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Discussion Starter · #1 ·
I have a 1974 CB200 that lost total compression on one side while riding. Below is a pic of the carnage.

I just replaced the piston with a new old stock original honda piston and put all new rings on both. Slapped it back together and it's running like new.

I understand that this happens when a carburetor is running too lean and therefore too hot. I thought I had the carb adjusted well but clearly I did not. Is this somewhat common on these older twins? What are some things I can do to help assure this does not happen again?


DSCN4894.jpg
 

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I'm surprised that with a hole that big you didn't do any other damage to your engine? Did you change the oil and clean the centrifugal filter to check for any bits of the piston? What about your valves? You say the engine now runs sweet, have you checked compression too? If it's all ok you've been very lucky!
Nigel.
 

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Discussion Starter · #4 ·
I'm surprised that with a hole that big you didn't do any other damage to your engine? Did you change the oil and clean the centrifugal filter to check for any bits of the piston? What about your valves? You say the engine now runs sweet, have you checked compression too? If it's all ok you've been very lucky!
Nigel.
Changed oil and there were for sure lots of tiny metallic bits but nothing large. I have only started up the bike and let it idle so far. I haven't ridden it yet. Still need to do some more fine tuning and setup before I ride it. Will do compression test soon.
 

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Discussion Starter · #5 ·
One thing that is a bit curious to me is that only one piston developed a hole. The other piston is perfectly fine. I had both carburetors adjusted the same of course. Seems really strange to me that only one piston would develop this damage and not the other.
 

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One thing that is a bit curious to me is that only one piston developed a hole. The other piston is perfectly fine. I had both carburetors adjusted the same of course. Seems really strange to me that only one piston would develop this damage and not the other.
Why do you think you'd get an air leak in both manifolds at the same time ?
 

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You checked the ignition timing ? Could be too advanced
 

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Discussion Starter · #8 ·
Air leak at the manifold? You mean where the carbs connect to the intake manifold? As far as I know I didn't have any leaks there. However, I am suspicious that it's possible I head a head gasket leak. This photo is from during flat sanding the head. See the unsanded areas? It took a good deal of sanding to finally remove all of that area on the far right side of the photo to the right of the cylinder. I'm fairly competent but somewhat new to this kind of motorcycle work. Is a mild head gasket leak possible? And it could have leaned out that piston? Eventually creating that piston hole? If there was a head gasket leak it was mild because I was flying around at 75mph on that little bike without any clear symptoms of a serious problem.
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Air leak at the manifold? You mean where the carbs connect to the intake manifold? As far as I know I didn't have any leaks there. However, I am suspicious that it's possible I head a head gasket leak. This photo is from during flat sanding the head. See the unsanded areas? It took a good deal of sanding to finally remove all of that area on the far right side of the photo to the right of the cylinder. I'm fairly competent but somewhat new to this kind of motorcycle work. Is a mild head gasket leak possible? And it could have leaned out that piston? Eventually creating that piston hole? If there was a head gasket leak it was mild because I was flying around at 75mph on that little bike without any clear symptoms of a serious problem.
View attachment 197537
The carbs are attached to the head via a rubber manifold, over time rubber cracks, cracks leak, leaking air in after the carb will give you a lean mix, a lean mix burns hotter, hotter combustion chamber temperatures melt pistons
 

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If the timing is too advanced and one of the two carbs is a little lean then it could explain the hole in the piston.
 
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