ONAN 18HP Decarbon?

[nekovt 0]

Hi Everyone, I just bought a 518-H with a 18HP Onan engine and an Eaton 7 Hydro pump that was replaced about 2 years ago. (It's in very rough shape but the big things are in good shape).

I was talking to someone who said I should "De-carbon" the engine because the engine carbons up which will damage it should the carbon build up become to much.

I have never done this before and I hear there is a manual out there that shows you how to take out the cylinder heads, tourque specs, etc. Once the heads are off and the carbon removed, then the valves need adjusting as well?

I also hear there's a "de-carboning kit" (gaskets) out there for these engines. Is there a good place to get this?

If I go through this procedure, is there anything else I should do?

Thanks!

Brad

[P3x 56]

Hi Brad,

I have a 520H with the Onan 20HP and read the same info regarding Decarbonizing the heads. Although I have not performed the procedure yet, I've purchased the Valve Grnd/Carbon Removal Gasket Kit (Toro PN - 57-9370 and the Onan PN 168-0189) in preparation. The cost was 42.50 from OnanParts.com.

I believe the same Kit PN applies to both 18 and 20HP Onans; you should verify this before shelling out your hard earned cash. The folks at OnanParts.com kindly sent electronic copies of the Maintenance and Parts Manuals for my engine as well which details the steps.

I'll be following the thread to learn any tips on do this procedure.

Good luck!

p3x

[sorekiwi 761]

There is a link to the Onan manual in the "Links" section of this site. There is also an JD Onan Manual with a lot of photo's available for download in the "Manuals" section at www.mytractorforum.com .

You could try this place for aftermarket Onan gaskets. http://www.gphope.com/store/cat/57

I have this set on my 520, but it has only about 10 hours on it since I installed them. The kit doesnt include exhaust gaskets though, I ordered them through my Toro dealer.

[nekovt 0]

Thank you! I have ordered the kit from Onan parts and they sent me the manual. Thank you!

[mavfreak 11]

What octane fuel is recommended for the Oman?if the onan is prone to carbon build up this could be prevented with proper fuel?

[zieg72 209]

standard 87 octane but I run a 50/50 blend of regular unleaded/premium unleaded. Much less fuel used, sounds better and seems to have more power. I have tried the mid grade straight but the 50/50 blend works the best for me... My $.02

[300zx 7]

This is the best product I have found for removing carbon from my heads, pistons and blocks. It is 3M's Roloc bristle discs. I first saw them when a Nissan mechanic was using them to remove the gasket material from the aluminum block on my car. He said Nissan recommended the yellow for aluminum and the green for steel/cast iron. The white is a medium grit.

3M's Roloc Discs

[mavfreak 11]

carbon deposits are caused by unburnt gas. If you have a high compression engine then a higher octane is needed but if its a low compression engine then the low octane is best. A high octane gas doesn't burn as well as lower octane. A higher octane can be compressed into a much better burn by a higher compression engine but wont ignite in a lower compression engine therefore leaving unburnt fuel in the cylinder causing carbon build up. And if running to low a octane in a high compression engine will cause the fuel/air mixer to ignite before its supposed to causing engine to knock. If your finding that an engine runs better on a higher octane than its was designed for then your starting to carbon up, The carbon deposits are giving you a higher compression ratio. To many times I have seen engines carboned up cause the owner believed more is better.

JMEO

[mavfreak 11]

if your engine does have carbon deposits in it the carbon is porous that causes hot spots and can cause the air fuel mixer to preignite causeing the engine it knock and ping this is because the piston is being forced down before it is intended to.

[P3x 56]

I run Regular (87 Octane) with Seafoam in my 520 and it seems to run fine. I tried a 50/50 mix of Regular and Premium to see if it would be a little less thirsty and did not notice any difference. Onan has to use 87 Unleaded.

I have a riding mower with a B&S 16HP Opposed in it which recommends Decarbonizing the heads at set intervals. I was having oil leakage issues and it ran like crap when warmed up. After ovrthauling the carb and fuel pump, it would not start after the initial leak check run. So I figured I'd decarbon it as had the parts and planned to do the Onan at some point. Simple procedure and glad I did it.

