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Old Apr 17, 2012, 09:59 PM
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USA, MN, Blaine
Joined Feb 2011
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Help!
.25 FX OS Max keeps burning out glow plugs

We're starting a "B" combat class at our club. I bought and I'm running a secondhand O.S. 25 FX Max with a Macs (Macks?) pipe on my Battle Axe B plane with a 9x4 Master Airscrew prop. I dont know how old it is but it turns over fine and it doesnt feel like its grinding or anything is gritty inside.

The problem I have is I cant go one whole flight without melting my glow plug, or to be more precise, melting the element in the glow plug. I get it running in what seems to be the optimum range give it a toss and off she goes. It flys great and then about 4 minutes in she sputters down to a low idle then quits. I pull the plug and the element is melted.

I went from a No.8 plug to a No. 6 (A3) plug thinking I needed a hotter plug and I melted two of them tonight so that didn't solve my problem.

For fuel I was using Wildcat 15% which I am now out of and will be switching to Omega 15%. I didnt get to run any Omega through it because I already burned 2 plugs and I didn't want to burn through more until I got some advice.

I don't think I'm exceeding the RPM of the motor, I tached it on the ground at about 17500ish but obviously something isnt going right. I can richen it up but then it just doenst have the same speed and agility.

I haven't taken the drastic step of unmounting and disassembling it too see if there is something clogged but I'm not afraid to either. I was just hoping it wont come to that.

Anyone else ever have this issue? Any suggestions? Bigger Prop like a 10 x 4 maybe?
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Old Apr 17, 2012, 10:09 PM
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United States, KS, Overland Park
Joined Jan 2010
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Quote:
Originally Posted by Newman12 View Post
We're starting a "B" combat class at our club. I bought and I'm running a secondhand O.S. 25 FX Max with a Macs (Macks?) pipe on my Battle Axe B plane with a 9x4 Master Airscrew prop. I dont know how old it is but it turns over fine and it doesnt feel like its grinding or anything is gritty inside.

The problem I have is I cant go one whole flight without melting my glow plug, or to be more precise, melting the element in the glow plug. I get it running in what seems to be the optimum range give it a toss and off she goes. It flys great and then about 4 minutes in she sputters down to a low idle then quits. I pull the plug and the element is melted.

I went from a No.8 plug to a No. 6 (A3) plug thinking I needed a hotter plug and I melted two of them tonight so that didn't solve my problem.

For fuel I was using Wildcat 15% which I am now out of and will be switching to Omega 15%. I didnt get to run any Omega through it because I already burned 2 plugs and I didn't want to burn through more until I got some advice.

I don't think I'm exceeding the RPM of the motor, I tached it on the ground at about 17500ish but obviously something isnt going right. I can richen it up but then it just doenst have the same speed and agility.

I haven't taken the drastic step of unmounting and disassembling it too see if there is something clogged but I'm not afraid to either. I was just hoping it wont come to that.

Anyone else ever have this issue? Any suggestions? Bigger Prop like a 10 x 4 maybe?
bigger prop, less nitro, richer on the mix. thats got to be wicked hot to melt a plug.
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Old Apr 17, 2012, 10:29 PM
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Asheville, NC
Joined Mar 2009
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As the saying goes, "Speed costs money, how fast do you want to go? "

My thoughts:

1. If you're running lean mixtures, use a COLDER plug. The A3 is the hottest of the OS plugs, the #8 is two ranges colder.

2. You need to run a little richer mixture. It'll lean out in the air as the prop unloads so if you start on the edge of lean, it'll go over-lean in the air.

3. A bigger prop will just load up the engine even more and cause it to run hotter. You won't go any faster since the pitch is the same.

It's a trade-off, power vs. reliability.

Good luck!

Mark
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Old Apr 17, 2012, 11:59 PM
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I would try something like a Merlin 2006 and make sure you are not running lean.
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Old Apr 18, 2012, 12:08 AM
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Umkomaas, South Africa
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I would say try a colder plug like an OS R5.
If the mixture and plug is the right temperature and it keeps blowing then the bearings are on the way out as small pieces of metal from the bearings are blowing the plug. It is one of the first signs that the bearings are starting to go.
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Old Apr 18, 2012, 12:23 AM
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Joined Mar 2012
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Do you hear anything that sounds like eggs frying? That would be predetonation and will waste a plug very quickly.

