Kubota D850 rebuild, prins off-road forklift

[Theemskerk30]

Hello, new member here and coming in with the best intentions not to start a thread that’s been touched on in the past.
I have a late 1980’s d850 motor in a forklift that I had machined .5mm over due to cylinder wall wear. New Kumar bros pistons and rings installed. I am struggling to gain any more than 300 psi in each cylinder when doing a compression test.
I had the head cleaned and the valves lapped.

Wet compression test added about 30-40 extra psi bringing me to 350ish

Battery fully charged

On a leak down test of every cylinder it is well within the allowable limit and has almost no leaks.

Looking to see if anyone has any ideas as to why my compression is low, leak down test showed to be good? Valves lapped, head cleaned. New head gasket installed and then removed and then the original Kubota head gasket installed to rule out any difference in thicknesses

 

[RJ Power]

Hello I have the same exact problem with a v1200 in a hi lift. Any body been here ??

 

[Russell King]

I would suspect the rings are incorrect or incorrectly installed.

Are the valves adjusted properly and closing properly? I think there are tests that remove the valve train but I have not gotten into that so not sure if it is possible or a good idea on Diesel engine.

The head gasket should probably be new and Kubota brand (if you have to remove head again) when replacing the old one. There is anecdotal information that other brands cause your problem with lower compression. Not sure what reused head gasket could do with the compression test results.

Do you have a work shop manual that has a trouble shooting guide?

 

[RJ Power]

I would suspect the rings are incorrect or incorrectly installed.

Are the valves adjusted properly and closing properly? I think there are tests that remove the valve train but I have not gotten into that so not sure if it is possible or a good idea on Diesel engine.

The head gasket should probably be new and Kubota brand (if you have to remove head again) when replacing the old one. There is anecdotal information that other brands cause your problem with lower compression. Not sure what reused head gasket could do with the compression test results.

Do you have a work shop manual that has a trouble shooting guide?

 

[RJ Power]

I am a engine pro of many years but limited Kubota industrial engine work.
I recently did a V1200 4 cyl for a hi-lift machine. Bored , honed for chrome rings, good valve job, etc.
The thing was hard to start and I found low comp. 310-320 on all 4. No blow by at all, revs nicely when it
does start, no smoke at steady speed, but low on power. I used the Kumar pists & rings but Kubota
head gasket. Pists. above deck matched manual, gasket thickness matched manual. but pist to head was
about .010 more than manual. The cylinders have ok leak down for rings that are not seated 15- 20 PCT
I thought maybe the cam timing could effect it if a tooth off but that's a stretch. I saw 1 other post
with the same symptoms but it got no replies. Any ideas ? thanks RJ

 

[kubotafreak]

Nobody has made mention of ring gaps or ring in ring land tolerance. My suspect would be the brand of piston. Is the piston head machined too much on the valve reliefs?

 

[whitetiger]

I have had a lot of bad experiences with that supplier's parts and filters offerings.