M. Michael Meyer

I just graduated to my second plane A Sig 4*40 with a SuperTigre .40 engine. I was told today at the flight field that it sounds like it has an air leak. At high RPM the engine will surge or pulse. We looked for any visible signs of an air leak and found nothing at the cylinder head or the hose connections to the engine, back plate that would indicate any "blow by." Is there anything out there that would tell me what steps I should take to troubleshoot the problem and isolate the air leak?

2slow2matter

My Feedback: (1)

Some typical places for air leaks (two strokes)--backplate, carb throat, fuel tubing.
Do you have bubbles in the fuel line? If not, then you can eliminate everything up to the carb. Try loosening the carb set screw, and re-seat the carb into the throat.
You said you checked the backplate.
There could be an air leak without visible signs of "blowby."
It's weird that you only get it at high speed, though. It idles good? Usually an air leak will keep you from getting the needles dialed in on both low and high ends. You are sure it's not just out of tune? Is the engine brand new? Was it broke in properly? Might just need to richen both needles up a bit.

horace315

My Feedback: (15)

check around the carburator,check the oring there and push it and hold it down with your finger when you tighten the bolt.

broke_n_bummin

Also check / replace the glow plug grommet.

Montague

You might just be too rich on the high end. What nitro fuel are you using and what plug? A couple of times now I've helped out someone with a new ST engine and they've all acted a bit odd when rich, not the typical smooth blubbering 4-cycle a lot of engines do, instead they surge and are hard to keep running. I'm not totally sure, but I think it's because the ST seem to run a bit higher compression. I suspect (but haven't tested) that the surging might also go away with lower nitro fuel, like 5%. I can't say for sure that this is the problem, but it might be worth a try.

Btw, you did watch the fuel lines while running at full throttle, right? No air bubbles visible going in to the engine?

combatpigg

My Feedback: (3)

I was showed this trick to troubleshoot airleaks in weedeater engines and it works with model engines, too. Take your engine and tape off the exhaust port to make it water tight, then place a drinking straw in the intake [make sure the intake hole in the crank is open] and seal around the straw with clay, or anything else that will seal off your straw, you can also use a length of fuel tubing. Now dunk the engine in water, blow into the straw to generate some pressure [it doesn't have to be alot] and observe where any air bubbles are escaping. More crankcase pressure leaks can be found coming out the front of the crankshaft than people realize. A good engine will pass this test with flying colors. Air pressure for the test can also be introduced via the exhaust port, with the carb sealed off.

M. Michael Meyer

I am using a 15% nitromethane mix. It is the same fuel I use with my OS .46 AX. The mfr recommends 0% to 15%. The manual does say that "[a]lmost all of the fuels on the hobby market with between 0% and 15% Nitro content will be fine, but you may get better results from a Premium fuel as opposed to a Sport fuel." (I don't know what that means.) It goes on to say that "[h]igher levels of nitromethane may require adjustment of the compression ratio to prevent detonation." I don't have the slightest idea how you adjust the compression ratio on an engine.

I did check very carefully this afternoon when I ran the engine. It seems to be better, but not 100% and there are no air bubbles in the fuel line.

bruce88123

You lower the compression by putting a shim gasket under the head. Shim gasket is just an extra head gasket.

Montague

The "sport vs premium" thing, I'd just ignore that. Use what you like. There are differeneces in fuels, but for the most part, most pilots won't notice, and I don't think it will matter all all in your case. What brand are you using?

I still think you're a bit rich. Try starting the engine, then slowly leaning it out until you hear the rpm drop a little. Don't leave it there, richen back up ASAP, just until you hear the RPM come down a little from the max value. See if that helps. Move the needle slowly, needle changes aren't instant, espcially when leaning out an engine. The engine needs time to blow out any excess. For that matter, if you lean it slowly, the surging might just go away as you lean it out. Stop leaning just when the engine holds it's RPM

To lower the compression ratio on an engine, you install additional head shims/gaskets. The engine might or might not have come with them (I bought my ST .75 used, so I don't know what's normally in the box). OS engines run a lower compression ratio than some other makes, btw. (at least they used to, I honestly haven't checked an AX, though I do own a couple of them). You can also cheat a bit by using a shorter plug (not a short, just a shorter one, like an OS #8), or add a 2nd washer under the glow plug. This will lower the compression ratio just a hair. A differnet plug brand might help as well, though I still think you are just too rich.

btw, most all glow two-strokes will show bubbles out of the front bearing if you do that water test. I personally wouldn't bother, but if you really think there's a leak, it might help show where. Also keep in mind that different parts of the engine are blocked off at different times as the crankshaft rotates, so if you do the under water thing, you'll need to rotate the crank as you do it. And be sure to get all the water out, and oil things up, espiecally that front bearing. I personally would want to run the engine for a bit after a dunking to make sure all the water is either blown out or boiled off.