Nitro essentially advances the firing timing of a glow engine as well as producing more torque than straight 80/20 due to its lower auto ignition temp than methanol.

You may need to add a head shim to lower the compression a tad if it's preigniting and you insist on running a higher nitro fuel.

You may find it actually runs better on 5% or 10% nitro and stops eating plugs.
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Old Apr 18, 2012, 06:33 AM
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Thanks everyone, that gives me a lot of options. I'll try a few and then report back.

I appreciate the input.
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Old Apr 18, 2012, 09:12 AM
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The likely suspect here is the tuned pipe. How was it tuned?
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Old Apr 18, 2012, 10:07 AM
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Honestly I have no idea how it was tuned. I got this motor secondhand and that's what was on it. It's also my first tuned pipe,if it can even be called a tuned pipe. It's the Mac's black tube pipe and it doesn't look like there is anything I can adjust on it at all.
I have had two other guys from my club who I would consider as having a great deal of knowledge about engines and tuned pipes helping start and run the engine and they didn't mess with or indicate that any tuning of the Macs muffler needed to be done. Nor did they think the engine was running funny either.
What kind of tuning should be attempted on the macs pipe?
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Old Apr 18, 2012, 10:11 AM
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I believe you have the Macs muffler and not a true tuned pipe. You may also look at the fuel system to see if you have a partial blockage that only affects the engine when you unload. Blowing out the carb fuel passages and lines is a good practice.
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Old Apr 19, 2012, 08:10 PM
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Add a shim to the head. We had one of the older ones before schneurle that needed about .020" before it stopped burning them. ($30 worth of plugs for a $15 swap meet motor) That was open exhaust. A pipe only makes it worse. Just run pretty rich and make sure the top end especially is rich enough with most pipes.
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Old Apr 19, 2012, 10:40 PM
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Took it apart before doing anything else...it aint good news...

Well I thought before doing anything else I'd take it apart and see if there was any metal or debris in the motor and here's what I found:

The pisiton head has what looks like a pockmarked silver/aluminum coating of somesort and its got major scratches/gouges down the side. So bad that you can see it no longer has a smooth edge

At the rod end it looks like it was banging up against the side and the walls are all scratched up

I hope that the pics will be good enough for you all to see how bad it is

I appreciate all the help everyone but I only spent $60 on this motor so I think I'm just going to move on and use my back up motor. Its not worth me throwing more money in to it.
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Old Apr 19, 2012, 11:20 PM
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This engine has been badly abused. The piston indicates it was run very lean and had detonation. The crankshaft is covered with rust which can damage a plug. If it has a true tuned pipe and it wasn't set that too can cause the piston damage from detonation or running lean. These can also blow a plug every run.

Too much nitro methane will also cause this. With high nitro you want a colder plug. When I ran boats I used 50% nitro and a colder Enya#5 plug. Enya plugs are stronger and the heat range is easier to figure out. #3 is the hottest and #6 the coldest.

The pitch being the same has little or nothing to do with the speed. A small prop with a 3 inch pitch on a 1/2A C/L speed plane will move the plane around at over 160mph. A 6x3 will barely fly the same plane at 100mph. Length has a big influence on speed as it changes rpm.

Looking at the pictures you showed, I would say you need a need piston and liner And probably bearings. The rust should also be removed. The fact that the rod has hit the sides of the case is a sure indication the bearings are shot.

Unless you want to spend a fair amount a money it's probably junk. Changing the bearings alone can be beyond some modelers abilities.
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Old Apr 20, 2012, 07:54 PM
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I would say its a parts engine now.
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Old Apr 20, 2012, 11:30 PM
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Neenah, Wisconsin, United States
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Meltdown! I have never seen a glow engine that bad. I melted a piston out of a snowmobile before and coated the the spark plug with melted aluminum.
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