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Discussion Starter #1
Hello!
I have a Stock 2000 Honda 400ex that has seemed to start smoking recently and also low in power. It runs good till it gets hot then it doesn't want to idle or start without some throttle. I don't know if this has to do with anything; but the exhaust cracked right at the head, I thought that was very weird. I suspect its rings and its probably going to need a new top end rebuild. Anyone else have a problem like this?? Let me know. Thanks!
 

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The smoke could be rings, but I'd say the power loss has more to do with the header being cracked.
 
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Your topend is probably completely worn out. Don't shortcut this overhaul. There are a lot of people here that know this machine well. Bore, valve job, check rockers, cam, cam chain, chain guides even the crank. This is a good time to clean the carb and fuel tank.
 

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Discussion Starter #4
The smoke could be rings, but I'd say the power loss has more to do with the header being cracked.
Thats what I figured. Thanks for the help!
 

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Discussion Starter #5
Your topend is probably completely worn out. Don't shortcut this overhaul. There are a lot of people here that know this machine well. Bore, valve job, check rockers, cam, cam chain, chain guides even the crank. This is a good time to clean the carb and fuel tank.
Thats what I didn't want to hear! LOL. Thanks for the help!
 
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Start by doing a compression test. Anything close to 100 or under start pulling the engine apart. Start by pulling the topend off. Check the rod for any up and down play. And i mean any. If you can feel anything the crank needs replaced. After that take the cylinder to a competent engine machinist. They will measure and tell you what you need to do to make it run right. Now is the time if you are looking for more power to go 416. If not just follow the instructions of the engine guy. If he says he dosnt need the piston turn and run he dosnt know what he is doing. If he does then he knows that every piston is sligtly different and he will taylor make that piston fit. I suggest wiseco for piston and ring kits. And cometic for gaskets. Have the head and jug skimmed to be sure the surfaces are perfectly flat. And for god sake use a torque wrench and follow all the suggested torque specs. If you do go 416 i highly suggest the gt thunder hd studs. Also i would upgrade your stock timing chain for the crf450. It drops right in. No modification necessary. I would also have your valves laped and put the new valve stem seals on while they are doing that. My .02
 

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Discussion Starter #7
Start by doing a compression test. Anything close to 100 or under start pulling the engine apart. Start by pulling the topend off. Check the rod for any up and down play. And i mean any. If you can feel anything the crank needs replaced. After that take the cylinder to a competent engine machinist. They will measure and tell you what you need to do to make it run right. Now is the time if you are looking for more power to go 416. If not just follow the instructions of the engine guy. If he says he dosnt need the piston turn and run he dosnt know what he is doing. If he does then he knows that every piston is sligtly different and he will taylor make that piston fit. I suggest wiseco for piston and ring kits. And cometic for gaskets. Have the head and jug skimmed to be sure the surfaces are perfectly flat. And for god sake use a torque wrench and follow all the suggested torque specs. If you do go 416 i highly suggest the gt thunder hd studs. Also i would upgrade your stock timing chain for the crf450. It drops right in. No modification necessary. I would also have your valves laped and put the new valve stem seals on while they are doing that. My .02

Thank you for the very helpful information!! I really appreciate it. When I get time I'm going to start tearing her apart. I'll keep you guys updated! Thanks!!!
 
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