B7100 blow-by question

[AOW162435]

Hello again. Last year I posted about the new-to-me '79 B7100 I purchased (B7100).
Following a thorough going-through & servicing, I took it out to our property in WV and put to work pulling a 5' mower every few weeks. At the time, the tractor had 928 hours on the clock.
The tractor starts correctly, runs fantastic, has plenty of power, and generally does what I need it to do. When I first purchased it and ran it, the blow-by from the valve cover vent tube was minimal at best (completely normal from my experience). A few weeks ago, I suddenly noticed a much greater amount once bringing it to idle. It will rev up butter smooth and run at full RPM without an issue, but once dropped to idle, the blow-by will immediately start.

I have read many, many posts & write ups online, both from here as well as on other forums. The end result is that I'm no closer to figuring out if this is truly problematic, or more of a nuisance. I did read that the breather has a screen that can become clogged - I'm happy to remove the valve cover and do so.

But based on my write-up, can anyone offer a fresh perspective on the matter? At this point, the tractor has around 950 hours.


Andreas

 

[TheOldHokie]

Hello again. Last year I posted about the new-to-me '79 B7100 I purchased (B7100).
Following a thorough going-through & servicing, I took it out to our property in WV and put to work pulling a 5' mower every few weeks. At the time, the tractor had 928 hours on the clock.
The tractor starts correctly, runs fantastic, has plenty of power, and generally does what I need it to do. When I first purchased it and ran it, the blow-by from the valve cover vent tube was minimal at best (completely normal from my experience). A few weeks ago, I suddenly noticed a much greater amount once bringing it to idle. It will rev up butter smooth and run at full RPM without an issue, but once dropped to idle, the blow-by will immediately start.

I have read many, many posts & write ups online, both from here as well as on other forums. The end result is that I'm no closer to figuring out if this is truly problematic, or more of a nuisance. I did read that the breather has a screen that can become clogged - I'm happy to remove the valve cover and do so.

But based on my write-up, can anyone offer a fresh perspective on the matter? At this point, the tractor has around 950 hours.


Andreas

Fix it till you break it

Dan

 

[85Hokie]

Blow by as you as you know full well is gases that are getting by the rings.

This may sound a bit counter intuitive - but running it harder, longer will make for a better engine.

Low speed = low temps - low temps are very bad!! As my physics teacher in HS said - if you could run a car at much higher temps, it would be more efficient. BUT we cannot.

The blow by cannot be "fixed" in an easy way - a rebuild is the true answer. BUT, if compression is good enough cold = then run it and don't worry about it UNTIL such time it does not start easily.

 

[TerryKing]

I suddenly noticed a much greater amount once bringing it to idle. It will rev up butter smooth and run at full RPM without an issue, but once dropped to idle, the blow-by will immediately start.

Hi Andreas, Not sure what is happening here... Hmmmm..

I've worked on many (older, like me) cars and trucks with engines worn enough to have some blow-by. Rebuilt a few.

I usually (always?) saw blow-by increase under load (like revving a gas engine). Higher combustion pressure.

MAYBE the rings are imperfect, but the higher combustion pressure under load "seats" the rings better??

I will try to check my B7100HST when I run it again.. So right now: Hmmmmm...

 

[AOW162435]

Would it be of any help if I take a video of what I'm seeing?

Andreas

 

[AOW162435]

Over the last few days, I mowed for around 5 hours total. Very steep fields & meadows (our property and two neighbors). I kept 85Hokie's comment in mind about running it hard... The tractor definitely got a workout cutting 4' high growth and going up slopes I can barely walk up.
I made sure to stop every so often to clean the radiator screen of accumulated milk weed fuzz, etc., and topped off coolant every morning.
I think I was hoping for some sort of miracle but the blow by is just as bad or even more. As stated earlier, it's not really visible while plowing along at full steam, but as soon as I throttle down and come to a stop, it sends a steady stream of vapor out of the hose.

Would really like a few more thoughts on what could have brought this on out of the blue. It really is a nice example (~960 hrs) and runs so well. While I'm not up for tearing the engine down myself, I'd have no issue pulling the front axle / frame away in order to fully expose the engine.


