Smoke when hot after valve job - Honda RC51 Forum : RC51 Motorcycle Forums
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post #1 of 30 (permalink) Old 07-10-2017, 07:04 PM Thread Starter
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Smoke when hot after valve job

Hi everyone,

This past week I did the regular scheduled valve job, and I ended up changing shims on many of the valves. I also changed the mechanical seal for the water pump, which meant I had to drain the oil and coolant and remove the radiators. Bike seems to be running great, smooth and lots of power, but I noticed the temps above 60 mph seem noticeably lower than before. Riding below 60 mph; temps seem regular. I figured this was the new blue fluid Honda sells and/or the valve job but... I was stopped at a light when the bike was hot 210f-220f (where the fan usually comes on and off), and smoke started coming out of the exhaust. I rode the bike home and smoke only came out of the exhaust when the bike was at a light for a minute and it got hot, then it would go away. I took a video of the smoke when I got it home. Please take a look:

https://youtu.be/MvMCfz5YYdo

There is no coolant in the oil, and I can't see any coolant or oil leaks from the outside. I will need to wait until tomorrow before I can bring it in and take a look. I had the bike sitting without oil and coolant for about a week in my apartment while I worked on it, I don't know if I could have dried out a gasket.

What could I have done to cause this? Any help would be appreciated!
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post #2 of 30 (permalink) Old 07-10-2017, 07:27 PM
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What does the smoke smell like? Fuel, oil, or water?
Black smoke is usually very fuel rich, and smells so.
White smoke that dissipates quickly can be water with not much smell. But if it lingers, can be oil, but also smells like burning oil.

How full is your crankcase vent catch tank? You have to drain it via the hose plug periodically. I usually check and drain mine every oil change.
If it's really, really full (though unlikely), some oil residue may be getting sucked into the intakes and getting burned.
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post #3 of 30 (permalink) Old 07-10-2017, 10:09 PM Thread Starter
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Quote:
Originally Posted by SubSailor View Post
What does the smoke smell like? Fuel, oil, or water?
Black smoke is usually very fuel rich, and smells so.
White smoke that dissipates quickly can be water with not much smell. But if it lingers, can be oil, but also smells like burning oil.

How full is your crankcase vent catch tank? You have to drain it via the hose plug periodically. I usually check and drain mine every oil change.
If it's really, really full (though unlikely), some oil residue may be getting sucked into the intakes and getting burned.

I drained the crankcase vent hose last week when I was working on it. Checked it again a few minutes ago and just a dribble of oil came out. I looked inside the air box and found a very small amount of oil in a few corners, so I don't think that is where it is coming from. I tried to look down the throttle bodies with a flashlight and I think I see some oil residue down where the valves open.

I started it back up and it took quite a while for it to heat up but when it got to 220f it started smoking again. The smoke is on the dark side an smells a little burnt, but not like gas. I thought it might burn off if it was air box oil, but the smoke did not go away after 6-7 minutes.
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post #4 of 30 (permalink) Old 07-11-2017, 02:05 PM
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Off the top, possible valve stem seals or rings comes to mind.
Or possibly the PAIR seal under the valve cover allowing oil to leak pass into the exhaust port.
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post #5 of 30 (permalink) Old 07-11-2017, 03:37 PM
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Sounds like a leaking valve stem seal. Had this on a friends bike some years ago. Bluish puffs of smoke mostly when accelerating off from idling at traffic lights or street corners. We took the heads off and found nothing wrong but some oil leakage into the exhaust ports. The exhaust valves (and seals) has the hardest life due to the high temperature.
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post #6 of 30 (permalink) Old 07-11-2017, 05:58 PM Thread Starter
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Thanks for the feedback I have received so far, I really so appreciate it.

In solving this problem how do I go about pinpointing the problem in the best way? I'm thinking I should check the spark plugs to find the problem cylinder. After that I am a bit unsure. Should I remove the air box and throttle bodies to look for oil on the valves? When a valve seal is leaking how will I tell, will it be covered in oil? Also, how do I tell if the PAIR is leaking?

Yesterday I looked down the intake throttle bodies all the way to the tops of the the valves and I think it looked oily, but I think it was both cylinders. Will there naturally be a small amount of oil residue around the intake valves?
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post #7 of 30 (permalink) Old 07-12-2017, 02:00 PM
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If your valve stem seals are worn and leaking, your valve stems and valve heads can be wet with oil film residue instead of being dry.
On the exhaust valve, the oil film can get carbonized from the heat and build up on the valve head if heavy enough.

You might be able to look down into the intake port to check the intake valve.
Unfortunately, about the only way to check the exhaust valve is to remove the exhaust headers and look into the exhaust port.

If the stems seals are shot, the two ways to replace them are to either remove the cams and heads, the use a valve spring compressor to remove the springs and replace the stem seals. Then reassemble.
The other method is to remove the cams and spark plugs, then use compressed air to hold the valves closed and use a special tool to compress the valve springs and remove the collets, the replace the stem seals and reassemble, all the while using compressed air to hold the valves closed.

Either method might prove to be difficult to do with the engine in the frame (especially the front head, due to the frame and steering head proximity).
It may require engine removal to complete the jog. Not insurmountable, but a pain in the butt, and requires special tools to remove and install the swingarm pivot and adjust bolts, and engine mount adjust bolts (if an SP1).
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post #8 of 30 (permalink) Old 07-12-2017, 04:26 PM
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As "Subsailor" says, it is unfortunately a quite big job. But look at it in this way: If it is the valve stem seals being the culprits, well then your best bet is to replace them all anyway. If it is something else that is wrong, you have to go even deeper. One thing is certain though, if you leave it as is, you are going to have lot of problems later on.
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post #9 of 30 (permalink) Old 07-12-2017, 06:08 PM
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This is an interesting thread.
Question:
If valve seals are leaky, will a compression test or a leak-down test help in diagnosing?
Would you typically get that same puff of smoke upon starting a cold engine?
Am I off in outer space with these questions?

Good luck GWS.

John, 2000 RC51 #000100

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post #10 of 30 (permalink) Old 07-12-2017, 06:49 PM
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A compression test will not determine this because of the fact that the valves shut regardless of any leak in the seals. As for starting up with a cold engine I am not sure because when temperature rises the seals will expand more than the valve stems (rubber against steel) and it may occur more clearly when the engine is warm enough. On the other hand when you have this problem and shut off the engine after a ride there will be oil above the valve and it may cause a puff of smoke when you start up again. A leak-down test is usually done by poring down petrol in the bores and determines if the piston rings in one or more of the cans are leaking.
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