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Large Glow Engines and Nitro Content

Old 08-25-2021, 03:41 PM
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thailazer
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Default Large Glow Engines and Nitro Content

Fellow flyer has a fairly new Magnum XL 180 and it idles great, but will die suddenly when advancing the throttle often. When it does get up to full power, it seems to have plenty of power and is not lean or too rich. Fuel is 15% nitro content which I am thinking may be the issue, as my Webra 120 needed a short plug and extra glow plug washer to be able to run 15% nitro. Does high nitro fuel affect detonation timing at mid-range that could kill the engine suddenly? There is no sputtering as it just plain stops. All advice appreciate.
Old 08-25-2021, 05:42 PM
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15% is not a problem. I had a Magnum 180 4St and I used 20%. your friend could have the wrong plug. but I did have problems with it's carb.
sO,
what plug is he using, brand and number ?
and what prop ? yes a prop can be the problem IF it is way to small. I saw a 160 run bad, the owner said it never ran well. he had a super light 13" wood prop on it, I sold him a 15" APC to put on it and it ran great instantly, because his engine had more "flywheel effect" with longer prop.
Jim
Old 08-25-2021, 08:25 PM
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Jim... Thanks for those points. The prop seems heavy enough, and we did try a OS #8 and an OS #6 with no difference in the quitting. We first thought that it was loading up at idle so we leaned out the low speed mixture so a pinch on the fuel line has less than a 10 second run. It seems like something is quenching the plug or there is detonation at the wrong time as it stops so suddenly.

You mentioned that you had problems with your carb.... Can you elaborate on that?
Old 08-26-2021, 05:56 AM
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Hi!
It sounds as typical pre detonation due to too much nitro!
Use 5% nitro and see what happens. All modern glow engines run good on 5%.15% is just waist of money! at least if you fly at sea level.
What prop size do you use? A 15x8, 16x6.17x6 or 18x6 APC depending on model is what I would try.
Old 08-26-2021, 06:21 PM
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Originally Posted by thailazer View Post
Jim... Thanks for those points. The prop seems heavy enough, and we did try a OS #8 and an OS #6 with no difference in the quitting. We first thought that it was loading up at idle so we leaned out the low speed mixture so a pinch on the fuel line has less than a 10 second run. It seems like something is quenching the plug or there is detonation at the wrong time as it stops so suddenly.

You mentioned that you had problems with your carb.... Can you elaborate on that?

my engine was very very finicky when the tank was half full or lower. if I ran it richer to compensate for it I lost a noticeable amount of top end power. the problem was rather bad. I have heard of people having this problem in smaller engines but never on an engine this large, some of these people raised their tanks.

now this^ reminds me, please check the high speed needle, Magnums are very well known to have loose high speed needles, just wiggle it to see how loose it is. if it is loose an OS 140 or 160 needle may fit well, this just may be your problem ?

sorry, I can not remember what OS needle it was that would fit my Magnum 180 4St, if I had to guess I would guess it was the OS 160 twin 4St ???

BTW, as for the 15% fuel, the engine may have had the Head Shim removed if it was worked on. "no shim" would give more compression, this could be the problem or just some of the problem, please first check that Needle.

good luck.

Jim

Old 08-30-2021, 09:21 AM
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Hi!
First try a lower nitro content. 5% is just what all glow engines need. If a glow engine starts ,runs at low and medium speed but stops suddenly when you try to set the high speed...then it is most likely that the fuel contains too much nitro.
Old 08-30-2021, 06:09 PM
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yes, I have seen that before with the bigger Super Tigers, Tartan you can only run 0%. this is because their compression is so high. but out of the box Magnum's 180s can run 15%. unless their compression has been raised I ran 15% and 20% in mine stock out of the box

Jim
Old 08-31-2021, 11:02 PM
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Originally Posted by thailazer View Post
Jim... Thanks for those points. The prop seems heavy enough, and we did try a OS #8 and an OS #6 with no difference in the quitting. We first thought that it was loading up at idle so we leaned out the low speed mixture so a pinch on the fuel line has less than a 10 second run. It seems like something is quenching the plug or there is detonation at the wrong time as it stops so suddenly.