Anyway, the problem will also probably lessen as the engine breaks in, at least it did for the engines I've seen that tended to surge.

Now, it's also possible that your local guy is right, and that you're leaking air, probably around the carb somewhere. This usually doesn't cause surging when the plane is stationary and level, but does cause the needle to be harder to set, and will cause the engine to lean out if the plane is moved at all. However, it can cause erratic things to happen, maybe surging. Usually instead of pulsing up and down in RPM, the engine will "run away" and suddenly leap up in RPM, or leap up then go overly lean and sag or die.

Now, to be totally contridictory, it IS possible that you are also set a little on the lean side. If the engine overheats, cools, then overheats again, you could get an up-and-down pattern, but it's usually slower in happening, since it takes time for the engine to cool off.

jaka

Hi!
If the rpm "pulsates" you are running too lean!

bkdavy

My Feedback: (1)

In my limited experience, pulsing is normally a sign that the engine is to lean, either from and airleak or wrong needle setting. Make sure you're high end needle is set 300-500 rpm below peak to the rich side.

Also, make sure your high speed needle isn't clogged. Debris in a fuel tank can sometimes lodge in the needle. Not always enough to block it, but enough to make it run lean. A very good test for this is to check the engine response to needle adjustments. A partially clogged needle valve will have little or no effect on RPM as its adjusted. If the needle is clogged, take it apart, flush it out with fuel or air, visually inspect all passages are clear, and then reassemble and retune the high end.

If it still surges after that, try slightly richening the LOW end (yes I said low end) and then retuning the high end.

Good Luck,
Brad

combatpigg

My Feedback: (3)

Even a hairline crack in the casting that is invisible until the engine is warm has been known to happen. Any random loss of air pressure within the crankcase will cause inconsistent running and make a guy think he doesn't know how to set a needle. Another cause of inconsistent running is bolting the engine to mounts that aren't absolutely flat and parallel, causing the case to distort. I like to set the high speed with the nose of the model pointing straight up and just momentarily pinch the fuel line to make sure that there is some reserve fuel in the mixture. Learning a couple of simple ways to diagnose a problem quickly is how the pros at the lawnmower shops feed their families and what they know applies to our hobby 100%.

BuzzBomber

My Feedback: (4)

I will not say that you do not have an air leak, but consider the source of the observation; the problem may have another cause. I know there are a few guys at my field who go around diagnosing every issue other flyers have, without even taking a good look/listen at the affected plane or engine. Typically, they have the same answer for everything...

If you have eliminated most possible air leaks, don't rule out the possibility, but look for other causes. A needle setting that is not near optimum could be the culprit as was suggested above, or it is possible there is debris in the needle assembly as well(this is less likely, but still possible). I've had bad fuel cause an engine to run in a very peculiar manner as well, but then again, that was with one particularly ornery old Fox engine...

bkdavy

My Feedback: (1)

Clogged needles are not as unusual as you might think. I've seen three in the past year. I filter my fuel going in, but sometimes things just find their way in (debris from the pressure line, breakdown of the silicone tubing, debris left from manufacturing despite rinsing the tank thoroughly). Two were on my own plane, and one was on another club members plane.

The remote needle valves are the easiest to deal with since you can hook them up to your fuel pump and just flush fuel backwards through the needle with the needle backed out. most of the time this will clear any accumulated debris.

Brad

BuzzBomber

My Feedback: (4)

Not to digress too far from the topic at hand, but I'm really surprised to hear your needle clogs that often even with a fuel filter. I (typically) only filter the fuel once, when it it pumped out of the bottle into the tank, and I've only had a clogged needle twice in the past three years. Oh well, I guess that's where the old phrase "YMMV" comes into play.

On a side note, I really hate those remote needle setups. I've only had them on two recent OS engines, a .10LA and a .40LA, and they are a pain. They take forever to settle out when you change the setting, and seem prone to inconsistent fuel draw. Can't for the life of me figure out why more engines don't use a direct mounted needle set at an angle to keep fingers away from the prop, ala the larger Norvels and several c/l engines......just my 2ΒΆ.

ELTIGRE

if its surging at high speed its possible you have a tiny leak & it will only show up at high power setting ie; when the engine is drawing max fuel. the fuel line itself can be less than tight on the inlet nipple causing air draw. also check your fuel tank setup. vibration getting to a tank could cause the same reaction esp. at high speed.