Engine life expectancy (Multiple Threads) - Page 8 - Honda CBR250R Forum : Honda CBR 250 Forums
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post #71 of 81 Old 05-12-2016, 03:44 PM
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I have replaced the top end twice due to piston knocking, at ~15k miles and ~30kmiles (at a shop), and I am preparing to do another top end rebuild myself (first time) with about ~50k miles on her. I've taken everything apart and the piston has a thick deposit of carbon on it, the cylinder has wear from knocking, the combustion side of the head is pretty toast, and the camshaft lobes are worn.
I'm no expert but all of this doesn't sound right. I believe that the knocking is from the carbon build up. The carbon is increasing the height of the piston giving it a higher compression ratio. But there shouldn't be that much build up. What I want to know is what kind of riding do you do? A lot of city riding at low RPM's can contribute to carbon building up. Also lugging the engine can do the same. I noticed that you also have an after market pipe. Do you have it tuned at all or stock map? If it running rich carbon with definitely start to form. Also how often do get up near red line and ring her out?

It just doesn't seem right to me. When I did my valve adjustment at 12,000 I looked in through the spark plug hole and my piston looked basically spotless.

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post #72 of 81 Old 05-12-2016, 04:37 PM
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That definitely doesn't sound right. I've got 16.500 miles on my now and no problems whatsoever.

My computer is fighting Aids and Cancer in it's idle time, and yours?
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post #73 of 81 Old 05-12-2016, 05:10 PM
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Thanks for the reply. That must have felt nice to peek at your piston and see that.

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The carbon is increasing the height of the piston giving it a higher compression ratio
Interesting point. But wouldn't the CR be lower with carbon build up? A deposit layer would add to the top of the piston and decrease the stroke from bottom dead center to top dead center? ...also not an expert haha


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What kind of riding do you do? A lot of city riding at low RPM's can contribute to carbon building up
How does low RPM contribute to carbon build up? My riding style has been very dynamic throughout my ownership...I've done a lot of city riding where I shift a lot, but I also have done a lot of weekend riding in the hills, but in all cases I usually ride her +75%. I redline fairly frequently. I also got back from and 8000 mile trip about 3000 miles ago (last September) where I cruised at avg 8500 rpm.

Hahaha, I need to update my mod list; I got rid of that sucker a while ago. It was annoyingly loud to me. I reset the ECU after I put it on and after I re-replaced it with the stock pipe
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post #74 of 81 Old 05-12-2016, 05:13 PM
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Schroeder, have you taken a look at your piston?

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post #75 of 81 Old 05-12-2016, 05:28 PM
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Originally Posted by efoo View Post
Interesting point. But wouldn't the CR be lower with carbon build up? A deposit layer would add to the top of the piston and decrease the stroke from bottom dead center to top dead center? ...also not an expert haha
No; the stroke doesn't change due to carbon build up. The stroke will remain the same as the carbon is simply lengthening the "rod" (consisting of the piston and connecting rod), not changing the distance it travels. But, since the combustion chamber is sealed, adding material to the top of the piston reduces the volume inside the combustion chamber, generating more pressure when the piston comes up to compress the fuel and air mixture. Too much pressure, predetonation/pinging happens.


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How does low RPM contribute to carbon build up? My riding style has been very dynamic throughout my ownership...I've done a lot of city riding where I shift a lot, but I also have done a lot of weekend riding in the hills, but in all cases I usually ride her +75%. I redline fairly frequently. I also got back from and 8000 mile trip about 3000 miles ago (last September) where I cruised at avg 8500 rpm.

Hahaha, I need to update my mod list; I got rid of that sucker a while ago. It was annoyingly loud to me. I reset the ECU after I put it on and after I re-replaced it with the stock pipe
Low RPM exclusive riding doesn't allow the combustion chamber to experience full pressures and stresses, full operating temperature, and also the engine turns at a slower speed. Therefore, it's more difficult for the combustion chamber to "self-clean" if it's not rung out occasionally.

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post #76 of 81 Old 05-12-2016, 06:02 PM
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Schroeder, have you taken a look at your piston?
No, I'm not much of a mechanic. I had my bike serviced at a Honda dealer and they said everything was ok (they changed the spark plug though at 16.000 miles even though I think that wasn't necessary). The bike runs great like on day one.

My computer is fighting Aids and Cancer in it's idle time, and yours?
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post #77 of 81 Old 05-12-2016, 08:32 PM
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No; the stroke doesn't change due to carbon build up. The stroke will remain the same as the carbon is simply lengthening the "rod" (consisting of the piston and connecting rod), not changing the distance it travels. But, since the combustion chamber is sealed, adding material to the top of the piston reduces the volume inside the combustion chamber, generating more pressure when the piston comes up to compress the fuel and air mixture. Too much pressure, predetonation/pinging happens.
Ahh, I get it; the intake of fuel/air is mapped for a certain volume, therefore decreasing that volume increases pressure, predetonation, etc.


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Low RPM exclusive riding doesn't allow the combustion chamber to experience full pressures and stresses, full operating temperature, and also the engine turns at a slower speed. Therefore, it's more difficult for the combustion chamber to "self-clean" if it's not rung out occasionally.
So high heat and high presure help "clean out" the combustion chamber by utilizing intake as efficiently as possible, which would in turn maximize exhaust volume?

Thanks for your insight. Do you have any thoughts on transmission maintenance for this bike, like inspection intervals?

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post #78 of 81 Old 05-12-2016, 08:34 PM
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No, I'm not much of a mechanic. I had my bike serviced at a Honda dealer and they said everything was ok (they changed the spark plug though at 16.000 miles even though I think that wasn't necessary). The bike runs great like on day one.
That's great. How many miles do you have on it now?

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post #79 of 81 Old 05-12-2016, 08:55 PM
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That's great. How many miles do you have on it now?
I just noticed that I had a calculation error on my side. The plug was changed at the 24,000km service which is just ~14,900 miles, not 16,000. I have now ~26,700 km which is ~ 16,590 miles.

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post #80 of 81 Old 05-12-2016, 10:33 PM
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Originally Posted by Schroeder View Post
I just noticed that I had a calculation error on my side. The plug was changed at the 24,000km service which is just ~14,900 miles, not 16,000. I have now ~26,700 km which is ~ 16,590 miles.
Okay cool. I think your next valve check is around 50km, probably 48000km. Have them take a peek next time if its easy. Although by the sound of it, that won't be for another 24000km. Hahaha the imperial system is such ********************. I just spent a month in Northern Europe and was loving all the kilometers and liters. It's sad to be back in that regard

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