1957 Petter AVA1 Diesel Engine "Overload Stop"

If anyone having an older Petter diesel engine has read the starting procedure in the manual, they've no doubt come across something called an Overload Stop. Folks may not understand what this mechanism does. I was one of those folks when I bought my 1948 Gibson D tractor that has a Petter AC1Z diesel engine on it. There was this funky lever that the instructions (and previous owner) said to move down and release. I (and the previous owner) had no idea what it did but I found out later that it is called "Overload Stop". And not until I got this new-to-me AVA1 engine did I realize what the Overload Stop actually does. On the AC1Z engine, the fuel injector pump rack is enclosed so I can't see what is going on, but on this AVA1, the pump and rack are hanging out there for all to see, and the Overload Stop is also right there. Well, if my engine had one, it would be right there, but unfortunately the Overload Stop contraption was missing. Not having anything better to do while recovering from rotator cuff surgery, I decided to make one today on the Smithy mill.

The purpose of the Overload Stop (as I understand it) is to keep the injector pump from sending too much fuel to the engine during normal operation. Basically the Overload Stop keeps the rack from opening up all the way when the engine is running.

You may have noticed the Overload Stop gizmo is hinged, allowing the adjusting screw to pivot. It's because during a cold startup, the Overload Stop screw can be swung up and over the stop bracket, which allows the injector pump rack to open all the way, which provides more fuel to assist in cold starting. Once the engine is at normal governed RPM, the Overload Stop screw will drop back into position by gravity.

I won't be using the Overload Stop as intended, but will use it to bring the engine below governed RPM. This engine didn't come with a variable speed control. It was designed to run at a constant 1800 RPM (producing a massive 6 horsepower).

After installing the gizmo onto the pump rack, I found the screw holding the two parts together hit the pump housing, so I made a pin to hold the two parts together, as shown below.

This next photo shows my new gizmo installed on the fuel pump rack, with the screw contacting a home-made bracket. The screw is cranked in quite a bit, but it can be adjusted to any RPM desired.

This next photo shows the Overload Stop disengaged from the stop bracket, allowing the injector pump rack to move leftward to the full-open position for cold starting.

This next photo shows a factory Overload Stop that I modeled mine after.

I did a test run with the new part installed, video link below:

Update 3/25/2022 - The injector pump doesn't regulate fuel very well at low RPMs. Sometimes the engine would die and other times it would rev up, so using the Overload Stop as a throttle control was not what I expected. I decided to make a somewhat factory-type variable throttle control, shown in the next two photos. I still need to trim things up a bit but it seems to work fairly well.