# Unusual B275 throttle problem



## marek916 (Aug 17, 2013)

I have a b275 with an unusual throttle problem I cannot seem to diagnose.
The problem is with the throttle diaphragm.
The problem is after winter storage, tractor will not rev up at all.
I've attached some pictures to show exact model and set up. 

Observations:
Nothing wrong with linkage. 
Butterfly is free and still attached inside.

If I pull of upper vacuum hose, rack moves and full throttle is applied. 

I can blow through both both pipes to throttle/venturi body, doesn't seem to have any restrictions. 

With both pipes connected diaphragm is pulled fully out against return spring and stays there. Rack oscillates back and forth very slightly at extended end of travel, engine revs fluctuate around idle accordingly. 

Air cleaner doesn't seem to be blocked, symptoms are identical with oil trap removed.

If I block one pipe on diaphragm body and suck on other there doesn't seem to be a leak (i.e. not a perished diaphragm).

If I remove the diaphragm body, I can blow and suck through both pipes.

If I replace pipes (with white hose) it makes no difference. 

With engine running there is vacuum on both pipes from throttle body/venturi. 

No one has touched the machine since last time it ran perfectly.

Diaphragm moves freely by hand.


I don't understand what could have changed that would account for the diaphragm being pulled out fully. If there was a leak or perished diaphragm I would have maximum throttle since the diaphragm would be fully pushed in (by the spring) and be at full demand. This would also be the case for a split hose (not the safest of designs!).
The pipe on the end of the diaphragm housing I think is an anti stall device that is attached to a valve that in the end of the housing that gets closed as the diaphragm reaches the far end of its travel. I think it's job is to reduce the vacuum just before stall allowing the rack to increase throttle. 

Does anyone have any idea what could of changed or explain this behaviour??!

Thanks!


----------



## marek916 (Aug 17, 2013)

Pictures:


----------



## marek916 (Aug 17, 2013)




----------



## RC Wells (Dec 26, 2008)

Check your fuel filter and the inlet from the fuel tank. This past two years has seen a real spate of algae growing in diesel, and that clogs the filter and the small fitting in the bottom of the fuel tank.


----------



## marek916 (Aug 17, 2013)

Hi, thanks for the suggestion. I can get full throttle if I push the rack in or take the vacuum pipe off so I suspect there isn't a fuel blockage. The tank return is free flowing also.
I'm pretty sure there is too much vacuum being created in the inlet venturi but I can't figure out why that would have changed!


----------



## RC Wells (Dec 26, 2008)

At this juncture I would remove and clean the pneumatic governor valve assembly attached to the intake manifold. If there are no problems with that, you can see the components of the related system in the pump itself by selecting the appropriate slide on this page: http://www.oldengine.org/members/diesel/Injection/Fuel1.htm

The most common pneumatic governor problem, outside of the diaphragm, is the valve in slide #3 becoming slightly rusted and sticking so the port release does not open which results in vacuum building up on the wrong side of the diaphragm. 

As far as safety, these are quite safe as the default in the event of failure is to slow the engine.


----------



## marek916 (Aug 17, 2013)

I took the venturi valve off and it seems to rotate easily. I took a look at the link you sent, which slide do you mean by #3? Do you mean CAV venturi page 3? Which item is the valve you are talking about?

Going back to the safety thing, the pump is at full throttle when the diaphragm is pushed in by the return spring. The vacuum actually retards the throttle demand. When you rotate the stop control it again manually pulls the diaphragm out.
This means if the vacuum pipe fails the spring pushes the diaphragm to full throttle. I figured this out from experimentation, is this not correct function?

Cheers!


----------



## RC Wells (Dec 26, 2008)

You are headed in the right direction with the CAV pneumatic governor slide if you look at the picts for Governor 3. You should have linkage to the arm on that valve, and I do not see any in the photo, just a rusty arm.

Also, in the third posting you did of the photos, there is an adjustment valve to control bleed off that is located behind the bleed port (the guy with the metal filter). Inside the housing look at the condition of that internal port to be sure some partial blockage has not occurred.

In the next slide down, you will find a small spring tagged with the number 67. If that spring gets moisture it will rust and then the internal diaphragm unloader will not function. Same is true of part numbers 70 and 71. 

But I would first disassemble the vacuum valve you took off the manifold and look for blockage in one of the ports or one of the screws to the butterfly valve to have fallen off and ended up in one of the ports. 

When I see these sort of problems, it is generally because the equipment was power washed and then put away and stored after being covered with one of those infamous blue plastic tarps that trap moisture and allow high humidity to rust the works.


----------



## w4dsb (Mar 13, 2017)

*i have the same problem!*

did you solve your problem? i seem to have the exact same problem any help would be wonderful


----------



## marek916 (Aug 17, 2013)

If I remember correctly, it was the right angled rubber inlet hose where it comes out of the inlet manifold had been squashed flat or had moved and was either restricting flow or letting in air.
I couldn't see it at first but I kicked myself when I found it!!

Let me know how it goes...


----------

