Smoking at start-up

2 replies to this topic
  • RyJames

Posted April 15, 2011 - 06:50 PM


Just looking for some input. I'm assuming I need to have the head redone on my 98 YZ400F due to what is likely a worn valve/seal. When I first start the bike after it has sat for a little while, I'm burning a little oil. After only a few seconds this goes away and there's no smoke. Is this likely caused by a little oil pooling on the piston due to a bad valve or valve seal? I've never had to rebuild a 4T head and would appreciate any advice or info you can offer.

What is the level of difficulty for this job for a regular guy who likes to learn and is fairly competant mechanically? I'll have to grab a manual right away.

Thanks in advance.

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  • grayracer513

Posted April 15, 2011 - 07:43 PM


If the valves are sealing, you may be able to replace only the seals. The valves themselves can't be refinished, so if they are worn, replacement is the only option. In that event, you need to have the valve seats refinished by a professional machine shop familiar with motorcycle heads. Most of these will charge you the same amount of money whether you have them disassemble and reassemble the head, or give it back to you in pieces, so you'd just as well have them put it together.

Remember that everything has to go back where it came from. The lifters need to go in the same bore they came out of, and if you reuse the valves, so do they. You'll need a valve spring compressor.

The level of difficulty in removing, dismantling, and reassembling the head is in the range of a B+ to A- on an A-D scale. B- manual skills, but A level attention to detail.

  • RyJames

Posted April 16, 2011 - 08:50 AM


Thanks, Gray.

Will start looking for someone I can trust to do any machine work if it is needed. Hopefully it's just the seals. I appreciate your help.

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