Vibration & Glow Plug Failures

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BobBonahoom

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Mar 6, 2009
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I have some engines in which plugs last a long time and other seemingly identical engines, set up exactly that same, with the same needle setting, same pipe, same prop, that eat glow plugs. They usually look fine, but the wire breaks down inside the plug. It seems like a major contributor to this may be vibration (?). I know that bad bearings cause plug failures. Is that because bad bearings set up more vibration?

Recently, I have been going to great lengths to try to eliminate run-out at the collet, again to eliminate vibration. Stu told me that CMB started putting slugs in their cranks in an effort to eliminate plug failure (due to vibration?).

Lastly, what about engine mounts? Can a rubber isolated engine mount be too "soft"? I think we all know that the solid mounts of years ago were too stiff, but is there a sweet spot with just the right durometer rubber?

We have a lot of smart engine guys and mechanical engineers on this site, so I just wondered if we could discuss this a little.
 

Mike Cathey

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The glow plug wire breaking is why a lot of people get their crankshafts balanced. Stu Barr is really up on this. I think he worked on this issue with Ron Bouche.

For nitro I have always used the 18 lb shear isolators which strike a nice balance between vibration dampening and durability.

I got a Gizmo motor mount set up for the Zenoah and the isolators that came with it are definitely harder. Most of what I have heard from the gas guys is the gas motors do have more vibration.

My two cents
 

BobBonahoom

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Just curious, what plugs you running?
I run 289s in my 45 and 67s and Nova #6 in my 21 and 12s. Now that I think about it, I have the problem mostly in 21 engines that run the Nova plugs, but I run the same plugs in my 12s and they never fail. I wonder if there is a resonant frequency failure mode associated with the RPM range that 21s happen to operate in?
 

dwilfong

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Chased this problem with a friend and it came down to the needle could not flow right at top RPM. It would run good but when you leaned into it and the eng turned on the plug would fail. Changed heads mounts pipes props plug type tank you name it. when it turned on it took the plug down deep. Changed the needle all good now. Some times it is the simplest thing right in front of your face.
 

Will Gallagher

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Nov 29, 2003
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I am sure there will be many opinions on this, BUT, I think David is correct. Engine comes to temperature and goes lean. Especially in the corners under increased load...

O'donnell 289 or 287T are my plug's of choice...
 

Carl Van Houten

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Hey Will,

What engines are you running those in? I have a friend at the pond that runs that brand (not sure what heat range) and I have seen them burn out when I thought they should have hung in there. It could be his fuel or setup causing the failures.
-Carl
 
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Rudy Formanek

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Chased this problem with a friend and it came down to the needle could not flow right at top RPM. It would run good but when you leaned into it and the eng turned on the plug would fail. Changed heads mounts pipes props plug type tank you name it. when it turned on it took the plug down deep. Changed the needle all good now. Some times it is the simplest thing right in front of your face.
Too small a pressure fitting through hole can do this also. Stinger IDs can vary, causing back pressure differences.
 

Will Gallagher

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Nov 29, 2003
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Hi Carl,
I was running MAC engines for years but now I run CMB 21's (O'Donnell 287T) and CMB 67/80 and 91 HR's (O'Donnell 289). I have never been as scientific as a lot of the guys on here are, but I actually run much faster with richer fuel settings using the O'Donnell plugs.

Rudy is correct about stinger ID and I will add Header ID, Head Clearance and lets not forget Squish Band to the mix.... So many variables is what makes Nitro Engines fun!

I sure miss Jack O'Donnell. He would have loved to chime in and share his knowledge on this topic....
 

dwilfong

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Some needles reacted different under different conditions. Remember it is the pressure drop across the needle that controls the fuel flow. Spray bar size venture bore size. line size .stinger size all have to work in unison to keep the eng fuel mix happy. The needle design has a lot to do with how the fuel flows across it under pressure drop. Some are not as sensitive to fuel flow as others and some are to sensitive. There potential is limited so the fuel demand is also limited. When you push the limits with high RPM setups and good flowing eng and pipe set up the fuel demands will change a lot and quick. How QUICK can your needle react or not react?????? you will find out toot sweet........LOL
 

Grimracer

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Do tell...

"it is the pressure drop across the needle that controls the fuel flow"

Grim
 

dwilfong

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Have you used a spray gun? As the air stream moves across the siphon tube it creates a vacuum in the tube lifting the paint out of the can and into the stream. Same thing happens with a spray bar in the carb as the air moves across the spray bar it creates a vacuum. this lets more fuel to flow at the same pressure built up in the tank as the pressure drop is higher. The pipe stinger will control the pressure build up in the tank but the spray bar controls the vacuum signal to the needle. the size of the carb also controls the velocity that the air moves across the spray bar creating the vacuum that make the pressure drop at the needle.
So it all has to work together.
Now some needles will flow better than others. I am not talking # on a meter. But how thy flow in reality. How easy do thy let the fuel move over the needle set. Think of it as how much HP do thy have. Two eng of the same size will flow different depending on how thy are designed. A needle acts the same.
OK that enough time to work on the E6........LOL
 

Carl Van Houten

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Hi Carl,
I was running MAC engines for years but now I run CMB 21's (O'Donnell 287T) and CMB 67/80 and 91 HR's (O'Donnell 289). I have never been as scientific as a lot of the guys on here are, but I actually run much faster with richer fuel settings using the O'Donnell plugs.

Rudy is correct about stinger ID and I will add Header ID, Head Clearance and lets not forget Squish Band to the mix.... So many variables is what makes Nitro Engines fun!

I sure miss Jack O'Donnell. He would have loved to chime in and share his knowledge on this topic....

Will,

Fuel delivery certainly has a lot of variables in itself and a poor fuel supply on the top end will dictate what plugs, pipes and props work best with your engine. Is the CMB 21 engine you referred to an inboard or outboard?
 

John Beardslee

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Apr 20, 2011
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I don’t recall plug issues back when we ran rigid engine mounts other than just squeezing it to much and going lean. When we had a wire breaking issue it was always an internal engine problem ie: breathing’s, bearings and bearings. Jim Wilson would change the bearings if he took out 1 plug because the wire pieces would attach themselves to the bearing components. You can’t feel them but eventually they generate vibrations ina high rpm engine.
Thanks John
 

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