Comments on the MD31A engine:
4 cylinder 2.4 liter naturally aspirated engine with 62 hp@ 3500 RPM
......Very robust engine for 62 hp, as the same block in turbo/aftercooled version produced 130 hp. The engine was designed and built by Volvo Penta
and produced between 1986 and 1994.
Your existing MD21B engine was based on an Pugeot/Indenor block, so there will be no parts interchangeability, with perhaps the exception of the transmission
Depending on your vessel, getting the MD31 to fit where the MD 21 currently sits might be tricky. The MD21 engine was fitted with a fairly small diameter flywheel, and that resulted in a relatively small diameter bell housing. The MD 31, on the other hand had a larger diameter flywheel and SAE 4 bell housing. I have seen instances where the MD31 flywheel housing couldn't fit down between the MD21 engine beds, and the beds had to be rebuilt.
If the engine was properly prepared and preserved when it went into storage
I think it should be OK coming out of storage
, but you should hear it run for awhile before making a decision. You mentioned it was used as a generator
.....so what if anything was done to the injection pump to allow it to run as an 1800 RPM
gen set engine? Can you find out? If you hear it run it should be able to hit about 3900 RPM at hi-idle as a marine
As I recall
, the MD21 engine had a max. rpm of 4500, and the MD 31 has only 3500 rpm, so there will need to be either a gear
ratio change, or a prop change, or maybe both. A good prop shop can help out here.
Hope this gives you something to think about.