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Onan 6.5 NHDFB De-carbonization


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  #21  
Old 10-03-2015, 08:13:33 PM
Sunnyside Sunnyside is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

Thanks all for the continued input------

Got some cleaning done today, and as it was predicted, wasn't much of an ordeal. I will get some pics of the bore tomorrow--didn't feel any lateral movement in the piston. I will also be getting pics of how the valves are seating--now that I have the carbon mostly removed, I can see the mating surfaces, and I don't think the valves are being pulled down sufficiently. Perhaps more carbon on the stem pinching in the valve guide? Pics worth many words--I'll post them tomorrow.
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  #22  
Old 10-03-2015, 08:19:59 PM
len k len k is online now
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Default Re: Onan 6.5 NHDFB De-carbonization

Valves may be sticking, squirt some marval mystery oil on the valve stems and exercise the valves by rotating the crank.
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  #23  
Old 10-04-2015, 01:30:59 AM
Jim Marcozzi Jim Marcozzi is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

With the engine warm, pour a steady flow of water into the intake- enough until the engine starts to bog down. Compensate the RPM dip by opening the throttle to hold at a normal RPM and continue the water flow. Don't stall the engine. Once out of water, let the engine stabilize and allow to run normally for a few minutes to cool, as the decarbonization process produces heat.

This will de-carbon it unlike you could believe. If you'd pull the head afterwards, you'll find a shiny silver combustion chamber and piston top. An alternative to H2O is to use GM Top Engine Cleaner, but you'll produce a ton of white smoke.
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Old 10-04-2015, 01:12:15 PM
Sunnyside Sunnyside is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

Carbon is relenting as you all predicted, and I see nothing that mechanically prevents valves from seating where they should. Do these less than perfect pictures allow opinions as to whether these are closing properly or not?

I can easily believe that this engine will still need some type of chemical or water 'post-assembly' treatment, but at least for what is pictured, based on your previous posts, de-carbonization is almost complete. So far I haven't pulled out the big guns, a wooden stick and 80G Scotchbrite with WD-40 doing the work.
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  #25  
Old 10-04-2015, 01:16:01 PM
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Jim McIntyre Jim McIntyre is online now
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Default Re: Onan 6.5 NHDFB De-carbonization

What does your valve clearance measure?
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  #26  
Old 10-04-2015, 01:26:21 PM
len k len k is online now
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Default Re: Onan 6.5 NHDFB De-carbonization

Looks like you cleaned them better than I would have bothered with. Looks like new!!!

Mentioned cleaning around valves since I once saw one so carboned up with fuzzy carbon around the openings I wondered how air flowed thru it.
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  #27  
Old 10-04-2015, 03:30:53 PM
Sunnyside Sunnyside is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

Jim- I don't know. I only have access to the tops of the valves and pistons--engine is still together.
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Old 10-04-2015, 03:37:06 PM
ArodaPowerCo ArodaPowerCo is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

From what I can see in the pics, the valve faces look clean and sharp with lots of margin. I'd say you're good to go. If you can't see any pitting on the face or seat, they'll seal just fine. Pic attached to clarify terminology.
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  #29  
Old 10-04-2015, 04:01:34 PM
sdowney717 sdowney717 is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

Valves will stick up from the block like that, it is normal appearance. Looks fine.
When cylinders go oval, it may not be enough to feel, only way to know is measure the bores. Engines with lots of wear get ovaled cylinders, but I would think it would then show a ridge you could feel at the top of cylinder where the rings don't slide. Even if worn it will run but it would burn more oil.

With lots of run time, valves can recede into the block, which will tighten the valve lash. On my MCCK I can hear the valve lash when it starts cold. Valves get hot, they get longer. Not enough valve lash, then your motor when it gets hot, it will not be sealing the valves and they can burn and you will lose power.
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