The compression test showed good for both cylinders. I had to do it cold as I could not get it to run for more than a couple of seconds. Both heads had quite a bit of carbon built up with the head gaskets leaking. The No. 1 cylinder leak was pressurizing the case after the engine was warm contributing to leakage and the running problems. Used a plastic scraper on the piston and mating flanges and Oven Cleaner spray on the heads which works good to remove the carbon build-up and followed with scotch brite on all. The valve seats had some lead built up. I used a brass tool made many moons ago to remove Orings from jet engine HP bleed air couplings ago and it did the trick.

After adjusting the Carb and linkages, it is running top notch with fuel consumption noticibly decreased on the maiden mowing job. I hope I get the same result after doing it on the Onan. The 520s do use the gas...

[416/520 0]

My 520 H has 900 hours on it and I was afraid to run it anymore. It was noisy enough that I just put a Honda clone[ 13 HP ] in to keep the grass mowed.

The front piston had just a blush of carbon on the end of it and some accumulating carbon on the head. The rear piston had at least 1/16" carbon on the piston end,and a much heavier accumulation on the head.

I could find no evidence of any ridge from the piston rings. The bore looked very good,but without using a bore gauge,I am not sure how much wear we are talking about. I have a notion to clean up the valves,put it back together,and run the engine with the spark plugs out and grounded with an electric motor slaved to the shaft to see if I can hear any noises related to valves or bearings. It would not take too much extra time to do this,and I just might learn something of value.

When I pulled the heads off,one was pretty loose,so if it resulted in low compression,that could explain the extra carbon buildup on the rear piston.

I am also looking for a good rebuild shop in the Dayton OH area...

[canam1991 2]

SEA-FOAM FOR ME VALVES AND PLUGS BURN NICE AND CLEAN/WHITE :hide:

[COMMANDO1 23]

10-4 on that, i run seafoam in everything. If it has a motor------seafoam it.

[nekovt 0]

Thanks for the replies everyone! I to have discovered Seafoam.

I was just going to ask the forum why I put a brand new spark plug in my onan engine and ran it for about an hour to find the spark plug has carbon build up on it.

It sounds like running 87 octane is the way to go for this engine, I have been running 93 thinking it was a good idea.

This may seem like a stupid question, but I have to ask it - if it's running to rich you would get carbon build up right?

I want to make sure I solve the carbon build up because I'm about to try the Onan Decarboning proceedure.

Thanks!

Brad

[dwcphoto 7]

Would you guys recommend using Seafoam in the oil to clean out the gunk, as well as in the gas? thanks, dwc

[gwgdog66 23]

Would you guys recommend using Seafoam in the oil to clean out the gunk, as well as in the gas? thanks, dwc

Yes, it works well for this also :grin:

[dvr100 0]

A word of caution on Sea foam in the oil. I only put it in right before an oil change-4 oz. and let it run at full throtle for 30 min. reason is the next door neighbor put in his dad's 71 Mustang at oil change- when he drained the oil it had a lot of metal shavings in it-might have been it just cleaned up years of wear but I don't want to take the chance of ruining the Kohler.

This is my first post but I have been lurking since I did a trade for a 310-8 last fall. Will be posting pics and asking lots of questions at a later date. i have already found this site so very helpful and hope I can be of help to others.

Have a wonderful day all. Doug

[dwcphoto 7]

Doug,

Thanks for the word of caution. I'm new here, too. This site has been a wealth of information in only a few days. It is a great community to be part of. Much appreciation! best, dwc

[sorekiwi 761]

I've never put Seafoam in the oil, so I cant comment on whether its effective or not. But I do use Marvel Mystery Oil and it works well. There was a thread about MM Oil about a month ago.

[dwcphoto 7]

Thanks! dwc

[dvr100 0]

I should have clearified my statment on the Mustang -the engine only has 75M miles on it so I wouldn't think there wouldn't be that much wear-it was parked for 15 years so that could be the reason for the meatal shavings. When I did my Sea Foam treatment the oil was dirtyer then any oil I've seen even from a diesel and I was raised on a farm with multipule diesle engines. Still trytig to figure out how to post pics because I know how much everyone likes them. :)