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Weird CR250 Starting Problem

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So I had a harescramble last weekend and ended up starting 2.5 min behind my class because of a starting problem with my 2005 CR250. It was starting great all morning and then on the starting line after the national anthem I kicked it over, it ran for about 10 seconds and then died.

After it died the engine felt like it had no compression, there was virtually no resistance in the kickstarter (it was easier to kick than a 125). At this point I thought my race was done. After 2 minutes of kicking someone push started me and the bike started right up. After I got going I was able to restart the bike fine if it stalled and after the race everything felt totally normal including the compression.

It was a very hot day around 90+ degrees. Does anybody have any idea what could've caused this problem? Everything seems fine now but I want to get this sorted out before my next race. Oh and the topend was rebuilt about a month ago so I doubt that's the issue.

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Sounds like you may have a ring that is sticking. Take off the top end and check out the pinston and rings

Stuck ring? I've never heard of this problem, I did a search and it didn't yield anything either. Can you elaborate alittle?

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Carbon build up or more likely damage to the piston top causes the ring groove to squeeze the ring and not allow it to float in the ring groove. This causes it to pull away from the cylinder wall and let all the compression gases by causing effectivly no compression at kicking speeds. Once you get it going it will rung sort of OK and sometimes it will un-stick and run fine for a while or longer

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Also, when a bike sits unused for periods of time, the top end rings can actually build up condensation and a slight layer of oxidation can form in the rings---enough so that you end up with a smoker and/or low compression.

Your rings can occasionally free up and all seems normal, but if the oxidation layer remains, you will likely encounter this again until addressed properly. I have found this mostly as a 4-stroke issue, but do not know if it is exclusively so.

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The thing I'm having trouble understanding is if a stuck ring were the issue it would be a chronic problem wouldn't it? I literally started and turned the bike off about a dozen times that morning with no issues. Then it started fine, with good compression, and died after running for a few seconds. After it died I had no compression. Could the ring have really gotten stuck while running?

I'm not doubting the diagnosis I'll pull it apart and check, I'm just questioning whether a stuck ring would happen suddenly while an engine is running? The sudden occurrence of the problem is what has me wondering. It was literally fine one second and then lost compression the next.

Also with regards to "it will run sort of OK" the bike ran totally normal once started.

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If you haven't already pulled your spark plug out check it too see if it is loose and there if you have already swaped plugs take it back out and make sure there is no dirt between the sealing washer and the head. Also you could check the reeds to make sure 1 is not chiped or not sealing the intake off. If you want to take the exhaust off you could look in through the exhaust port to see what the rings look like.

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When you think about it, what else could it be? 2 stroke compression is about as simple as it gets. Piston, rings, cylinder, head. Piston and rings move, cyl and head don't. Piston and rings seal to the cylinder, cylinder seals to the head. If your head gasket is not bad and the spark plug is not falling out and the piston does not have a hole in it. then it must be the piston/ring seal to the cylinder

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When you think about it, what else could it be? 2 stroke compression is about as simple as it gets. Piston, rings, cylinder, head. Piston and rings move, cyl and head don't. Piston and rings seal to the cylinder, cylinder seals to the head. If your head gasket is not bad and the spark plug is not falling out and the piston does not have a hole in it. then it must be the piston/ring seal to the cylinder

good point, do you think overheating could have anything to do with making the ring stick? I just wonder because I've never had the problem before and this was by far the hottest day I've been on the bike this year. Sitting on the start line idling the bike was definitely getting hot (but not boiling over). Plus the problem went away once I got air flowing through the radiators. Just thinking aloud here...

Oh and I haven't had a chance to pull it apart yet b/c the bike's at a friend's house. I'll check the rings as soon as I can.

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