Wondering why you can’t get your engine to idle correctly? Engine running lean? You’re sure you have checked everything and still can’t figure it out?
Well you probably have an air leak somewhere.
How do I find out if I have an air leak that’s effecting my engines performance?
Well, the best way to figure out if your two-stroke RC airplane engine has an air leak. Test it!
These leaks include anything coming from the crank seals,cylinder head gasket, base gasket, the spark plug threads, intake manifold, or exhaust manifold. No matter where these air leaks originate, they affect the performance of your engine, making the engine difficult to tune, and are not normal for a healthy engine and will eventually end up costing you your engine if not the whole plane.
Performing a leak down test is the best method for determining if and where air leaks are coming from. A leak down test is fairly simple to perform test (if you have the right equipment) and can provide tremendous amounts of information about the health of an engine. I typically like to perform a leak down test on an engine if I’m having trouble tuning the engine consistently, if I’ve bought a used engine with a suspect past history,or when I’m finishing up assembling a new engine. The biggest mistake I see allot of people make is when they rebuild an engine with new parts and skip the leak down test. I cannot count the number of times where I have rebuilt an engine with new parts and then performed a leak down test to find a gasket that was not sealing properly or another part that was causing a problem. The leak down test is imperative and ultimately becomes a form of cheap insurance against the time and money that has gone into the build...
How a leak down test works
How exactly does a leak down test work? A leak down test works by pressurizing the crankcase cavity and cylinder. A sealing plate is bolted onto the intake manifold and another sealing plate is used on the exhaust to create an airtight cavity. A small air pump is used to pressurize the cavity to 8to 10psi and the pressure is monitored in the cavity for around five minutes. If the pressure gauge holds steady at 8/10psi over the course of five minutes the engine is air tight it has passed the test. If the pressure ends up dropping over the five minute period, then there is a leak somewhere in the system which must be found and fixed.
Although performing the leak down test is fairly simple,setting it up may be a little more difficult. Leak down test kits are not common or even unavailable in the RC airplane engine market. You can try to cobble one together and sometimes these home made parts can be part of the problem in getting an accurate leak down test.
At High Point we have taken the cobble out of your home made parts and now offer a number of Pro Leak Down Test kits to let you get on with the test.
Let’s get started,
Assuming you have a bicycle pump, air retracts pump, Mighty Vac or some kind of LOW pressure hand pump with a Low pressure gauge so you can pressurize the crankcase cavity as it allows precise controllable pressurization.
Do not attempt to use an air compressor! since the pressure rises quickly and you run the risk of blowing the seals out.Fixing blown seals can be an expensive and time consuming affair.
Now that you have the High Point leak down test kit affixed to your engine. Double check that the carb plate and exhaust plate or plates are cinched down. (if you like you can use some oil or grease on the rubber side of the plates)
Once your kit setup is complete, pressurize the cavity to 8-10 psi and start timing. If the pressure is lost quickly there is a large leak in the system and now you need to find it. I like to use a squirt bottle with soapy water to hunt for air leaks. It is also possible to hear the audible hissing sound from the leaks, which will point you in the right direction. Once you identify a leak you will have to determine what is causing it and remedy that problem before trying the leak down test again. This part of the process can be frustrating and time consuming, but it is definitely worthwhile. Be sure to spray your block off plate and carb manifold plate for leaks, you want to make sure they have been bolted on to clean surfaces.(remove old gasket gook)
Pump up the Schrader valve to 8-10 psi. (I prefer to test twice, TDC and BTDC)
If you’re able to hold the 8-10 psi for 5 minutes you’re looking good. Sometimes this standard is not always achievable, and when that happens I am comfortable settling for a 1 psi drop over five minutes. Even with brand new parts I am not always able to maintain 8 to 10psi and believe this is due to parasitic leakage past some of the seals or gaskets. In my opinion,anything more than a 1psi drop means you should still be chasing that leak.
8 to 10psi or 16 to 20inHg? What? (Pressure or Vacuum?)
Some engine builders I have worked with will recommend testing vacuum for five minutes at 16/20in Hg (equivalent to 8/10psi of pressure) as well which will test the seals under vacuum. I have yet to find a seal that has passed under pressure but not under vacuum so I wouldn’t get all freaked out if you don’t have a means to pull vacuum when testing.
Good luck with your leak testing.
1 - Tune the engine when it is warm, and double check the tune after a flight.
2 - On the DLE there is a little silver screw on the carb that is an "idle set screw", it has a spring on it and it sets how low the low throttle arm will physically go down. Take that screw OUT and deposit it in the trash can. Do not remove the black H and L screws obviously.
3 - Start the engine and run it for a good few minutes with varied throttle and some full throttle run ups to get it nice and warm.
4 - If you have not done so, adjust the servo travel for a consistent idle, does not need to be perfect as long as it is low. 1600 is too high to properly tune that low, it should be much closer to 1200.
5 - Tune low needle first. Now run the engine up to clean it out with a couple full throttle run ups and then go to low idle. Listen to the engine.........how long does it take to start to "load up" or start to slow down in RPM? If it is rough right away you are likely way to rich.Normal starting point on 100cc engines is 1.5L and 1.5H. I have had to go as low as 1 3/8 on the low needle on the DLE carbs. Adjustments on gas carbs are VERY small, like the width of a screwdriver blade or 1/16th of a turn or less.
A good "safe" tuning on the low needle you should be able to clean it out with some run ups, return to idle and it should have a nice steady low idle for at least 15-20 seconds before it starts to load up. If it does this it is still just a "touch" rich but will not die on you and it will run nice and smooth.
6 - How low should react. Now that you have it running,clean out the engine and let it go to idle. Let it sit for 10 seconds or so and then slam the throttle to full, not slow, nail it!! Listen to what it does. If it dies or almost dies, you are too lean. If it stumbles and works its way up(may see smoke in exhaust) you are rich. As tuned in number 5 above it should run up quickly with very little or no noticeable stumble.
7 - Now tune the H needle. There are several ways to do this with RPM but the simplest way for a safe needle setting is this. With the engine warm and the low needle tuned run the engine up to full throttle for about 5 seconds, now chop it in one quick motion to low. Listen to what it does then.
- If it returns to a steady idle, you are very close.
- If the idle goes way low and works its way up to a steady idle you are too rich.
- If it stays at a high idle and then slows down you are too lean.
- If it dies you are either way too rich or way too lean.With the H at 1.5 you should not be too lean.
8 - When H is set return and double check the L and then double check the H. Then it's time for a flight. Listen to the engine carefully for all of the noted symptoms while you are flying. One engine test I do in-flight is for the H needle. Get the engine nice and hot, maybe a high speed pass or something. Now do a full throttle straight up line. Listen! Engine should be smooth and steady until you can't go higher. If it starts to sag a tall, land immediately and richen the H needle just a tad and try it again.
This fade can also be caused from overheating not tuning related so make sure your engine is properly baffled if you now the tuning is correct.
There you go, pretty simple and really only takes a few minutes once you get it down.