My Tractor Forum banner
1 - 4 of 4 Posts

·
Registered
Joined
·
141 Posts
Discussion Starter · #1 ·
I have a B7100. On top of the engine (head) there is a "down spout" sort of looking thing with a hose attached to it (my B5100 has this same thing). The hose basically just hangs down under the tractors. I was assuming it was some sort of vent. Anyway, on the B7100 this hose stretches all the way to about mid frame and exhaust comes out of it!! I mean A LOT of exhaust. It's like having an extra tail pipe. Exhaust does not come out of this same hose on the B5100. Any idea what's going on?????
 

·
Machinery enthusiast
Joined
·
4,256 Posts
Sounds like it's exactly what you guessed, a crankcase vent.
Back in the day before PCV's, an engine either had a vent tube like you have or the oil filler cap had the vent built in it.
All internal combustion crankcases have to be vented. On each piston's down stroke they basically compress the air, oil/fuel vapors into the crankcase.
The compressed vapors need a way of venting or it'll work against the pistons down stroke. Theres also the need for the engine to make a partial vaccum which venting helps achieve using pressure differentials etc.
In the 60's, automotive engines that vented the crankcase vapors directly into the environment were found to be a polluntant so the positive crankcase venting (PVC) system was devised to reroute these vapors back into the engine to be reburnt. Ag engines weren't required to incorporate the PCV system until much later.

re. lots of smoke coming from the vent tube. Odd's are what you're seeing is what is called "blow by".
Blow by occurs in well worn but still servicable engines. Whats occuring is the piston rings are worn to a point where they are allowing exhaust vapors to compress past them into the crankcase where it exhausts through the crankcase vent.
Basically, it is an indicator that the engine while still good, is going to need new rings and possibly new pistons in it's near future.
Now near future can be anywhere from 10 to 1000 hrs. I know, thats a wide range. Theres folks that go beau-coup hr's on engines with blow by.
It's best to just keep a good watch on it when operating it. If it starts smoking more or begins to make unusual sounds, it's time for a rebuild.



Very worn valve guides can also cause some bypass but normally it's the pistons.
 

·
Registered
Joined
·
141 Posts
Discussion Starter · #3 ·
Thanks for the info. I was kind of thinking it was rings. Don't these Kubotas have cylinder "sleeves"? So instead of boring the block you would just buy a new sleeve? I'm thinking this winter might be a good time to take the engine down, if it's not too expensive. I have practically no money in this tractor, so I could definitely justify spending some on it. While I haven't priced the parts yet, I've heard that Kubota parts are surprisingly affordable. I tend to get a little scared when going to buy any parts for a foreign machine. I guess I spent too many years buying parts for Honda 3 wheelers. It's really a pretty sweet tractor and perfect size for my little 5 acre place. If I could restore it and get the engine right, it would be just what I need.
 

·
Machinery enthusiast
Joined
·
4,256 Posts
Some are sleeved and some aren't, I'm not sure on your model but you may not need to go that far in any case. You may possibly get away with just new rings. Alot depends upon what it looks like once you get the head off.
Theres been a few thats posted pics and their experiences in rebuilding these older Kubota's, try doing a search for "rebuild" or "rings" etc under "search this forum", you shuld get some hits and good info.
 
1 - 4 of 4 Posts
Top