You mentioned that you had problems with your carb.... Can you elaborate on that?
I believe nearly 10 seconds with a pinched fuel line is way too much! I t should not run more than 4 seconds when pinching the line close to the carburettor.
While in mid range both needles feed the engine. I think your engine is running too rich (both needles), and the plug is getting cold in mid range. 15% of nitro is a waste with so big engines.
Old 09-01-2021, 03:37 AM
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When doing the pinch test, you want to see a change in RPM pretty quickly. If you don’t, you’re too rich. Too lean and you’ll hear an RPM drop pretty quickly. Slight rise in RPM is just right. anything else is too rich.

You can use high nitro in high compression engines, you just need to adjust for it with the glow plug. A lot of people use high nitro and hot plugs and compensate for this by running too rich. I see this in trucks and buggies all too often. Correct oil ratios come into play here a little bit as well.
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Old 09-03-2021, 03:30 PM
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Originally Posted by thailazer View Post
Fellow flyer has a fairly new Magnum XL 180 and it idles great, but will die suddenly when advancing the throttle often. When it does get up to full power, it seems to have plenty of power and is not lean or too rich. Fuel is 15% nitro content which I am thinking may be the issue, as my Webra 120 needed a short plug and extra glow plug washer to be able to run 15% nitro. Does high nitro fuel affect detonation timing at mid-range that could kill the engine suddenly? There is no sputtering as it just plain stops. All advice appreciate.
First of all try and adjust the idle needle assuming there is one. Idles great is very subjective. When the idle circuit is either too lean or too rich the carburetor will not be able to adapter to the sudden fuel/air mixture change when opening the barrel and the engine just dies. You can have an engine idling very nicely with a combination of idle needle adjustment and carburetor barrel opening. To test it as mentioned in other posts you need to do a fuel line pinch test while the engine is idling. The engine needs to increase in RPM for a short time and dies. If it dies right away without increasing RPM your idle mixture is too lean. If it increases RPM for a long time before dying you are too rich.

No you won't have detonation issue at mid throttle. Detonation will occur when the engine is running full bore. You will hear a "fry egg" sound as per Clarence Lee. If you need to continue to run 15% fuel and think that the engine is detonating all you have to do is to put another head gasket on the engine.
Old 09-03-2021, 05:09 PM
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one way to tell if your idle is way too lean is, if you come off the throttle quickly to an idle, you may hear your engine make a sound "NAA NAA NAA NAA" and most likely your idle will be high and then it slowly come down to where it should be. and the sound will go away.
I have seen this many times in all kinds of 2st, from our glow engines to weed eaters, to motorcycles.

he still has not told us what prop is on the engine,. he said it was heavy... well heavy (and more pitch) is one thing, it loads an engine, but a longer prop produces more angular momentum

Jim

Last edited by the Wasp; 09-03-2021 at 05:11 PM.
Old 09-03-2021, 06:01 PM
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>>one way to tell if your idle is way too lean is, if you come off the throttle quickly to an idle, you may hear your engine make a sound "NAA NAA NAA NAA" and most likely your idle will be high and then it slowly come down to where it should be. and the sound will go away.
Old 09-04-2021, 08:09 AM
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If your rpm hangs when you come off a high rpm run, your high speed is too lean most likely. Most people would richen the low speed, but this wouldn’t be correct if the idle speed is stable and the transition is nice. An over advanced ignition will over heat the plug and a further lean low mixture will keep it hot. Richening the low side will just quench the plug and band-aid the problem.

Old 09-04-2021, 04:33 PM
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If your rpm hangs when you come off a high rpm run

yes I agree, but it can happen at start up too. the air is thinner in the north. I have seen/heard this on RC Helis at startup with their super light clutches and light hubs, you can not start a Helis at high RPM because of the long blades. and the European motocross bikes were notorious for it with their Bing Carbs back in the 70s

Last edited by the Wasp; 09-04-2021 at 04:39 PM.

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