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Onan 6.5 NHDFB De-carbonization


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  #1  
Old 10-02-2015, 08:47:53 PM
Sunnyside Sunnyside is offline
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Default Onan 6.5 NHDFB De-carbonization

These pics show an Onan 6.5 NHDFB SN A 943109475 Spec 1H


The generator sat in a basement since 1999, with the heads off. I have a note from the original owner explaining to his associate that the "shop says the engine has too much carbon, not worth the expense to clean it out, cylinders are oval, best we buy a new one".

BUT, knowing these engines were generally better than the people that operated them , I want to ONLY clean the carbon mess without disassembling the engine, put the heads back on, get it to run, and come to my own conclusion as to what the problem is- with the help of this forum, I hope.

In reading about mechanical carbon removal, I'm leaning towards less chemicals, no lye, long-term soaking in gas/WD-40/etc., and brass brushes. I haven't found Scotchbrite mentioned yet--OK at say 150? Anything better than Scotchbrite? Any suggestions would be appreciated.

I realize that only a tear down would allow getting at all the carbon, but if I get say 80%, especially at the valve seats, it might run, and some carbon chemical cleaning would be possible--as long as the source of the problem is identified. The unit was on a service truck, used for a few minutes , and, used irregularly. Probably built up carbon from the get go, then someone 'fixed' the carb to get more power, or altered the timing to get rid of the pinging.

Am I on the right track?
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  #2  
Old 10-02-2015, 09:05:28 PM
Kimbra Dean Kimbra Dean is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

I am sure there may be some better methods but when I decarbon small engines I usually just use a scraper, a screwdriver as a scraper, and maybe a wire brush. Of course you need to be careful not to scratch up the gasket surfaces and cylinder wall. I do my scraping and wire brushing with the cylinder at TDC to prevent any damage to the cylinder wall. You can work around the valves with a Wire brush while they are open to clean some of the carbon from the back of the valve and the valve seat. Just remember that the piston and block are aluminum so be careful not to scratch them up too bad.
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Old 10-02-2015, 09:18:37 PM
len k len k is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

The piston, valves and head look like many other small engines. Try not to scratch it much, but on other engines I use a flexable putty knife to clean the broad wide sections. A wooden popcycle stick with end broke off flat to clean valves and seats. Then for the circular sections of the head I've found light use of a ~ 2inch dia wire wheel ($3) on a air powered die grinder ( ~ $30) fits just about perfect, and does a great job without any noticeable scratching. And a razor held perpendicular to the surface to scrap off any carbon/junk.

If you need a narrower putty knife I've taken a butter knife and ground the tip of it into a flat scraper. On other engines like lawn mowers and snow blowers I reuse the head gasket. Just look it over to make sure there as no gaps. If it ripped apart during disassembly, I just make sure I put it back exactly as I took it apart. So the ripped edges mate, then I may tighten head bolts a LITTLE tighter than usual. Never had any problems.

And as Dean says, do this with piston at TDC to keep the junk out of cylinder.
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Old 10-02-2015, 09:18:38 PM
David C David C is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

The piston that is pictured doesn't have enough carbon to worry about. I would carefully clean around the valves when they are closed with a small wire brush (tooth brush size). I would carefully clean the back side of the valve with a small screwdriver. If the head gaskets are still around and they are the laminated style, you should be able to reuse them. Torque the bolts according to the manual. It would probably do wonders on the generator to get it up and running and load it up to rated capacity for a hour or so.

David C.
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Old 10-02-2015, 09:32:34 PM
len k len k is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

For next time there's a water trick that supposed to de-carbonize it. After it's warmed up trickle a SMALL stream of water into carb inlet while it's running. Take it easy so you don't get so much water in it that you hydrolock the piston. As it flashes to steam it greatly expands and supposed to remove/blow-out the carbon. After water treatment I'ld run it at least 15 minutes maybe 30 to dry out things to make sure the valves, piston , rings and muffler are dry and don't rust.
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Old 10-02-2015, 09:47:14 PM
JohnnyC JohnnyC is online now
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Default Re: Onan 6.5 NHDFB De-carbonization

ATF (automatic transmission fluid), water, and ground up walnut shells will work. I like using ATF because it really pisses off the neighbor if their house is nearby . Also, keeps the mosquitoes under control in the summer.

