Looking for options to address mid-range burble in a Homelite 30cc magneto engine.
Top RPM with a Master Airscrew 18-6 prop is about 8,300.
It will idle forever at 1,500 rpm (see video below).

Fuel is 93 octane ethanol free, with 32:1 STP oil mix.

When reducing throttle on the ground, burble will start around 6,000 rpm.
In the air, the burble will start around 7,000 rpm.

Low end is leaned for good transition,
Further leaning of the low end produces too much hesitation.

I thought the problem was solved by leaning the low end on Sunday morning.
But as the day warmed up, the burble returned.

Since the engine has been so reliable, I can live with the burble if it can't be easily fixed.
Any suggestions will be appreciated.

Thanks!

There is a trick, but I really don't recommend it in your case since your idle and transition are so nice, and the trick is a bit difficult to do without screwing the carb up.

In your case, the numbers suggest you are making good power and you need more loading. What I recommend is going to a heavier prop load. If you would like more speed, go to an 18 x 8 prop. If you would like more static thrust, go to a 20 x 6 prop. In either case, I recommend switching to a Zoar prop. They are very good props, and also load the engine more than other props, which is what you want. You will think your engine sounds a bit lame, or less powerful, but just wait till you fly it; you'll be impressed. Do retune as normal for the new prop. Without taking the time to calculate it, I'm guessing that your prop tips are going supersonic at full throttle right now, which is both noisy and inefficient. (It does sound cool though, at least to me.) Anyway, expect the new combo to be more quiet, yet fly more powerfully, and probably help or solve that pesky burble. Most good setups prop for no more than 7500 on the ground, with many going for lower yet; like 6500. Personally, except for my highly souped up hot rods, which are a different story, I go for 7500 +/-.

AV8TOR

Thanks Av8tor.

I have an APC 18-6W, a Zinger 18-8, and a Zinger 20-6/10 in inventory..
I prefer the static thrust route, so if I have enough ground clearance,
I'l try the 20-6/10 first before deciding which Zoars to purchase. .

With the 18-6 Master Airscrew at full throttle, it sounded like the tip speed was in the 450 mph range.
But I do wear foam earplugs and ear muffs when messing with the engine, so my guess is probable way off.

Hmmm... I was wrong. Your "ear guess" is spot on, though I think you're pulling my leg. Your prop speed right now is around .6 Mach, or 450 mph more or less. Of course, that's static and it unwinds in the air.

Anyway, my recommendation is still good. The Zinger props are a fairly light load; it will be interesting to see how that Zinger 20 x 6/10 works out.

AV8TOR

Lol, you got me. The RC Karma gods have already exacted their retribution on my engine for that attempt at humor.

As a first step, I replaced the Master Airscrew 18-6 with a Zinger 18-6 for an apples to apples RPM comparison.
The Master Airscrew 18-6 was turning 8,300 rpm. With no needle adjustments, the Zinger 18-6 maxed out at 7,400 rpm.

As I recall with the smaller 10" to 14" props, the Zingers usually had more bite than the Master Airscrew props..

As you predicted, changing props seemed to help the mid-range burble on the ground.
The Zinger hasn't been flight tested yet.

However, the Zinger did exhibit the "Porky Pig" sound a couple of times during slow transitions (see first video below).


Then while slowly decreasing throttle from full, the engine just died (first time ever!) as if the kill switch had been pulled (see second video below).
Then the engine would not restart, even using the tried and true Sea Foam as a starting fluid.


I have replaced the Bennet's kill switch. It seemed to test normal when off the airplane and without vibration.
The spark plug looked clean. I will try to get the engine started again tomorrow.

Any idea what causes that Porky Pig hesitation sound?
I'm wondering if it was an early indicator that the switch might fail, if it in fact failed.

Thanks again for all your help..

This morning the Master Airscrew 18-6 was reinstalled.
The engine started and ran normally.

Wish I knew for sure why the engine quit running yesterday.
This airplane is likely not to survive a dead stick at an inopportune time.

Do kill switches fail very often? Or maybe the plug fouled?

That clearly looked and sounded as if the engine was just electrically shut off. And yes, kill switches do fail. Your plug looks great. If further testing happens to reveal a sketchy mag coil, don't buy one; I have a couple right here with me for you.

Plugs can go bad suddenly though, so you might try that too. I favor NGK over Champion. I used to be a Champion plug fanatic, but they are not as good as they used to be. And as the old saying goes; "Never be afraid nor unprepared to change the plug on a two stroke." I personally like changing them as a matter of standard practice after breaking in a new engine anyway. (Contamination due to rich mixtures, metal particulates, etc.)

