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SadieCoonhound

520H Backfires under load

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SadieCoonhound

Hi guys. I've been reading your forums for years you guys are amazing. 

I've finally joined and am eagerly hoping for advice. I recently picked up a 1989 520h. A very pretty tractor. I'll try to get right to it. 

It starts and runs great until it's under load. Underload it starts popping a little and a little missing. But under high throttle it has a very loud backfire about every 10 seconds or so. Again, before the tranny is engaged, I can hold that rev up to its max and it sound great but when I engage the tranny the above symptoms occur. I used sea foam, cleaned the float bowl, gave it a compression check (120psi both cylinders) replaced the rear spark plug (very sooty)

I'm a new guy and compared to you gentleman, not to bright. Can you lead me in the right direction?  Thanks

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Ed Kennell

:text-welcomeconfetti: to the  :rs:  Sadie.      When were the valves adjusted in the back cylinder?

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SadieCoonhound

Thanks for the welcome. I have no idea when or if the valves were ever adjusted. It backfires through the muffler by the way, not the carb. 

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Ed Kennell

It may be time to pull the heads and decarbon.      Other Onan experts should be here soon to offer advice.

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SadieCoonhound

Yes it is an onan p220. I've been fooling around with the lean/rich screw hoping it would be something simple. Yes, I know it never is. 

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Tinbender

An explosion through the exhaust is called Afterfire or Afterburn. Normally this is caused when fuel/air mixture is too rich. Although, I have a 520H I have never touched the carburetor so I can't help with adjustments. Before you look at the carburetor install new spark plugs, bad plugs can cause some weird happenings. Sorry that's all I can offer.

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SadieCoonhound

I put in new plugs this morning. And continued to try and adjust the fuel mixture. When I throttle it up it continues to miss and will backfire. But when I close the choke almost all the way it runs well. Can anybody help? Thanks. 

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Ed Kennell

Could be air leaking in at the carb or intake gasket or around the throttle shaft.

Does this tractor have a working vacuum gage?

Edited by Ed Kennell

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SadieCoonhound

Are you suggesting a leaking intake manifold gasket? Or the carb gasket.

Yes the vacuum gage works. Thanks

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Ed Kennell
48 minutes ago, SadieCoonhound said:

Are you suggesting a leaking intake manifold gasket? Or the carb gasket.

Yes the vacuum gage works. Thanks

Yes, when they run better with the choke closed, that is an indicator that air is getting in some other way.   Leaking carb and intake gaskets and throttle shaft clearance are  possibilities.

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WHX??

First off :WRS:  Coon Dog,

Take a can of WD-40 and spray it around the lower carb area and intake manifolds to check for ir leaks. If the engine changes pitch when doing this it might be a possible indication of air leaks. It's still possible you may have a piece of gunk in the carb so a total carb cleaning may be in order. make sure you have a good fuel filter on it. 

Any idea of how many hours are on this tractor and any other maintenace history?

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SadieCoonhound

There's 850 hours on it. I don't know much history other then new fuel filter, fuel pump, hydraulic hoses. 

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SadieCoonhound

Thanks for the welcome. 

When you say total carb cleaning do you mean removing the whole carb?  Or just the top half?

I'll try to squirt the WD tomorrow 

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WHX??

:text-imsorry: but to do it proper the whole carb must come off. If fuel pumps and filter have been replaced one never knows what was intoduced into the fuel system. Might be something as simple as a clogged main jet.

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SadieCoonhound

Thanks. Makes sense to me. Hopefully I can get to it shortly. 

I really appreciate you guys. Will let you know my progress. 

The coon dog sleeps 19 hours a day. 

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WVHillbilly520H

With the Onan the intake manifold is 2 parts that was sealed from the factory with a "silicone" style gasket ,over the years this seal becomes brittle from the heat cycles and sitting, you can take the whole manifold off and reseal it, but to check for a vacuum/air leak with the engine running spray carb cleaner around the carb base and manifold in small spurts if the RMPS rise then you have a bad seal/gasket if not then it's something else, and my father changed out the coil on his mower and forgot the bolt holding the condenser so it wasn't getting ground all the time bouncing off the block when running causing a very similar issue like you're describing, Hope this helps, Jeff.

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SadieCoonhound

Thanks Hillbilly. 

I'll try the spray. I have another WH 520HC. Maybe I can swap condensers for a test. 

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6bg6ga

Its an 89 so I will make the assumption it will be electronic ignition. With the electronic ignition there is a condenser on the positive side of the coil not negative like a points type ignition. Having the condenser off the engine or not grounded will not cause this problem. The condenser's purpose is filter the DC from the battery and to supposedly curb any AC signal riding on the DC. Looked for something with my scope and found nothing.

 

If you have a timing light I would put it on and see where the timing is. Note*  Personal experience here.... I had a bad module make my Onan P216V run the same way shortly before  the module gave up the ghost and died completely. So, this COULD also be a timing issue. Without hearing it or being there this post like others is trying to put out the best information possible.

 

Also, no WD40?  You can also use a more messy way to test for intake leaks and that is oil shot into suspected areas. If leaking it will draw in the oil and you'll get some oil smoke.

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SadieCoonhound

No, I dont have a timing light. The module looks new.

Will try these spray. Are you saying that it cannot be the condenser?

and you're right it's got electronic ignition. 

Really appreciate your advice. 

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SadieCoonhound

I tried the WD and it didn't make a difference. Looks like the complete carb cleaning next. 

although i might just try to clean the top half one more time. Two questions if you don't mind. Do I have to remove the muffler in order to remove the intake manifold?

And should i bother to switch condensers?

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HokieAg07

The Nikki carbs on these engines are very sensitive to cleanliness, clean it thoroughly. Exhaust comes off prior to intake manifold. 

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Turftech

Greetings,

Have you checked the RPM @ full throttle? The factory spec is 3600RPM

Picked up a 93 520H last fall. Had a noticeable miss. Found max RPM @ 4200, set to spec, problem went a way.

Hope this helps.

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ezeastside

Just a quick thought,

Sadie coon mentioned it runs great until under load?

Could a safety switch be making and breaking like PTO, Seat or neutral safety?

This would shut it off turn it on shut it off and cause extra gas in the carb then combustion with a back fire through exhaust!!

Try eliminating the safety circuit with a jumper to test,.

2 cents!!

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SadieCoonhound

Appreciate you guys

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SadieCoonhound

Thanks again guys. 

Removed and cleaned carb. Removed heads and decarboned. New head gaskets from boomer, etc. adjusted valves. The tractor is running, no back fire or missing any more. So far, so good. Really want to thank you guys for all you advice and input. 

 

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