Andreas

 

[TheOldHokie]

Over the last few days, I mowed for around 5 hours total. Very steep fields & meadows (our property and two neighbors). I kept 85Hokie's comment in mind about running it hard... The tractor definitely got a workout cutting 4' high growth and going up slopes I can barely walk up.
I made sure to stop every so often to clean the radiator screen of accumulated milk weed fuzz, etc., and topped off coolant every morning.
I think I was hoping for some sort of miracle but the blow by is just as bad or even more. As stated earlier, it's not really visible while plowing along at full steam, but as soon as I throttle down and come to a stop, it sends a steady stream of vapor out of the hose.

Would really like a few more thoughts on what could have brought this on out of the blue. It really is a nice example (~960 hrs) and runs so well. While I'm not up for tearing the engine down myself, I'd have no issue pulling the front axle / frame away in order to fully expose the engine.


Andreas

My thoughts:

You just worked the snot out of your 35-50 year old tractor cutting 4 foot high vegatation on mountain goat trails. The tractor performed flawlessly but when done you observed visible vapor at the crankcase breather.

 

[InTheWoods]

...The blow by cannot be "fixed" in an easy way - a rebuild is the true answer. BUT, if compression is good enough cold = then run it and don't worry about it UNTIL such time it does not start easily.

Yep - no easy fix.

Just keep an eye on your oil level, as it is possible to blow out crankcase oil with excessive blowby...

 

[85Hokie]

one thing that will scare you - IF you fill the radiator up TOO much, it will spew OUT via the hose that goes to the whistle ....and you will smell radiator fluid.......

The fluid in the radiator should be about an inch below the fill top - covering the top of the fins.

Remember this - the fluid works on expansion caused by heat = as the water gets hot it will rise, comes up the top hose and falls through the radiator.

Get you an infrared thermometer - get the tractor good and hot, run the snot out of it.

SHOOT the top neck of the radiator while running - should be 195-210 F......... shoot the bottom neck, should be about 80 to 100 F LOWER.

And that vapor trail the Old Hokie mentioned ............. hell that is a good sign!!

If it starts easily with glowing and runs well - keep the radiator clean and it will go ANOTHER 960 hours!!!!

 

[GeoHorn]

Don’t really “shoot” the radiator top and bottom or you will definitely have a radiator leak problem.

It may be that a stuck piston ring (or several) may be the cause of low-speed blow-by. Working the engine hard May resolve this after some time. There may be some folks who will encourage you to try various “mystery oils” or “snake oils”… but I’m not a proponent of those.

I think you have a good running tractor and that if you regularly check and maintain fluid levels for safety…. the little tractor may “cure” itself by simply being used regularly. Don’t be too hasty to tear into it unnecessarily. IMO

 

[AOW162435]

Just a small update - I started the tractor up today after more than a month of sitting. Other than some initial 'surging' at idle, it started up fine. The blow by was evident almost immediately. I let it warm up for awhile and then let it run at higher engine speeds for 10-15 min. There is no question that the blow by is just as bad (or worse) than when I used the tractor hard for a few days in late October.
It now consistently drips oil from the tube, and if I cover the tube with my finger for a few seconds, it hisses once I remove it.

Other than checking on it today, it will be another month or so before I can trailer it home to Maryland. As suggested by a few members, I will pull the valve cover and properly clean the vent screen. That said, I have the sinking feeling that the problem is stuck rings, broken rings, valve stem seals, or a combination of these.

If this is what I'm facing, what is the best way to find someone who truly knows these engines and can properly handle what's needed? I am very capable mechanically, but am not keen on tearing into this engine as a first timer. I'm happy to pay someone fairly to wrench on it with me in my garage, or I'd be fine trailering it to someone in MD/VA/PA/WV.

Andreas

 

[InTheWoods]

Dang. That sucks. I saw your earlier thread on this (otherwise fantastic) tractor.

Seeing the oil on the gravel would convince me this needs to be addressed. With only 900 hours, seems like this thing suffered some trauma - maybe a serious overheat at some point? Ingested some water/coolant and hydrolocked? Ran without oil? Some form of abuse anyway...

What would I do?

Hmm...

I might just sell it as-is and look for a replacement. Not sure what your needs are, but I recently augmented my old B7100 with a B7510 with loader. Money well spent, for me anyway.

Or maybe...

...pull the head and measure the bores, pull the pistons and check the crank, inspect the head, to see if an 'in-frame' fix is in the cards. I put new rings in an abused D950 (generator engine) and it wasn't too bad of a job. If your block needed machining, I'm not sure I'd bother. A B7100 w/o a FEL isn't that hard to replace. Sad to say, but a 'proper' rebuild is going to run into enough money to make me ponder whether it's worth it. Plus, as you elude to - where to find someone to do a good job at it...

