B7200HST engine surging below 1200RPM

[lawrenre]

I'm baffled... B7200HST D950 motor with 1200hours, ran very smooth at 900rpm but over the last two months it now surges below 1200rpm and will not idle down to 900 anymore, it dies.
Air filter cleaned, was not dirty very minimal dust/dirt
Cleared tank no water replaced fuel filter
checked exhaust for good flow, removed muffler from exhaust manifold, engine surges below 1500rpm.
replaced fuel lift pump (Amazon)
replaced injectors (Kubota)
replaced glow plugs (Amazon)
new fuel lines (Amazon)
checked valves set to .007
Compression check 310, 310, 330
cold start compression relief checked and set correctly
removed the injector pump, checked the rollers and spring tension.
All three rollers looked good, springs feel good all three have the same tension, replaced O rings on the top nuts where fuel exits Injector pump.
Removed throttle arm and replaced o ring and re-installed arm, feels snug.

What am I missing????
Too view YouTube video clink on link below.

B7200HST Surging

This is a B7200HST Kubota that has a surging issue below 1200RPM

youtube.com

 

[TheOldHokie]

I'm baffled... B7200HST D950 motor with 1200hours, ran very smooth at 900rpm but over the last two months it now surges below 1200rpm and will not isle down to 900 anymore.
Cleared tank no water replaced fuel filter
checked exaust for good flow
replaced fule pump
replaced injectors
replaced glow plugs
new fuel lines
checked valves set to .007
Compression check 310, 310, 330
compression relief checked and set correctly
removed and visually checked injector pump
All three rollors looked good, springs feel good all three have the same tension, replaced O rings top end of Injector pump
What am I missing????

Check the throttle linkage - it is known to loosen and start to slip.

Dan

 

[lawrenre]

Check the throttle linkage - it is known to loosen and start to slip.

Dan

Thanks Dan, I did check linkage and also removed throttle arm and replaced the o ring on the shaft under the arm. No change.

 

[TheOldHokie]

Thanks Dan, I did check linkage and also removed throttle arm and replaced the o ring on the shaft under the arm. No change.

Its been quite a while since I last looked at the throttle mechanism but IIRC its pretty damn simple. Have you looked to see if its doing anything strange? Can you alter the behavior by manually manipulating the arm on the speed control plate? Diagram beliw.

Dan

 

[Mountainman]

I dont see air filter and intake tube in the list of things youve checked/changed. Also a visual inspection of an injector pump doesn't tell you much other than nothing appears to be obviously broken.

 

[North Idaho Wolfman]

Where did you get the injectors?
90% of the injectors on the market right now are junk.

Agree with @Mountainman that an external inspection of the pump doesn't do a lot of good, if the internal seal / seals (gasket) are bad on the Injection pump you won't get consistent pressure.
The top O-ring is just a low pressure dirt and fuel seal.

Another thing to check is the 2 throttle springs and 1 idle spring, if one goes then it will cause all sorts of issues.
Yellow (throttle springs)
Blue (idle spring)

This is where Spring 110 attaches in the front.

 

[TheOldHokie]

Where did you get the injectors?
90% of the injectors on the market right now are junk.

Agree with @Mountainman that an external inspection of the pump doesn't do a lot of good, if the internal seal / seals (gasket) are bad on the Injection pump you won't get consistent pressure.
The top O-ring is just a low pressure dirt and fuel seal.



View attachment 143113

Another thing to check is the 2 throttle springs and 1 idle spring, if one goes then it will cause all sorts of issues.
Yellow (throttle springs)
Blue (idle spring)



View attachment 143114

My money is on something in that linkage

Dan

 

[lawrenre]

My money is on something in that linkage

Dan

Injectors are from Kubota

 

[North Idaho Wolfman]

Injectors are from Kubota

Did you get them from a dealer, or online?

 

[lawrenre]

Did you get them from a dealer, or online?

