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Anyone know how I managed to do this?
- Thread starter Jaymod
- Start date
The last time that happened to me was on my '69 Triumph 650 Tiger. It had a single carb and was told it was a more common failure on the dual carb Bonneville's with one carb going lean.
The failure happened at highway speeds. Mechanic could not figure out what caused it but back in those days we had to rebuild the engines every 2yrs so didn't think much of it but with today's bikes, I would certainly question things the way you are doing.
The failure happened at highway speeds. Mechanic could not figure out what caused it but back in those days we had to rebuild the engines every 2yrs so didn't think much of it but with today's bikes, I would certainly question things the way you are doing.
Well presumably he went lean as he ran low on gas and then switched to reserve. I've done that countless times though and never holed a piston. Interesting mystery. Maybe tried to keep the speed up with lots of throttle for a few seconds before flipping the valve?
Mad Mike
Well-known member
My guess is timing is set too far advanced - check timing and make sure the woodruff key is still in the flywheel. Could also be the wrong sparkplug,
Running lean can damage pistons too and that's a possibility -- but lean tends to heat everything and your end up with damage at the edges or cause a seize -- not usually with a hole. Lean condition also leaves a lot of coal on the piston, the whole top would be black with carbon, your piston top view doesn't suggest lean caused that hole..
I thought a sooty piston was a sign of a little too rich and his clean piston was a sign of running lean.
Are you sure this was related to reserve? You may have been running lean for a long time and the power loss that caused you to think reserve was required may have been the start of the hole.
Center electrode eroded completely away is strange. Normally the tab electrode takes the beating first.
Lean under load. That piston is too clean. Running lean at high speed and load to begin with, and then running out of fuel leaned it out further and put it over the edge. It is also possible that the spark plug was running too hot and acting as a preignition source.
Blown head gasket would lead to steam cleaning ... and coolant temp isn't a good indicator of piston temp. Did you know the head gasket was leaking before this happened? The usual symptom (been there, done that) is that it pumps coolant out into the overflow tank and won't draw it back in.
I revise my hypothetical sequence of events. Leaking head gasket allowed coolant level to drop too low, leading to local overheating in the cylinder head, which when combined with the lean condition from running out of fuel, cooked the piston.
I revise my hypothetical sequence of events. Leaking head gasket allowed coolant level to drop too low, leading to local overheating in the cylinder head, which when combined with the lean condition from running out of fuel, cooked the piston.
I never noticed anything before, coolant was behaving normally.. even after, I started it in the driveway to confirm it was f’ed. had the rad cap off no bubbles.. then I pulled the breather hose from the crankcase that fed back to the carb, man there was a good amount of smoke.. that’s when I knew for sure it had to come apart lol.
So definitely a prolonged lean condition, plug too hot, and maybe the head gasket leaked at those elevated temperatures?
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So definitely a prolonged lean condition, plug too hot, and maybe the head gasket leaked at those elevated temperatures?
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Looks OK to me. I would be tearing down carbs prior to running it again. Figure out what is different about the bad cylinder.
This is the other piston
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Mad Mike
Well-known member
Don't you think this would create damage at the edges rather than the center of the piston? Typically lean leads to piston swelling and seizure.
When I've pulled coolant-washed pistons (head gasket leak), they have been uniformly clean.
When I've pulled lean run pistons, they are usually carboned up entirely and show damage on the edges.
I would expect detonation to show up as a "peppered" look near the edges. It is possible to overheat (and melt) a piston without detonation ... Whether it seizes in the bore or not, depends on the amount of piston-to-cylinder clearance available for it to expand. If it has low compression (it's an EX500) and operates on decent fuel so that it doesn't detonate, and has enough piston-to-cylinder clearance so that it doesn't seize ...
There's plenty of ways to break stuff. When my cbr125 ran lean at WFO (which is standard operating condition on that bike) due to a failing fuel pump, I cracked the piston all the way across the top, from one pin-bore to the other.
Too rich can break stuff, too. If it doesn't wear out piston and cylinder walls by excess fuel washing oil off the cylinder walls, or cause bottom-end wear via fuel dilution of the oil, the carbon build-up will get into the ring-lands, seize the rings in the grooves, and score the cylinder. Badness follows.
There's plenty of ways to break stuff. When my cbr125 ran lean at WFO (which is standard operating condition on that bike) due to a failing fuel pump, I cracked the piston all the way across the top, from one pin-bore to the other.
Too rich can break stuff, too. If it doesn't wear out piston and cylinder walls by excess fuel washing oil off the cylinder walls, or cause bottom-end wear via fuel dilution of the oil, the carbon build-up will get into the ring-lands, seize the rings in the grooves, and score the cylinder. Badness follows.
I just wanted to thank everyone for their input. Just so I understand, it was most likely a prolonged lean condition? I will check that everything is right when I get it back together. Maybe get a wide band O2 sensor and a way to read it
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I like wideband gauges. Be careful with the results. As your bike has two entirely separate mixtures, rich on one cylinder and lean on the other could look ok on the gauge. With one carb feeding an engine, there will be some variation but you can't have the huge difference between cylinders that you can get on a bike. Now, if you put the sensor in the header before the merge you could get performance of a specific cylinder. Welding a bung into a shiny header will look like poo. I don't know of wideband gauges that do multiple simultaneous channels.
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For the 500 I have an extra set of headers, so just weld a bung into each pipe, the 250 headers already look like crap and you can’t really see them anyway, the dirt bike is purely functional so seeing a welded bung wouldn’t bother me. Then I get to spend next summer moving the system from bike to bike, and pulling carbs repeatedly lol
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