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Valve guides

jabber800

Extreme
Joined
Nov 22, 2007
Messages
68
Location
Coniston Ont.
Im in the process of repairing a Nytro engine that burnt one of the exhaust valves. The engine has what I would consider low mileage at 10,000 km's so the burnt exhaust valve was a bit of a surprise! Even more of a surprise where my results when I decided to check the valve guides on the exhaust side. Wear limiters where exceeded by over 8 thou near the valve pocket side. Has anyone seen this before on these engines? I will be installing new guides and cutting the valve seats on all the exhaust side. Just curious as to why this has happened with this little mileage. I don't even think the Yamaha mileage spec to have valve clearances checked was reached. Any input would be appreciated!
 

Stock, boosted, any modifications to the sled previously?
 
Totally stock sled. Major carbon deposits in and around valve seats and valves.
I'm going to replace all the exhaust side guides and seals. Just seems a bit odd to me.
 
I believe a burnt valve was the reason for the tear down.

Was it ran for an extended period with bad exhaust donuts? Was it ever overheated? What type of oil? I haven't heard of any Nytro motors having valve problems.
 
Engine had lost compression in 2 cylinders. I did a leak down test and verified I had valve leakage on the exhaust side. A burnt valve was what I was expecting to find. The bad valve guide on all the exhaust valves was a very big surprise.
 
Could of burnt a valve from poor fuel or broken flex pipe carbon might be from efi system trying too make up from the burnt valve causing a real rich fuel air mixture
 
Sorry to drag up an old thread, but I have an RS Venture that burned an exhaust valve. Can I just put a new valve in and go again?
 
Sorry to drag up an old thread, but I have an RS Venture that burned an exhaust valve. Can I just put a new valve in and go again?
 

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odds are yes but make sure the guide is good and there are no exh leaks once re assembled unless you like doing this job again.
 
Check the condition of the valve seats and give them all a good lapping. Then, reassemble and check for leaks in the combustion chambers by putting them chamber up and filling them with rubbing alcohol. Check the ports for evidence of leakage. Although I didn't have a burnt valve, I did have leaking valves along with a blown head gasket and this method worked a treat.

If in doubt, take the head to a machine shop.
 
Another valve burned up.
 

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Any idea on the cause?
I sent the head to Quebec and I asked what he thought may have caused it. He was thinking maybe somewhat defective valve right from new.
I did measure exhaust valve clearance and most were below spec. They are replacing all the exhaust valve guides, but intakes are good.
 


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