1999 YZ400F Update!

ok got the head and cylinder off this weekend and there is quite a bit of carbon build up-how should I go about cleaning the head,. sandblast? Also I could tell that there was oil getting in the gas but the rings look ok, does this mean anything? could oil be getting in from the valves?

maybe soda blast it?

yes oil could be getting in from the valves

Unless you plan to refinish the seats and replace the valves, don't blast it. Soak the carbon free with fuel injector cleaner and brush it out without marring the seats or the valve faces.

What indicates to you that oil is getting into the chamber?

just the fact that the rings didn't look bad to me. I'm used to seeing 2 smoke rings, this was my first 4-stroke top end rebuild

So because the rings don't look bad, you think oil is getting into the combustion chamber? I don't follow.

the rings don't look like they're shot so I'm assuming oil is getting into the combustion chamber through a leaky valve...is there a way to test for this?

The question was, "what makes you think there is oil in the combustion chamber?"

Did the bike use oil? Did it smoke?

the rings don't look like they're shot so I'm assuming oil is getting into the combustion chamber through a leaky valve...is there a way to test for this?

if you are that worried about it just replace your stinking valve stem seals. if you are going to rebuild it do it right. dont do the bare minimum

The question was, "what makes you think there is oil in the combustion chamber?"

Did the bike use oil? Did it smoke?

Yes, at the beginning of the thread I said it smokes and it smells like oil either the rings are shot or a leaky valve but like I said the rings look to be ok so I'll guess I'll just put this new top end in and see where it takes me. I don't have the experience to take apart the head and replace the valves and all that stuff so if it is a leaky valve i'll pop the head off and take it in next week.

You should de-glaze the cylinder and replace the rings before you assemble it. Take the head to a shop and have the valve guide seals replaced. They're too cheap to bypass that step.

You won't be able to see the faults that cause the rings not to seal in most cases. However, smoke in the exhaust can be used to diagnose the source of oil in the chamber. Rings normally tend to cause smoke under a load more than at other times. When they first start leaking, valve stem seals will cause a puff of smoke on startup after the engine sits a while. When they get a bit worse, you will see a puff of smoke as the throttle is opened after decelerating. If the seals are bad, but the rings OK, the smoke won't continue after the initial puff.

You should de-glaze the cylinder and replace the rings before you assemble it. Take the head to a shop and have the valve guide seals replaced. They're too cheap to bypass that step.

You won't be able to see the faults that cause the rings not to seal in most cases. However, smoke in the exhaust can be used to diagnose the source of oil in the chamber. Rings normally tend to cause smoke under a load more than at other times. When they first start leaking, valve stem seals will cause a puff of smoke on startup after the engine sits a while. When they get a bit worse, you will see a puff of smoke as the throttle is opened after decelerating. If the seals are bad, but the rings OK, the smoke won't continue after the initial puff.

From your description it sounds like its the rings....Anyway I got the cylinder de-glazed last week when I got it off and I ended up just doing the valves myself..not as hard as I thought, still would take it to a shop next time. I'm in another rut right now though...after getting the head and cylinder back on I put in the exhaust cam and go to do the intake but there is not enough slack on the chain to get it over the intake cam-suggestions on what to do?

The chain can't generally be installed with the cams in place. Position the intake cam in the center of the head with the engine at TDC, the exhaust cam aligned, and the chain over the E cam. Run the chain over the intake cam so that there are 12 or 13 pins between the two marks at 12:00 on the two cams, then roll the intake cam into place, seat it, and check the timing.

If there isn't enough slack to do this, you've probably drooped a link off of thr crank sprocket.

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