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Small Engines (Lawn Mowers, etc.)/Honda GX620K1 Blowing Oil Badly


QUESTION: My Honda 20HP horizontal shaft V-Twin, Model: GX620K1 QAF SN: GCARK-1080221 is blowing oil badly.  Apparently there is high crankcase pressure as it is even blowing the dipstick out of it's hole.  There is oil in the bottom of the carb. intake when running.  There is much oil smoke from the exhaust, especially under heavy load.  After running (mowing) a while the oil level drops a lot (down to add line in an hour).

History:  Last fall the engine started making noise.  When I disassembled it I found the exhaust valve seat on #1 head broke, part of it dislodged, and was beat between the piston and head.  The dents in the head and piston were not very deep (1/16" or so) and I did not see any cracks or what I thought was more serious damage to the head or piston. Both heads showed signs of overheating (cooling fins had been blocked) ... possibly why the valve seat dislodged).  Both cylinder walls were smooth.  Both heads showed it previously had loose head bolts when disassembled and signs of oil leakage between head & block.  The #1 head had more carbon ... possibly from head being loose, drawing in oil past the loose head gasket, and burning it.

Repair Action: I took the head to an automotive machine shop and had the exhaust valve seat replaced, both valves ground, and the head resurfaced.  Re-assembled with new gaskets and new crankcase cover seal (output shaft), since there were signs of oil leakage there before.  I did blow and suck on the crankcase vent tube and the valve seems to operate freely, but did not remove or clean it.  The engine starts and runs quietly and smoothly and has good power (but maybe a slight bit reduced).

What should I look for to stop excess crankcase pressure causing blowing oil?


ANSWER: I would check to see if the engine has vacuum.  Check out this video:

I would also perform a cylinder leakdown test on each cylinder.  See this video;

These two tests will let you know how much blow-by the engine is producing.  If there is no leaking from the intake or exhaust valve then the air must be getting past the piston rings and out the dipstick tube.  If this is the case you will have to re-ring and/or possibly bore the cylinders oversize and replace the pistons.

Let me know what you find.

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QUESTION: Thanks Eric.  Your reply was very helpful.  Before you replied I did a compression test on the engine.  I got 60# on number 1 cylinder (the one I had replaced the exhaust valve seat and ground the valves), and 70# on number 2 cylinder.  Normal range is 85# to 110#.  I shot oil in the cylinders and reran the compression tests.  I got 120# on #1 and 90# on number 2.  Indicating possible bad rings on both cylinders and possibly an additional issue on #2.  After reading your reply, I checked for vacuum ... opening the oil fill cap blew the cap off indication positive pressure ... the vacuum gauge verified that reading.  A cylinder leak down test on #1 was 45% leakage all to the crankcase.  #2 had 40% leakage from crankcase and from exhaust valve.  My thought is I need to replace rings on both cylinders and grind the valves on #2 head.  I do not have a micrometer to measure bore wear, but since I did not see and abnormal wear in the cylinder walls (no scoring, no ridge at the top, no dark spots or scoring) I don't think I will need to hone the cylinders.  Since I had oil leakage from the governor shaft and front seal with the high crankcase pressure, I plan to replace them as well.  Does this sound like a good plan?

It is always best to deglaze the cylinder to ensure the new rings seat well.  Since you are doing all the work yourself you could just replace the rings and hope they seal.  

Another option is to dis-assemble yourself and then take the block, piston, rods, basically all the internal parts to a machine shop and have them check the cylinders and rods.  It should not cost too much money since you spent all the time dis-assembling and clening the parts for the machine shop.

You could deglase with a ball hone, just scratch up the cylinders for the rings have a chance to seal.

I would also the cost of new pistons.  If the cost it way too expensive, probably is, you could just replace the rings but realize the new parts may not work too well.  Did you price a new engine?


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Eric A. Jones


Lawnmower Repair . Certified Master Service Technician from B&S. Have 23 years experience on B&S, Lawn Chief, Weed Eater, Echo, Peerless, Wheel Horse, Snapper, Atlas, MTD, McCulloch, Homelite and many other numerous brands. Specialize in electrical repair.


Born and raised in the midwest. Started tinkering with engines when I was about 14 on my Suzuki RM-80. I began lawn mower repair at a small hardware store. I knew absolutely nothing. I read lots of repair manuals and met an older fellow who taught me many lessons. I continued working on small engines through high school and paid my way through college working on mowers at the same hardware store. Decided to get away from the midwest and mower repair so I joined the Air Force. I repaired air traffic control electronic equipment and ended up in Hawaii where I got a part time job at Small Engine Clinic. I gained a lot of experience from the Small Engine Clinic and had a blast repairing small engines. I then took the Briggs and Stratton Master Service Technician test and earned my MST. I then traveled to Wisconsin where I attended the factory update training seminar and received formal training. Continued working on mowers part time as I completed 20 years of military. Retired from the military on a Friday and continued in the lawn and garden industry the next Monday.

MAS Aerospace Operations BA Mathematics AAS Electronic Communications AAS Electronic Technology

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