Diesel Power has a point (my post was from memory and Diesel Power's comment made me check my official M-B engine manual).

Engines using the pneumatic governor on the injection pump require opening the throttle all the way to check compression (see page 00.4 - 115/2). The pneumatic governor is found on some 615/616/240 engines and have throttle valves. You will have to look to see if there is a throttle valve in the intake manifold/tract.

Engines with the mechanical governor on the injection pump require that the throttle remain closed during the compression check (see page 00.4 - 115/2). Generally most 617/300D engines and some 615/616/240D engines do not have throttle valves.

Good Luck!
Tom

__________________
America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
Shop Foreman;
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.