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Old 10-06-2009, 03:38 PM   #1
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Default Please help me understand where I went wrong so I don't break another engine.

I was trying to tune my engine when I broke the con-rod where it connects to the crank shaft.

When I took the engine apart, the piston was wedged in the sleeve. I noticed that the top of the piston head and the bottom of the turbo button had scaring on them. They are now rough to the touch where before they were smooth. They are not rough as in jagged like pieces were chipped out.

During the tuning process I did need to replace a glow plug because it burned out.

Is this enough to know whether the engine was running rich or lean when the con-rod exploded?

What symptom could I (or should I) have found to indicate that I was running too rich or too lean before the con-rod got destroyed?
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Old 10-06-2009, 03:42 PM   #2
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Pics are pretty blurry but it looks like there may be a lot of pitting on the button. This would indicate detonation which is typically caused by a lean condition. Perhaps this was damage from the piston though?
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Old 10-06-2009, 04:14 PM   #3
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This would indicate detonation which is typically caused by a lean condition.
+1 the pitting is indication of running lean, plus the scoring on the exhaust side of the piston says it was hot due to being lean... also running lean will not give enough lubrication to the rod bushing and cause premature failure, which in your case was the ultimate failure amongst the detonation.
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Old 10-06-2009, 04:16 PM   #4
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how many gallons through the engine?

what fuel and percent?

what plug?

did you replace the rod after break-in?

did the plug that 'blew' leave the coil in the engine?
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Old 10-06-2009, 04:17 PM   #5
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Default Temps?

You were keeping a close eye on the temps...right?
You should read the tuning bible on this forum.
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Old 10-06-2009, 04:33 PM   #6
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You were running too lean!Lake of lube to the con rod bushing you should keep an eye on the temps 210-230 is a good temp that's were the parts start to expand. Especially around the piston which relieves stresses on the connecting rod.Make sure your blowing smoke on throttle after breaking (like a puff of smoke) this is an indicator that your low end is on the rich side which is always good!If you are going to over heat an engine 9 times out of ten it will because of a lean low end needle.These engines work by first collecting fuel in the crank case.The smoke is an indicator your engine is getting sufficient amounts of fuel.Another way you can do this is by pinching the fuel line you engine should increase in rpm for about three seconds before dying off.If it wants to die immediately then your too lean on the low end needle.
I hope this helps you!
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Old 10-06-2009, 05:48 PM   #7
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To all the people saying he was running too lean, that's possible but not necessarily true. Rods will often stretch or wallow out the lower bushing before they let go. When this happens, the piston comes up higher, increasing compression and causing the detonation that pitted the head and top of the piston.

As for what you did wrong, when a motor that has been running fine starts to kill plugs, STOP RUNNING. 9 out of 10 it's a rod about to go, the remaining 1 being either a rear bearing or worn out crank pin.

What motor was this and how old was it?
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Old 10-06-2009, 06:03 PM   #8
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Is it possible the crank pin or rod had wear and was requiring the tune to be adjusted over and over? If so you could reach apoint to where tuning would have been transient at best. sooner or later when you have an engine with transient tuning you will eventually make an adjustment and the timing will change as the rod slop increases , until failure is eminent. Just a scenario.
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Old 10-06-2009, 06:10 PM   #9
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+1 the pitting is indication of running lean, plus the scoring on the exhaust side of the piston says it was hot due to being lean... also running lean will not give enough lubrication to the rod bushing and cause premature failure, which in your case was the ultimate failure amongst the detonation.
If this is trully the problem, then is this fixed richening the HS needle or LS needle?
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Old 10-06-2009, 06:13 PM   #10
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how many gallons through the engine?
About 1-1.5

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what fuel and percent?
Cold fusion 30%

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what plug?
O'Donnell 97T

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did you replace the rod after break-in?
No haven't replaced anything on the engine yet.

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did the plug that 'blew' leave the coil in the engine?
I'm sorry - I didn't know to check for the coil. Is this important? I just threw the old plug out.
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Old 10-06-2009, 06:15 PM   #11
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Most all rods that break like that are from over revving.

As stated with "transient" tuning as the engine wouldn't run or idle because of a worn rod bushing it was tuned leaner and leaner until "POP!"


Garret, It probably needs to be richined on both ends.
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Old 10-06-2009, 06:18 PM   #12
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Originally Posted by elkman View Post
You were keeping a close eye on the temps...right?
You should read the tuning bible on this forum.
Yes I was. I struggle a little with this because I am not sure where to take the temperature. I have been told so many theories that I am confused a little bit.
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Old 10-06-2009, 06:22 PM   #13
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Originally Posted by wingracer View Post
To all the people saying he was running too lean, that's possible but not necessarily true. Rods will often stretch or wallow out the lower bushing before they let go. When this happens, the piston comes up higher, increasing compression and causing the detonation that pitted the head and top of the piston.

As for what you did wrong, when a motor that has been running fine starts to kill plugs, STOP RUNNING. 9 out of 10 it's a rod about to go, the remaining 1 being either a rear bearing or worn out crank pin.

What motor was this and how old was it?
Werks B7 Pro and it was only 1 - 1.5 gallons through it. I literally just finished breaking it in. It was practically new and had never been raced.
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Old 10-06-2009, 06:29 PM   #14
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Werks B7 Pro and it was only 1 - 1.5 gallons through it. I literally just finished breaking it in. It was practically new and had never been raced.
I can't keep up with Werks. Is that Picco based or something else?
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Old 10-06-2009, 06:33 PM   #15
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NVM, I found out myself. It's Sirio based so I am not surprised one bit it threw a rod. You have to break them in at low rpm's and replace the rod before you ever stand on it wide open.
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