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JohnF2015, Small Engine Technician
Category: Small Engine
Satisfied Customers: 1102
Experience:  Owner/Service Technician at S & J Small Engines
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I have a Generac 04760-2 15kw LP standby generator. The

Customer Question

I have a Generac 04760-2 15kw LP standby generator. The engine (model DE6219 GT990 ser#(###) ###-#### ***** started backfiring during startup. It also looks like there has been a fire in the air cleaner housing at some point. When it backfires it blows a plug fitting up and out of the plastic air cleaner intake housing, cylinder next to big side access door. These plastic plugs or rings look to be made into the housing base.
Spark Plugs look good. I can see oil in the cylinders when looking down into them and also some in the intake manifold but this engine seldom needs any oil top off.
Looking in the engine manual, I read about the LP regulator and wonder if I am getting too much LP build up after shut off. ??
Please point me in the right direction--
Submitted: 1 year ago.
Category: Small Engine
Expert:  JohnF2015 replied 1 year ago.

Hi Kerry, my name is***** will be assisting you.

I believe that you are headed in the right direction by checking and resetting the valve clearances. This is really the only thing that might cause the unit to backfire through the intake like that. You should be able to turn the flywheel manually by hand in order to check or reset the valve clearances. You could have an ignition coil misfiring that would cause the problem as well. I would check the valve clearances as a starting point and go from there.

Thank you it has been a pleasure to assist you. If you need additional assistance feel free to ask.

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Customer: replied 1 year ago.
John, while checking the intake exhaust valve settings, I discovered after running, Cylinder one valve cover is hot, cylinder two is cold. Denoting no fire condition. Plug is good. I think this now points me to a defective coil for that cylinder.
Valve adjustment is ok. No issue with those.
Expert:  JohnF2015 replied 1 year ago.

Yes, it does sound like the number two cylinder is not running or firing. Have you checked to see if you are getting a good spark to both cylinders. I know that #1 is probably okay but you can use it to compare to #2 and see what if any difference you have. I have seen the ignition coils on these engine be a problem. Normally if I were replacing them I would replace both of them at the same time. It could be that #2 is firing some of the time but not all the time which would explain the backfiring and load up of fuel in the intake and cylinder. By the way most likely what you are seeing in the intake a cylinders is fuel not oil.

Customer: replied 1 year ago.
This runs on LP gas.
Pulled both plugs and checked by putting plug ends next to the block while turning the
Engine over. Both were firing. The plug on #2 was mild dirty- the other looks almost new.
Expert:  JohnF2015 replied 1 year ago.

It sounds like the #2 is not firing correctly. You could have a compression problem, valves not sealing properly or a head gasket partially blown.

Customer: replied 1 year ago.
I'll do a compressing check tomorrow and report back!
Expert:  JohnF2015 replied 1 year ago.

That will be fine. Let me know what you find.

Customer: replied 1 year ago.
OK, just did compression check. Cylinder 1 had 190, cyl 2 had 185
So It's not a stuck valve. Seems more and more like the coil pac. Acts as if
it is firing at wrong time.
Expert:  JohnF2015 replied 1 year ago.

Compression looks good at the numbers that you have indicated so I would agree that it does sound like an ignition coil acting up and not firing properly.

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