Author Topic: Catastrophic engine failure - 91 Sporty  (Read 574 times)

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Offline schwaber77

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Catastrophic engine failure - 91 Sporty
« on: August 17, 2017, 09:23:10 AM »
Hey guys,

A buddy brought his bike over the other night to put it up on the lift and take a look at it and what was found was not good. He said he had rebuilt the carb and got it fired up and it sounded terrible which at that point he decided to bring it over to my place.

The bike is a 1991 Sporty 1200 with 8500 miles on it that has been customized big time, not just in appearance but it has had some pretty significant motor work done to it too. Unfortunately, what exactly has been done to it is unknown. It had been parked for years and my buddy got it from his buddy who had gotten it in some sort of a home foreclosure scenario.

I turned it over by hand and it seemed smooth. Attempted to fire it up and for the few seconds that it ran, it sounded like a machine gun going off.

So I did a compression test, on a cold motor: 120psi on the front cylinder and 190psi on the rear - obviously a significant issue

I did a leak down test and the front cylinder wouldn't hold pressure at all, air flowed freely through the vents to the air cleaner and the rear cylinder was better but still didn't hold pressure for long at all.

So I pulled out the bore scope camera and went in through the spark plug hole and found a lot of damage...see attached

Did they build this motor without checking for valve clearance and it turned out there was no clearance? Did they adjust the push rods wrong? Something else? I'm guessing it had to do with the setup as both pistons are showing damage....

So obviously a rebuild is needed but to what extent is the question...

The large missing chunk from the front cylinder piston is no where to be found and with all the dings and dents on the piston heads and vertical scratches on the cylinder walls, I'm guessing metal went through the motor. So then I have to believe that an entire engine rebuild is in order and not just the top end... It seems apparent that there has got to be at least a bent valve(s) and damage on the valve seats and surrounding areas.

I know, the answer to most of these questions would be found with tearing the motor down completely but I figured I'd post to see if anyone has any feedback and at least, for your viewing pleasure/sadness as it seems obvious to me, this is what happens when a motor is not properly put together. As of right now, I'm not gonna tear into it or work on it anymore until I hear back from my buddy as far as what he wants to do with it (ie. rebuild, drop a new-used motor in it, or scrap it all together...).

I appreciate any feedback,

Offline Pete_Vit

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Re: Catastrophic engine failure - 91 Sporty
« Reply #1 on: August 17, 2017, 10:46:12 AM »
wow that sucks, I'm wondering if the right pistons were used  :idunno:
at first sight it looks like the bore is off  :scratch: and or the rings never seated, gonna take another look
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Offline Panzer

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Re: Catastrophic engine failure - 91 Sporty
« Reply #2 on: August 17, 2017, 01:37:49 PM »
Yikes, not looking good at all.
You're doing the right thing on holding up till you show the owner.
Does the owner have deep pockets?
Like you said, you won't know until you do a tare down.........well, you knew that!
Due to conservation the light at the end of the tunnel has been turned off.

Offline aswracing

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Re: Catastrophic engine failure - 91 Sporty
« Reply #3 on: August 18, 2017, 06:43:27 AM »
The fracturing of a valve pocket like that is a classic detonation failure. That's where the piston breaks when you have detonation going on.

Offline doubletrouble

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Re: Catastrophic engine failure - 91 Sporty
« Reply #4 on: August 18, 2017, 07:49:47 AM »
I guess they removed the nitrous before he got the bike.

Offline Burnout

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Re: Catastrophic engine failure - 91 Sporty
« Reply #5 on: September 14, 2017, 08:00:04 AM »
Good chance the damage goes deeper, my guess is the rod bearing is out and the piston is hitting the head.

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Offline prodrag1320

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Re: Catastrophic engine failure - 91 Sporty
« Reply #6 on: September 15, 2017, 04:34:33 AM »
big time detionation,also wouldn't be surprised to see KB pistons in it too,seen this many times with the hypertecnic KB stuff