Kubota blow by

not aboard ship but at least a engineering question.
My neighbors '70’s kubota mid size tractor has massive blow by via the 3/4" vent pipe leading downward from a minaturized ‘catalytic converter’ looking thing on the side of the block leading into what most of us would assume to be the water jacket. I loosened the #1 injector on this 3 cyl. engine and could tell it was firing, just not doing much. he states it started blowing smoke and oil all over when he drove it up from the neighbors a year ago. it’s been sitting idle and i want to use it but told him i’d get it running in exchange … now i’m not so sure, i sure don’t want to take it apart!

Without knowing the year and model number it is kind of like asking “what is the red thing on the side of the engine” without at least a photo.

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the model # on the tractor is: M4950, M5950, M6950, M7950.
The principal thing is the blow by, you could pressurize a air tank with the blow by. after 1 hr . of running it, you’d need to put 1 or 2 quarts of oil in it.

The amount of blowby you’re describing, I’m looking at rings and liners. Can pull the exhaust mani and take a look at the ports to see where you’re having issues but if I’ve got a small engine apart with serious blow by I’m at least gonna re-ring all the holes and get a peek at the liner condition and bore mic them against the book.

Can you pull an engine serial number/got any pictures of the vent/‘converter’?

I’ve never worked on one of the tractors so I don’t know what they give you but have lots of little kubotas scattered around in various gensets etc. Straightforward well designed engines with good parts availability for the most part.


Yes, without the engine model and serial number it really is a “what is the red thing” question. Unfortunately it looks like Kubota and its parts people have monetized all other sources of information. I couldn’t find an open source maintenance manual anywhere. Not having the exact information about what engine to look for and even which side of the engine doesn’t help either.

Yeah for the tractor model itself it’s gonna be a lot more difficult. I have access to parts and overhaul manuals for several of the more common kubota models — I would assume even the tractor engines have an ESN independent of the tractor model/serial so hopefully Jimrr can share that.

I’ll try narrowing down the model number and something about the engine but if i could eliminate a valve stem issue i’d think we’d be right into the cylinder, and rings? i might get a pic of it running for you here but it may be a day or so?

Valve stem issues show up as blue smokey exhaust. Blowby is the result of high crankcase pressure. The word itself describes the usual source, cylinder pressure blowing by the piston.

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yes, i’m pretty much resigned to it being piston/ring bs. with the volume of blue ‘air’ and mostly a steady stream of oil out the vent it’s assuredly so even though I am unfamiliar with the pcv valve or whatever that appendage is on the LH side of the block. There is another tractor just like it down the road a ways, it may not of sold at that ranches auction in which case there may be another engine around but as for me, it looks like i’ll have to us the cat or something to level my foundation.

I looked at that engine guys and found no data plate, the book says “RH side” but even there i don’t know what RH side may mean in a japanese book. I looked all over using either to clean any suspect data plate and didn’t find any. it pretty much has to be rings or piston related. We’ve all been fooled before though. The owner still wants me to ‘‘make it so it’ll start’’ which i guess i’ll do on my own time but I’m reluctant to use it, If #2 cylinder drops then i have the spectre of me having ‘‘broken’’ it hanging on me. I can use the neighbors backhoe but it seldom starts after it’s warm.
I forgot to mention the gas temps. #1; 150F, #2: 275F, #3: 300F // after 3-4 min. run time measured with ‘ray gun’ top of manifold exhaust port.

It sounds like you got it to start and that is all he asked for, right?

If you want to confirm that a cylinder might be the culprit, loosen the nut on the injector line going to #1 and if nothing changes then you found the location of whatever might be causing the blowby.

If you are sitting on the driver’s seat, the engine’s right hand side is the same as your right hand. Take 2 photos and post them, they don’t cost much.

naturally it’s more complicated. I have to bypass the ignition circuit to activate the solenoid to turn the engine, i also have to hotwire the fuel shut off valve. it’s not a big deal but gaining access to the wires in the dash is a absolute forgettable task I am delaying.
and yes, as mentioned I did loosen #1 injector line.
so some wires are goofed up in the dash somewhere, i just need to commit to it so pete can crank his engine and hopefully quit buying all the ether in town. he intends to dig post holes despite the blow by. I’d like to borrow it but what if it blows some more rings or something? i’ll of wished I used a shovel !!