignitor still in place.

Let it sit for about 30 seconds. The engine should run somewhat smoothly.

Now remove the ignitor.

If the RPM's drop after removing the ignitor, the engine is too rich on the low end.

Adjust the LS needle until this stops happening. Make sure it has stopped altogether, then richen the low end by about 1/8 of a turn BUT NO MORE.

Now you should have a barely perceptable drop in RPM when you remove the ignitor.

Run the engine up and down to double check acceleration. That SHOULD now be fine too.

Another 'test' to find out if the fuel/air mixture is correct is to set the throttle at idle and pinch the fuel line to the carb. If the engine revs and dies it's too rich. If it quits immediately it's too lean. IF it runs a cpl seconds then quits it should be adjusted right.
Good Luck


Tom

The pinch test is particularly good at high throttle settings, but most engines are not responsive to a pinch test at idle.

I've seen properly tuned engines continue to run for 15 seconds and more with the fuel line pinched, where-as at high throttle the results are immediate.

The rather low and slow fuel flow at idle masks the result of a pinch test.

The ignitor removal lets you quickly focus on an approximate setting. Then you can use a pinch test to verify the results. Fully pinched off the engine should run, and then after a while briefly speed up and finally stop once tuned.

It should idle easily for 30 seconds on the starting bench with NOglow plug igniter attached. It should transition smoothly from idle to full throttle in approx. 1 second with no stumbles.
1. check glow plug - a glow plug may work but is not optimal. Iuse an #8 plug for all my 40 / 60 2-strokes. Can't tell ya how many times an "expert"tells me to swap the plug and "TaDa" - smooth operation. One of these days I will rememeber this and NOThave to be reminded by an expert.
2. are there any air bubbles showing in the fuel line between the high speed needle and the carb? Idon't know if this engine has separate carb / needle assembly with a connecting fuel line - if they are 1 piece - check the fuel line into the carb/needle assem ... If there are air bubbles between the needle & carb then air is getting into the fuel line between the engine and the needle assem OR between the needle assem and the carb OR thru the high speed needle assem itself (o-rings). Oh yea, had a situation where my fuel tank was solidly mounted without foam. Engine vibration caused foaming in the tank that then transferred to the needle assem and then to the carb. No fuel line problems just needed to isolate tank with foam. Some anal retentive types use stainless thin guage wire and safety wire the fuel lines to the various nipples to ensure "no air leaks" - be careful 'cause those little wires can puncture the fuel line (don't ask how I know this)
3. steal (OK- be nice and ask to borrow) some new fuel from a friend and replace your stuff with the new stuff.
4. All else fails - sell it to someone you don't like

BTW - Ihappen to use Hobbico power panel that has an amp guage for my wired glow igniter. Allows me to turn off the juice to the glow plug without having to physically remove the igniter.

I have used fuel engines since 1975, and I learned the only way to set the needles early in my "career". This have worked on all my fuel engines.
Despite the carburator is clean and everything else OK (fuel line, plug, fuel etc) I do it this way:

- Adjust full needle to max RPM and back off two clicks.
- Let the engine idle for 5 seconds, open the throttle abrupt and listen. If the responce is perfect I increase idletime to 10, then 30 seconds and check if it responds perfect.
- If it don`t the engine die/raise RPM in two ways: Either it stops/spin up sounding like it is leaning out. Then I richen the low needle (very small adjustments every time!), and readjust the full speed needle. Redo the idle test, and do further adjustments.
- If the engine die/raise RPM, making a noice like it is rich I do lean the low end needle, and do adjust the full speed needle and redo everything.

I have no idea how many times I have done this at the field helping beginners make their engines run reliable... Works every time!

But if the plug is wrong or the fuel is wrong or very old it is impossible making an engine run perfect. And some cheap china-motors is impossible to get reliable, usually due to poor carburators.

When adjusted my engines could idle for 2-3-4-5 minutes and still respond perfect every time