Purchased from Gold Country Kubota in Newcastle CA, The old ones looked to be the original injectors, after I replaced them I still had the same symptom. Did you get a chance to watch my video? I found a basic rebuild kit on ebay for the injection pump, springs, o rings and washers for $82 dollars! seems crazy to me for a few springs...

 

[North Idaho Wolfman]

A DYI injection pump rebuild is... well, your results will vary.
Good luck.

Have you looked at the governor and throttle springs that I pointed out?
You could have one that's stretched.

 

[lawrenre]

A DYI injection pump rebuild is... well, your results will vary.
Good luck.

Have you looked at the governor and throttle springs that I pointed out?
You could have one that's stretched.

The governor linkage is next on the list, I'm puzzled by why it surges only at low RPM if it is a linkage issue, I'm not a diesel mechanic by trade so no formal training, Today I eat ham and turkey! then tomorrow I dive back into investigating the best way to get to the springs along with checking to see how much and availability of the three springs. It appears I would have to remove the injector pump and open up the throttle housing??

 

[lawrenre]

This is the injection pump kit on ebay

Check out Kubota Fuel Injection Pump Rebuild Kit D650 D750 D850 D950 on eBay!
https://ebay.us/Vs0Kkw

 

[North Idaho Wolfman]

The governor linkage is next on the list, I'm puzzled by why it surges only at low RPM if it is a linkage issue, I'm not a diesel mechanic by trade so no formal training, Today I eat ham and turkey! then tomorrow I dive back into investigating the best way to get to the springs along with checking to see how much and availability of the three springs. It appears I would have to remove the injector pump and open up the throttle housing??

FYI: That's a pretty sad rebuild kit, it doesn't even included the most important parts the delivery pump gaskets and delivery pumps, those are the 2 parts that will leak!

You don't remove the injection pump to see the springs, you pull these two covers:

 

[Mountainman]

The governor linkage is next on the list, I'm puzzled by why it surges only at low RPM if it is a linkage issue, I'm not a diesel mechanic by trade so no formal training, Today I eat ham and turkey! then tomorrow I dive back into investigating the best way to get to the springs along with checking to see how much and availability of the three springs. It appears I would have to remove the injector pump and open up the throttle housing??

Should you need any of those springs let me know. Id be happy to send them to you.

 

[lugbolt]

I can't speak much for the 7200's but when I was putting the B7100's together, brand new off of the crate, in the early 1990's, they all surged at low RPM. RSM said it was normal. I tend to agree, hundreds of them did it so it wasn't isolated.

 

[ruger1980]

Item #110 in blue above is the start spring which help bring the rack to full fuel to aid in starting. The idle spring is marked in red below. If you just turn the adjusting screw in a little it should hopefully stabilize your idle. This is of course if there is not significant wear or slop in any of the governor components. You may have to readjust your low idle adjustment on the governor plate after this adjustment.

 

[lawrenre]

I pulled the side stop plate and cracked open the upper throttle plate today, all three of the springs were ok and nothing was broken, that is not to say all the springs are like new...
I will try the idle adjustment tomorrow.

 

[lawrenre]

OK well the idle screw seemed to do the trick, the springs have started to weaken I guess, probably 2 turns and it smoothed out nicely. readjusted to stop screw at the throttle arm and at the lever adjuster. Now if it just had 10 more HP it would be perfect!! Thank you all for sharing your expertise.

 

[North Idaho Wolfman]

OK well the idle screw seemed to do the trick, the springs have started to weaken I guess, probably 2 turns and it smoothed out nicely. readjusted to stop screw at the throttle arm and at the lever adjuster. Now if it just had 10 more HP it would be perfect!! Thank you all for sharing your expertise.

You know there is a really good reason that they put anti tamper seals over those right.
It's really never a good option to mess with adjusting those points, and it's Highly unlikely to change anything at all.

But yes have at it POP off the aluminum caps and adjust away.