OS XM Plug issue

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Kris Flynn

Well-Known Member
Supporting Member
Joined
Mar 11, 2002
Messages
4,371
Hello All,

I have 2 x OS Motors that started to hurt plugs just as they started to go well (ie spool up and rev out nicely) as I was leaning on them. The plugs look brand new, but do not light up. I have tried 4 different brands of plugs, probably 6 or 7 heat ranges and all have the same issue, brand new looking plugs that do not glow after a good run.

One motor is a Mongar Sport motor and the other is also modded. BUT both have been running for years as they are (one of them even won the WTC in 2022). Both run water cooled heads. I also did notice they were getting very hot compared to normal...even though they were not really running that lean or hard?

I was running 60% nitro (15 syn and 3% caster) and dropped back to 50% to see if that had become the issue, but no.

anyway...what can cause a plug to break and still look brand new? too much compression? as I mentioned, they have been running fine, now all of a sudden they start doing it? I tried different fuel tanks and got the same result...they also do not share carby's, so that is another variable...

bearings are either brand new or still very good in both motors...both motors still had/have OK 'nip' or compression at TDC

Cheers
Kris
 
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What about the PTO bearings?
Yep, all good and fresh...particularly on one motor and the other are not that old either.

Do you think bad bearings could cause plugs to "break" like this?
If you look at the coil under magnification is it still shiney chrome or frosted?
The coils look brand new and shiney as they come out if the packet :-(
 
Vibration is usually what causes this in my experience.

Are your props balanced properly? Not just the blades but the hub as well? Mounting bolts etc. tight?

My F.A.S.T bro Mike Nowicki was eating a plug a run in his new JAE 21, figures he made the motor mount too rigid trying to keep good alignment for the wire drive.

Rod G swore that warming the plug for a minute (annealing it) helped but I don't think so...
 
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On a good running motor, the plug tends to want to get pulled out into the bowl. Might try annealing the new wire and pulling a coil or 2 into the bowl. Then see how they last. A slow pull is going to be better then yanking on in during the run. I would suspect at 20K it is pretty violent.
Mike
 
The only time I’ve had that issue with plugs it been a high rpm vibration and it’s always been a bearing problem. If you have not pulled the crank or pto and used brake clean on the bearings and then checked each bearing, you might want to.
John
 
I know you mentioned multiple heat ranges... are they all from the same manufacturer?
4 different manufacturers (OS, Rossi, Odonnell, BP)
then at least 2 but sometimes 3 heat ranges from each brand...going from super cold (ie thicker "stronger" wire, through to thin and 'weaker')


Where does the wire break?
Can you show us a picture, eventuelly pull the wire out to see where it is broken.
I will try to find the plugs from the weekend...but it must be right down at where they attach, inside the bore in the plug. I will pull the wire and have a good look at where it actually broke on these (OS # 8) plugs.

On a good running motor, the plug tends to want to get pulled out into the bowl. Might try annealing the new wire and pulling a coil or 2 into the bowl. Then see how they last. A slow pull is going to be better then yanking on in during the run. I would suspect at 20K it is pretty violent.
Mike
This is done to (effectively) get advanced timing on a plug, yes? IE in my thought process of what I have been told, be able to get a better durability of a colder plug but the performance of a hotter plug due to the 'advanced timing' (if you pull the coil on a #6 plug for example, you can get similar performance to a #5 plug)?


The high speed vibration does sound feasible...I will pull apart on of the motors and have a good look at its bearing again.

I am using these glow igniters from A-Main https://www.amainhobbies.com/protek...e-glow-igniter-1.2v-5000mah-ptk-7607/p1048998

Thank you all so far...as you can imagine, this has cost me a lot of money in plugs so far LOL
 
I was always told the plug breaking at the very bottom was a PTO bearing issue

New bearings are a myth, lol. Had a new set from factory pour full out the top. Definitely worth another check!
 
I always got the best performance from a K&B 1L or McCoy 59 and the deck height as close as it would go. My best running watercooled motor had witness marks on the head button from the piston hitting it. Plugs live forever on 66%. You have something going on. For 2 engines having the same problem is interesting. What is common?
 
Kris,

Have you checked the bearing pockets in the case yet? They maybe
worn out and allowing the bearings to
move too much, and then the crankshaft is moving and causing the
vibration problem. It is a vibration problem I believe.

Just Another Thought,

Mark
 
If they are breaking the coil away from inside the top of the plug it nearly is aways a vibration problem. You have probably hit resonace point that shakes the coil and breaks the top weld. If you gently push the coil back in it might glow again that proves its top weld gone. We use different grade rubber mounts in Europe to move the resonance to a different rev range.
 
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