Small Engine question

[AndyM]

I hope you will indulge a non-Kubota question (please). A note - I am a complete rookie at engine rebuilds (this is the first one I have had apart).

Some time ago I had asked about a Honda GX-31 that was leaking oil rather profusely from the breather tube. I gather a little is supposed to slop up and be regathered into the block, but this Floods the air filter and beyond . After checking that the oil return ports were not blocked and I could see no other obvious causes, I determined to follow up on the suggestion something was amiss with the rings (this was suggested on this site when I had first discovered the problem).

So I have it apart and can see nothing obviously wrong with the rings. (I still suspect them as I was burning oil and oil seemed to be flooding up the lifter channel).

I have attached some pictures and would greatly appreciate any ideas of where to go from here. I would note the engine has very few hours on it.

 

[1970cs]

Have you check the cylinder for wear or out of round?

Pat

 

[85Hokie]

When this engine was running - did you ever run it without the air filter on? If so - was there a change? A restriction in air flow will cause more air/oil to be pulled via the breather tube.

I had a twin cylinder that would choke itself out with the air filter on (and it looked clean!) but once removed, it would not suck as much via the breather tube.

I agree with 1970 - you need to mike the entire engine.....the cylinder and the piston.

 

[rentthis]

I have found that the best thing to do with that engine is to replace it.

 

[BruceP]

I can think of THREE possible reasons for the symptoms you describe. I have listed them in the order I would check them. (I am assuming the proper viscosity oil is in the crankcase.)

1) Too much oil in crankcase. (perhaps the dipstick is inaccurate?)
2) Too much restriction at air-filter. (sucking oil from crankcase)
3) Too much blowby past rings or valveseals.

Also, you did not mention if this is horizontal or vertical crankshaft.

 

[AndyM]

Thanks to all for the answers.

1970
I can measure the piston; I am in round at the top and out .02mm at the bottom. I will need to buy a guage to check the cylinder - next time I'm in town. I don't see a lot of visible wear on either the piston or the cylinder - though I did find a scratch on the lower part of the piston (it ends well below the ring channel).

Hokie
I have not run the engine for any length of time with filter off - I did have it running when I discovered the the issue and oil continued to run out with filter off.

Rentthis
Buy a new one because it's expensive to fix or just too unreliable? Given the price of stuff up here replacing is an expensive option. Plus I would hate to admit defeat so quickly on my very first repair attempt!

BruceP
Just before I started the teardown I filled the oil to about halfway up the dip stick - it still ran out it floods.
I had washed (gas/oil mix) and dried the air filter before my last test.
The blowby comment I would like to follow up on but am not sure what to do to prove it. Any guidance you can offer is much appreciated.
It's a verticle shaft on a mantis tiller.

Andy

 

[RCW]

Rentthis
Buy a new one because it's expensive to fix or just too unreliable? Given the price of stuff up here replacing is an expensive option. Plus I would hate to admit defeat so quickly on my very first repair attempt!
Andy

Andy -

Rentthis has A LOT of experience with equipment that he rents out for people to abuse. Not his first rodeo........

Not suggesting you give up at all, but it sounds like that Honda engine might have some gremlins......if Rent says it's a loser engine, you might want to take that into consideration as you move forward.

JMHO.....

Good luck!

 

[eserv]

I haven't much experiance with the Honda GX-31 but I do own two of them that have been pretty good machines. My first approach here would be to make sure you are not overfilling the engine! These only hold one tenth of a quart and will throw oil out if they are overfilled! The tiller must be laid over so the sparkplug is facing up to get the right oil level. Here is a operators manual http://cdn.powerequipment.honda.com/engines/pdf/manuals/31zm3613.pdf

 

[North Idaho Wolfman]

The biggest issue on these small engines is, even as nice as they are, they are still 100% aluminum and do NOT have a cast-iron sleeve like most other engines. This means it's a real challenge to rebuild one and get it right. The cylinder walls, piston and rings are all special treated with some really high-tech coatings to get around the obvious problems with wear.

 

[lugbolt]

Here, do it the easy way. Pull the rings off of the piston. Put the top ring into the cylinder, and use the piston to shove the ring about 3/4 of the way up into the bore. Look (measure) your ring gap. Write it down, then do it again for the 2nd ring. You're looking for roughly .010"-.030" end gap. If it's real wide you're going to want to measure the bore. If the bore is abnormally oversized (worn), throw it away and get another engine as they're just as expensive to overhaul and refinish the bore as they are to replace it.

Lot of guys just throw rings in 'em, but if the bore is out of round and/or tapered, a set of rings will last about 5 hours and it'll start pumping oil out again.

Also, look closely at the clearance between the ring and the piston. It should be pretty tight-.002" at the most. More than that and the ring lands and/or rings are worn where they contact the piston.

Finally, if the bore checks out round and not tapered, you may be able to re-ring it assuming everything else (the piston) is in good shape. But before you stuff new rings in it, I'd highly suggest lightly honing the bore, and after reassembly, dump a little bit of break-in oil into it, run it a few hours and then change the oil. New engine oils....roughly 1999 and newer...are really slick, and sometimes need a little "help" in the break-in process.

 

[Tooljunkie]

Many of the honda engines,oil is checked without threading the dipstick down. Unscrew it,wipe it and recheck. This would actually raise oil level compared to screwing it in. Wrong fill plug? Valve adjustment, or incorrect valve timing?

 

[AndyM]

Thanks all..

RCW
I know from reading the site regularly Rentthis has tons of experience on this - I hear you both loud and clear. In part I am pursuing this (maybe more than I should) as a learning process. (I have an old JD 110 that is another project in the background).

eserv
thanks for the link. I had always understood Honda engines were worth the premium they seem to demand. Sound like views are mixed.

NIW
Got it. At this point I might try to put it back together because my time is cheap and I need the practice. But I will keep my hopes in check

lugbolt
Thanks for the specific suggestions - I will give them all a go and see how I can make out. The engine probably has less then 10 hours of actual use - I am assuming it shouldn't wear too bad with those hours (or am I wrong?)

Tooljunkie
yup - checked with threads out. Plug is OEM. Did check the valve adjustment before I started the teardown. It was tight but within spec. (I did come by a WSM)

Again, all these comments are very much appreciated. A babe being led by the hand...

 

[Kubota Newbie]

So a ring set for that is pretty cheap - +/- $12. If you replace them there is usually a top and bottom to the rings. Get them in upside down and it'll really suck oil!
Piston - The only place a piston is supposed to be round is at the top/to the bottom of the oil rings. From there down they are machined to be slightly bigger 90 degrees to the pin boss than they are as measured on around. The skirt will also probably measure slightly bigger at the bottom than near the rings.
.030 would be a BUNCH of ring gap on an engine that size. The lesser the better normally down to about .004 per inch of bore. Unless you have a ring gapping tool I wouldn't screw with them if they're no tighter than that (your used ones won't be otherwise the engine would've seized).

Another thing to check is to make sure the baffles and etc. seem to be in place in the breather assembly. There's usually some sort of oil separator in place and a little reed or disc valve of some sort.

Scratches on the cyl. wall are more of an issue than a scuff or two on the piston skirt.

 

[AndyM]

Thanks for the info Newbie. One question - the breather assembly (which I am taking to be the the valve cover) seems to be a solid unit with three holes and a port. I used compressed air to ensure there was no blockages but don't see or have access to a reed or disc valve. Can I assume if I have air flow between the hole and the port all is well?

I did find a the rings in the US cheap enough; prices triple by the time you add shipping, duties, taxes et al up here. But, will have to do it as I can't even find a supplier online up here. Amazing.