Thanks Neil – We did check for fuel returning but only at varying no-load RPMs, not under load.
There should be no air or water in the system- we also ran it from a jury rigged tank, no filter, clean fuel, so I’m pretty sure those basics were eliminated.
No offense taken – most of the time people do miss these things.
We haven’t torn things down too far yet. Any way to check for a sticking governor?
Dave
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Dave,
Sure sounds like a fuel issue. The governor does not restrict flow, it sets the rotation of the plungers in the HP pump which controls how much fuel in delivered to the injectors. The fuel not used or which leaks by the pump plungers is what returns to the tank. Under light load you'll hardly see any return fuel, more as the load goes up.
The governor works by a balance between a spring and flyweights, the throttle lever adjusts the spring tension and engine speed sets the flyweight position. A quick look at the M20 tech manual shows the governor is right in the HP pump assembly, it's possible the governor is sticking and causing slow response but it may also be worn fuel pump. You'd have to pull the whole assembly and send for rebuild. Cost me about $700 for that on an M3-20B engine last year on my club launch.
Before that extreme, however, let me ask the more obvious questions (don't mean to offend, just asking): any water in the fuel? That could give a slow response, or maybe an air leak on the fuel supply hoses, though it sounds like you've checked that?
Neil Gallagher
Weatherly 35-1
Glen Cove, NY
On 9/7/2021 9:16 PM, Dave via CnC-List wrote:
Evening all. Trying to help a neighbour here with a diesel issue. His engine will rev freely at idle but with the transmission engaged in either forward or reverse, revs increase very slowly and only to 1800-2000 RPM on the tach. (whether tach is accurate or not, RPM under load is much lower than no-load, and RPM increases quite slowly.)
We have verified that neither the exhaust system nor the fuel supply to the high pressure pump is obstructed. Air Intake is clear. The speed lever works smoothly to its full travel. The compression release is not engaged. The engine runs a new facet electric fuel pump with plenty of flow.
When the engine is running, no fuel returns to the tank via the return line. The return line is not blocked, at least not downstream from the return barb at the injector. I am not sure under what circumstances the fuel should return to the tank in a diesel, as the return occurs after the high pressure pump.
All This leads me to suspect a fuel (volume) problem but i am not exactly sure where to look next. I believe all that’s left are injectors and high pressure pump. The high pressure pump has a governor, not sure if that functions by restricting fuel flow.
Any thoughts?
Many thanks! Dave 33-2
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