Or, If I wasn't in a hurry - keep an eye out for a donor machine...

Good luck!

 

[GreensvilleJay]

curious, found this..

https://www.amazon.ca/D750-Overhaul-Rebuild-Kit-15261-21110/dp/B0CS6NT1GP

say $550 CDN about $350 US$ ??
I'll assume engine has to come out/off to do the rebuild, others will know.
I'd expect x2 for labour, unless you're doing the 'grunt' work.
It's a LOT cheaper than the $5K I was quoted for my BUDA 4D-153 10 years ago and that's AFTER I'd removed it from forklift !

so 'ballpark' , about $1000 ? +- ??

 

[85Hokie]

curious, found this..

https://www.amazon.ca/D750-Overhaul-Rebuild-Kit-15261-21110/dp/B0CS6NT1GP

say $550 CDN about $350 US$ ??
I'll assume engine has to come out/off to do the rebuild, others will know.
I'd expect x2 for labour, unless you're doing the 'grunt' work.
It's a LOT cheaper than the $5K I was quoted for my BUDA 4D-153 10 years ago and that's AFTER I'd removed it from forklift !

so 'ballpark' , about $1000 ? +- ??

found? this thread?

 

[InTheWoods]

curious, found this..

https://www.amazon.ca/D750-Overhaul-Rebuild-Kit-15261-21110/dp/B0CS6NT1GP

What experience is there with these Chinese aftermarket parts?

 

[GreensvilleJay]

I found, well Google found, several rebuild kits... I just posted the one from Amazon Canada as a 'reference' for the original poster.
While I've never used this kit, you could check prices directly from Kubota, using the parts list.
I'm sure others sell kits but where do THEY get 'their' parts from ?
Perhaps someone here that has rebuilt same engine will post what they paid for the kit they used.
The actual rebuild isn't that difficult,though I'm assuming the sleeve/rings/piston is a drop in deal(normal for me) and NOT a 'must be machined' ordeal.

 

[North Idaho Wolfman]

I found, well Google found, several rebuild kits... I just posted the one from Amazon Canada as a 'reference' for the original poster.
While I've never used this kit, you could check prices directly from Kubota, using the parts list.
I'm sure others sell kits but where do THEY get 'their' parts from ?
Perhaps someone here that has rebuilt same engine will post what they paid for the kit they used.
The actual rebuild isn't that difficult,though I'm assuming the sleeve/rings/piston is a drop in deal(normal for me) and NOT a 'must be machined' ordeal.

The Chinese rebuild kits are a dice roll at best.
The Head gaskets are almost always junk and then half the other parts can be the same.
The really bad part is finding out the parts are bad after you've run it a week and then it fails.
The sleeves are NOT drop in, they 1: need to be pressed out, 2: pressed in, and 3: they need to be machined to size.

I've rebuilt quite a few of these and they are not cheap to do a good rebuild.
The last one I did was in the $3500 range to do.
Yes they can be done quickly if you have the skills.

If you get into needing other hard parts the price can go up really quick.

Any good rebuilder is going to be in the $2500 range in labor to rebuild it.

 

[North Idaho Wolfman]

Just a small update - I started the tractor up today after more than a month of sitting. Other than some initial 'surging' at idle, it started up fine. The blow by was evident almost immediately. I let it warm up for awhile and then let it run at higher engine speeds for 10-15 min. There is no question that the blow by is just as bad (or worse) than when I used the tractor hard for a few days in late October.
It now consistently drips oil from the tube, and if I cover the tube with my finger for a few seconds, it hisses once I remove it.

Other than checking on it today, it will be another month or so before I can trailer it home to Maryland. As suggested by a few members, I will pull the valve cover and properly clean the vent screen. That said, I have the sinking feeling that the problem is stuck rings, broken rings, valve stem seals, or a combination of these.

If this is what I'm facing, what is the best way to find someone who truly knows these engines and can properly handle what's needed? I am very capable mechanically, but am not keen on tearing into this engine as a first timer. I'm happy to pay someone fairly to wrench on it with me in my garage, or I'd be fine trailering it to someone in MD/VA/PA/WV.

Andreas

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Contact @Mountainman or Chris at https://www.dieselenginetrader.com/engines/Kubota/D750/106490