AV8TOR

After studying that plug some more, I would say your tuning/mixture is spot on, but it is in fact time to swap out the plug for a new one. Try .020 - .022 for the plug gap. You may have other problems for the sudden stoppage as discussed though; it's just that I would change the plug anyway. If the plug were the direct cause of the stoppage, I would normally expect much more misfiring before quitting.

AV8TOR

Thanks Av8tor. Changing plugs sounds like a smart precaution.

The local NAPA auto parts store had the Champion plugs.

I had previously purchased the NGK replacement 1270 for the Champion RDJ7Y,
but the NGK appears to need a different spark plug cap.

Which NGK plug do you use?

Whatever old thing lying around. (Just kidding.) I do know people like that though, and my brother is one of them. Then he wonders why engines don't play well with him....

I have a bunch of engines, so the answer varies. I don't remember the NGK specific numbers right off the top of my (old age) head. Besides heat range, there are two basic variables in the plugs for most of our engines; sealing surface and hex size. The sealing surface is either taper seat or gasket, and the hex is either 5/8" or 13/16". Hex size is important for engines running the newer ignitions with a metal cap as they only accept the size they were designed for, and sealing type is important on most engines. A Homelite will accept either sealing style plug though, and with the magneto, should accept either size hex as well.

I just looked at your pic closer. As I recall, that's a correct number, but there's something wrong with that plug. Either it had a mistake in manufacture, or the connector got broken off.

Of course, make absolutely sure no matter what that you use a resistor plug, especially with that unshielded magneto. (NGK's with an "r" in the number are resistor, as are most brands, but if in doubt be sure to check.) Sparkplugs.com is a good plug research site.

AV8TOR

Av8tor, thanks for the info.

Both of my NGK plugs have the same flat tip.
Don't remember which online hobby place I bought them from.

Anyway, just ordered the same NGK plug number from CH-Ignitions.
Can't wait to see whey they look like.

That's bizarre. I've never seen a flat tip like that. I'm rather sure it's a manufacturing mistake. The CM-6 plugs have a flat top, but they have a receiver groove in the flat top.

There are two types of the normal top connector. Some are made as part of the plug, and some are the same size and shape connector, but the connector itself screws onto a stud that comes out of the plug. If you have one of those lying around, maybe you could cut the stud off and drill and tap these plugs to accept the stud and screw on terminal. You would have to be very careful how deep you drilled into the plug tip though, so as not to punch through and damage the resistor inside. Probably not worth it for the price of a plug unless you're bored....

AV8TOR

Av8tor, you were right.
The extensive mid-range burble is helped by increasing propeller load on the engine.

Master Airscrew 18-6. 8,300. Burble up to 6500-7000
Zinger 18-6. 7,600. Burble up to 5000.
Zinger 18-8. 6,500. Insignificant burble, quieter, faster.
Zinger 20-6/10. 4,750. Did not attempt flight.

While it was nice to fly without the burble, the Zingers didn't really float my boat


Next I will try an APC 18-6W

Tested the APC 18-6W. Max RPM 6,800. Burble up to 5,000.
Strong climb, sluggish low end throttle response between 1500-4000.

I've ordered a Zoar 18-6, and I have a Zinger 18-5 I'll try last for grins.

Glad it's working out for you. You are going to like the Zoar prop; they make very nice, substantial and efficient props. Did you get your new spark plugs yet?

Keep an eye on engine temps if you are doing extended ground runs with these higher prop loads. (Should be fine in the air.)

Also, did you receive my latest PM from a couple of days ago?

AV8TOR

Av8tor, I just sent you an email.

I'm still using the new Champion.

Also, I flew the PT-19 again today with the APC-18-6W prop.
The engine starts easily and idle's reliably. It runs strong and climbs strong with no stoppages.
Today's tach said the top end was 7,000 RPM.
I can use one click above burble as a comfortable slow cruise throttle setting..

So I'm going to stick with the APC 18-6W for now.

Many people consider the Master Airscrew and Zinger props to be quite good; for stirring paint.....

Zoar is possibly the best of the wood props, APC is highly regarded for 18" and under, (I use their 16 x 8 a lot on my souped up 20 to 30 size engines), then of course there are the ultra high buck carbon fiber jobs. (Not my thing, though they possibly would be if I were both rich and into competition flying of some sort.....)

I have an ancient wood Top Flite 18 x 6 on an Echo 24cc engine in my big Great Planes Fokker Dr1 Triplane. The whole package is a perfect combo. (Tachs at 7500 rpm's.)

AV8TOR