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valves adjusting question

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Can anybody explain to me why it is that the valves tighten up. It would give me a better understanding on how check and fix them. So do you always put in a thinner shim than the one already in place? (to make room for a feeler gauge) Or could it need a thicker shim? I just dont understand, its probably simple right?

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The valve seats wear and as they do the valves ride higher in the head which then decreases the clearance with the CAM rockers. Thinner shims are needed to increase the clearance.

Typically, one the valve starts to move you don't have long before a head job is needed. Typically you might get 1 or 2 adjustments.

The good news is that the Yamaha's tend ot be pretty good on valves. My 2006 YZ450 valves have been spot on since I first picked up the bike. I check them twice a year.

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That is not quite correct. The valves wear and sink further into the head not the seats. The Titanium valves are softer than the seats and are exposed to more heat than the seats.

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Having the valves exposed more to the heat than the seats, does that mean, that the valves expand when the engine runs really really hot? This would mean that the valve clearance decreases with heat?

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Can anybody explain to me why it is that the valves tighten up. It would give me a better understanding on how check and fix them. So do you always put in a thinner shim than the one already in place? (to make room for a feeler gauge) Or could it need a thicker shim? I just dont understand, its probably simple right?

Lack of maintenance.

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Having the valves exposed more to the heat than the seats, does that mean, that the valves expand when the engine runs really really hot? This would mean that the valve clearance decreases with heat?

When the engine warms up everything expands, the head, valves, all of it. But not everything is made of the same material. Aluminum has a coeficient of expansion about ten-times greater than titanium (or steel). When the Aluminum in the head heats up it expands enough that the cams actually move away from the valve seats, increasing the valve clearance.

This is why, when the intake valves are worn to the point of zero clearance, an engine will not start. But, if you manage to get it started and warmed up, they will restart much easier if they aren't allowed to cool down.

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Lack of maintenance.

Or running valve clearance looser than spec for any length of time.

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A poor quality air filter or an air filter that doesn't fit correctly will let in debris and will wear the material off the valves prematurely

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A poor quality air filter or an air filter that doesn't fit correctly will let in debris and will wear the material off the valves prematurely

Yup. A lot of dirt passing will definately do a number on your valves, cylinder plating as well.

Always use an oiled-foam filter. Avoid oiled-cloth (K&N or similar) filters like the plague. Great for performance but horrible at stopping dirt.

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Also heat is a big culprit in valve wear. The valves must make contact with the seat to transfer the heat created from combustion. If there is no clerance the valve will hang open during combustion compromising the valve and the seats. That is why it is important to check the valves every 25 hours to get the longest valve life.

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Yup. A lot of dirt passing will definately do a number on your valves, cylinder plating as well.

Always use an oiled-foam filter. Avoid oiled-cloth (K&N or similar) filters like the plague. Great for performance but horrible at stopping dirt.

I use twinair filters. I think theyre pretty good at gettin the job done.

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Thanx for the replies on the heating issues.

I hope that the next question is ok, and not considered to be an attempt to start a new thread. If so, pls. let me know.

My YZ250F 2005 will start excellent and always first time at sea level when cold. It will do the same when semi hot. If it is really really hot, then it will NOT start again.

The other guys at the race track call my bike "The Mule". No matter how much I kick it, it will not move an inch.

Once every 25. time it will suddently start and run as if nothing had ever been wrong. Perfect idling, perfect torque etc.

Normally waiting 20-30 min. depending on the temperature it ALWAYS starts FIRST time, as if nothing had happened.

Shimming is within limits, the spark is new, the carb just had a cleaning, the fuel/air mix is perfect when cold, and I use my hot-handle as prescribes, I kick just an inch after TDC, I have an automatic cam.

Just for the fun of it. My son has an YZ450F 2006 with same specs. It will need 2-3 attemts but always starts cold, semi hot and really hot.

Could it be that the heat will change the distance and measurement, so that I should try a number tighter shims?

Any other suggestions? I'm going nuts. Next step will be demolition of the bike. I wouldn't even be cruel enough to try to sell it - I would be doomed to hell for ever.

Thanx in advance.

Ask

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Thanx for the replies on the heating issues.

I hope that the next question is ok, and not considered to be an attempt to start a new thread. If so, pls. let me know.

My YZ250F 2005 will start excellent and always first time at sea level when cold. It will do the same when semi hot. If it is really really hot, then it will NOT start again.

The other guys at the race track call my bike "The Mule". No matter how much I kick it, it will not move an inch.

Once every 25. time it will suddently start and run as if nothing had ever been wrong. Perfect idling, perfect torque etc.

Normally waiting 20-30 min. depending on the temperature it ALWAYS starts FIRST time, as if nothing had happened.

Shimming is within limits, the spark is new, the carb just had a cleaning, the fuel/air mix is perfect when cold, and I use my hot-handle as prescribes, I kick just an inch after TDC, I have an automatic cam.

Just for the fun of it. My son has an YZ450F 2006 with same specs. It will need 2-3 attemts but always starts cold, semi hot and really hot.

Could it be that the heat will change the distance and measurement, so that I should try a number tighter shims?

Any other suggestions? I'm going nuts. Next step will be demolition of the bike. I wouldn't even be cruel enough to try to sell it - I would be doomed to hell for ever.

Thanx in advance.

Ask

Idle is too rich when the bike is hot. Use your idle mixture screw to lean it out some as the temp changes during the day. You might have other issues as well; float level set too high, needle diameter too small, or pilot jet too big.

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2griimjim, i'm very glad you've joined this site as us oldtimers have answered these questions so many times that we just get lazy and say "do a search" or simply don't post a reply at all.

thanks for answering.

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2griimjim, i'm very glad you've joined this site as us oldtimers have answered these questions so many times that we just get lazy and say "do a search" or simply don't post a reply at all.

thanks for answering.

Cut-n-paste is a wonderfull tool:banana:

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ok, so it was hard to start before and still hard to start now. Just finished checking & adjusting the valves. All are within spec. Even checked the cams. Now when I go to start it, it backfires on me. Blue flame and a little pop noise....not every kick, but say every 5 kicks. Really easy to kick when it backfires. Drained the bowl to get rid of the gas thats been sitting in there. Same thing....:bonk: What should I do?

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ok, so it was hard to start before and still hard to start now. Just finished checking & adjusting the valves. All are within spec. Even checked the cams. Now when I go to start it, it backfires on me. Blue flame and a little pop noise....not every kick, but say every 5 kicks. Really easy to kick when it backfires. Drained the bowl to get rid of the gas thats been sitting in there. Same thing....:bonk: What should I do?

Hard to start when? Still starts easy cold and hard to start when hot? How old is the sparkplug?

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Hard to start when? Still starts easy cold and hard to start when hot? How old is the sparkplug?

hard to start before i adjusted the valves. (took about 30 kicks) Valves were out of spec, so I adjusted them within spec. Now it wont start.

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