JohnnyC
New Jersey
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Old 10-02-2015, 11:19:09 PM
Sunnyside Sunnyside is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

Thank you all for the info and ideas-----

You are more aggressive than I was anticipating. Good, the job will go faster. I certainly like hearing that the carbon build up is not at extraordinary levels. I will assemble my army of various scrapers tomorrow, get the piston TDC, and start the war--------
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Old 10-03-2015, 12:10:16 AM
ArodaPowerCo ArodaPowerCo is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

Not so much "war" as "skirmish"... I agree with the others. Doesn't look too bad. I've seen much worse.

My personal favorite tool for the job is a 3M Roloc disc on a small air grinder. The Scotch Brite pads work awesome. Use a light touch, and like others mentioned, piston at TDC. When done with abrasive, clean with carburetor cleaner and compressed air. Get all the carbon out. Move the piston a ways down in the cylinder, and wipe out any remaining debris. Some will get trapped between the piston and cylinder down to the first compression ring.
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Old 10-03-2015, 12:50:44 AM
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Default Re: Onan 6.5 NHDFB De-carbonization

As just mentioned above move the piston to BDC after they carbon is all removed, wipe out any material, then I also like to get a clean rag, soak it in eng. oil, liberally wipe down both cylinders while @ BDC then turn it over a full revolution to BDC again, wipe out any carbon on the cylinder wall, repeat until the rag comes out clean. Just another way to attempt to get and keep the cylinder & piston clean. Just my .02
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Old 10-03-2015, 04:37:21 AM
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Default Re: Onan 6.5 NHDFB De-carbonization

What the shop note really said. We have no idea what we are doing. So you should by a new generator.

The carbon you have is minor. Compared to some I have seen.
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Old 10-03-2015, 10:24:00 AM
PopsGarage PopsGarage is online now
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Default Re: Onan 6.5 NHDFB De-carbonization

your engine looks fine, the 3m disc's will polish up the cylinder heads pistons and valves a brass wire brush will help to clean it up. It is obvious that the person that made the diagnosis knew very little about engines. tidy it up re-assemble change the oil and run it.

Bob
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Old 10-03-2015, 01:03:21 PM
len k len k is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

Actually it looks like most of the carbon layer on the piston has already fallen off.
Can't see pic that well , but any thick layer of carbon around the valve heads and seats (including back side of valves heads) wants to be removed so it doesn't interfere with flow of air and exhaust. Want engine to be able to breath easily.
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Old 10-03-2015, 03:13:31 PM
sdowney717 sdowney717 is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

Shop says 'cylinders are oval'
Like they know that, come on really now. Maybe they say that because of the oily carbon or blowby.
This sounds like an excuse to convince people get new generator.
Is there a ridge?

Open the intake all the way and brush and scrape clean the backside of the valve.
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Old 10-03-2015, 03:36:22 PM
len k len k is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

Quote:
Originally Posted by Sunnyside View Post
..... 6.5 NHDFB SN A 943109475 Spec 1H..... The generator sat in a basement since 1999....
Gen was made in 1994, so at most it was available for use for only 5 years.

Likely doesn't have that many hours on it, so unlikely cylinders are worn oval. Can measure while you have heads off. Or maybe try to move piston sideways and feel for slap.
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Old 10-03-2015, 05:10:50 PM
Tracy T Tracy T is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

just my.02$ worth, it looks like it was ran under light or no load. clean it up and load it down, you may have found a diamond in the rough.
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Old 10-03-2015, 05:17:55 PM
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Default Re: Onan 6.5 NHDFB De-carbonization

Hmm. There's something missing from this story.

We're all in agreement that the carbon shown in your pictures isn't a problem. A little build-up is perfectly normally in an L-head engine running on gasoline, and what you show would not have caused any operational issues.

So why were the heads removed in the first place? Scraping off the carbon takes a few minutes. I'd be a little concerned there's another issue lurking...
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Old 10-03-2015, 05:22:27 PM
Tracy T Tracy T is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

either that or billy got it right in post #10.
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Old 10-03-2015, 05:46:48 PM
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Default Re: Onan 6.5 NHDFB De-carbonization

Well, yeah, but what was it doing in the shop in the first place?
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Old 10-03-2015, 05:56:15 PM
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Default Re: Onan 6.5 NHDFB De-carbonization

I know I have run into several low priority projects where I've taken something apart and found I needed something special or had a high priority thing come up and never gotten arround to putting it back together for a long time.
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Old 10-03-2015, 08:00:29 PM
Tracy T Tracy T is offline
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Default Re: Onan 6.5 NHDFB De-carbonization

Quote:
Originally Posted by Jim McIntyre View Post
Well, yeah, but what was it doing in the shop in the first place?
My guess, a shop that either was into it for the money or didn't know shat. Foweled plugs
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