1990 LT250R Quad Racer Melt Down

1990 LT250R Quad Racer Melt Down

This quarter liter LT has been through just about everything. When the piston melted the molten metal sprayed the inside of the engine and ruined the crankshaft and rod bearings, and it was already on its next to the last bore size. It may have failed from leaning out while running out of gas, though several other problems were discovered during the inspection.

It's easy to see the piston "vent." The top ring is stuck. There's almost no color at all inside the piston (under the crown). That usually means the engine wasn't run for an extended period of time in an over-heated condition.

Inside the left engine cover I found an inch deep puddle of oil and water. It was being sprayed inside the case as the flywheel mixed it up. Over time the painted surface of the engine has started to break down - it's showing some corrosion now..

This "upside down" shot shows the balancer was doing some spraying. Additionally, its seal was "shot."

The rotor lived too long in the wet case. Even with the rust it still performs ok.

The stator and associated parts started to corrode. Even in this condition the engine runs well and the lights work fine.

The spark plug has seen better days - as has the rest of the combustion chamber. Its sealing surface is very uneven.

Impression
Though it didn't happen as a result of the melt down this time, the head shows what looks like a part of a roller bearing that was squished between it and the piston. I noticed the spark plug was sticking through the head a little more than normal. This head has been cut a few times. In fact to help prevent this from happening again I cc'd the volume above the piston with the piston at TDC to determine the compression ratio - it was 17.6:1 (UCCR), and was running pump gas! I opened up the chamber to lower the compression ratio.
What It Is
This barrel is ported and it's fitted with a pipe from the same builder. It's a "National" motor, as witnessed by the wording carved into the outside of the cylinder. When indexed, the basic port timing looks like this:

Exhaust opens at 89 degrees ATDC for a duration of 181 degrees.

Transfer ports open at 119 degrees ATDC for a duration of 122 degrees.

Very mild timing. Very high compression ratio.

The intake is "open!"

The exhaust port is wide and smooth.

Rough Stuff
What I noticed more when looking through the exhaust ports is the rough cylinder wall in the background. There are some light vertical scratches on its surface from the warped, over heated, sticking upper ring, but the bore was in poor shape to start with. It seems the last time the cylinder was bored it may have been bored a little too close to the final dimension and may not have left the operator enough material left to properly hone it.

The boring bar leaves circular cutting marks after it passes through - the cylinder hone is supposed to remove them as it enlarges the bore just enough to leave the proper piston to cylinder clearance. It seems that this was not done right.

Again, the poor bore is noticed as well as the rough deck. The vertical scrapes are not lined up with cylinder studs.

The traditional looking port work. This engine has been taken apart many times - its lower deck was not flat.

Looking at it from above the LT seems to have lots of port area.

The transfer tunnels - the divider has a knife edge.

Blow Out
With the engine together and torqued up tight I set up the leak down tester to verify the work.The Suzuki power valve engine makes it a little tough to test just the top end since it allows air to enter the lower end during this kind of test. The result is having to pressure test the complete engine - crankcase and all. Putting a finger on the collar that's inside the output shaft seal will prevent it from popping out of the engine under the pressure.

The 6 psi I pumped in started coming out. It showed bubbles under the reed cage at the base gasket.

This air leak was a very bad one. Air was coming out of the engine just about as fast as it was going in.

As it turns out there was a very slight vertical alignment problem with the engine cases. Perhaps one of them was replaced at one time. The right case was .0015" higher than the left one - just enough to let air right out at the junction of the cases. I took the top end off and used a small amount of Three Bond 1211 to help seal the gap.

Was it a leak like this that caused this engine failure? Maybe it was the leaking main seals. Perhaps it was the poor bore. It could have been the ultra high compression ratio. With so many problems together in one place it doesn't really matter which one caused it, what matters is that it was fixed.

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This quarter liter LT has been through just about everything. When the piston melted the molten metal sprayed the inside of the engine and ruined the crankshaft and rod bearings, and it was already on its next to the last bore size. It may have failed from leaning out while running out of gas, though several other problems were discovered during the inspection.
It's easy to see the piston "vent." The top ring is stuck. There's almost no color at all inside the piston (under the crown). That usually means the engine wasn't run for an extended period of time in an over-heated condition.	Inside the left engine cover I found an inch deep puddle of oil and water. It was being sprayed inside the case as the flywheel mixed it up. Over time the painted surface of the engine has started to break down - it's showing some corrosion now..
This "upside down" shot shows the balancer was doing some spraying. Additionally, its seal was "shot."	The rotor lived too long in the wet case. Even with the rust it still performs ok.
The stator and associated parts started to corrode. Even in this condition the engine runs well and the lights work fine.	The spark plug has seen better days - as has the rest of the combustion chamber. Its sealing surface is very uneven.
Impression Though it didn't happen as a result of the melt down this time, the head shows what looks like a part of a roller bearing that was squished between it and the piston. I noticed the spark plug was sticking through the head a little more than normal. This head has been cut a few times. In fact to help prevent this from happening again I cc'd the volume above the piston with the piston at TDC to determine the compression ratio - it was 17.6:1 (UCCR), and was running pump gas! I opened up the chamber to lower the compression ratio. What It Is This barrel is ported and it's fitted with a pipe from the same builder. It's a "National" motor, as witnessed by the wording carved into the outside of the cylinder. When indexed, the basic port timing looks like this: Exhaust opens at 89 degrees ATDC for a duration of 181 degrees. Transfer ports open at 119 degrees ATDC for a duration of 122 degrees. Very mild timing. Very high compression ratio.
The intake is "open!"	The exhaust port is wide and smooth.
Rough Stuff What I noticed more when looking through the exhaust ports is the rough cylinder wall in the background. There are some light vertical scratches on its surface from the warped, over heated, sticking upper ring, but the bore was in poor shape to start with. It seems the last time the cylinder was bored it may have been bored a little too close to the final dimension and may not have left the operator enough material left to properly hone it. The boring bar leaves circular cutting marks after it passes through - the cylinder hone is supposed to remove them as it enlarges the bore just enough to leave the proper piston to cylinder clearance. It seems that this was not done right.
Again, the poor bore is noticed as well as the rough deck. The vertical scrapes are not lined up with cylinder studs.	The traditional looking port work. This engine has been taken apart many times - its lower deck was not flat.
Looking at it from above the LT seems to have lots of port area.	The transfer tunnels - the divider has a knife edge.
Blow Out With the engine together and torqued up tight I set up the leak down tester to verify the work.The Suzuki power valve engine makes it a little tough to test just the top end since it allows air to enter the lower end during this kind of test. The result is having to pressure test the complete engine - crankcase and all. Putting a finger on the collar that's inside the output shaft seal will prevent it from popping out of the engine under the pressure.
The 6 psi I pumped in started coming out. It showed bubbles under the reed cage at the base gasket.	This air leak was a very bad one. Air was coming out of the engine just about as fast as it was going in. As it turns out there was a very slight vertical alignment problem with the engine cases. Perhaps one of them was replaced at one time. The right case was .0015" higher than the left one - just enough to let air right out at the junction of the cases. I took the top end off and used a small amount of Three Bond 1211 to help seal the gap. Was it a leak like this that caused this engine failure? Maybe it was the leaking main seals. Perhaps it was the poor bore. It could have been the ultra high compression ratio. With so many problems together in one place it doesn't really matter which one caused it, what matters is that it was fixed.