Fokker V.23 + MARZ 2.5D

The fuel tank is about 35cc, so the run time of less than two minutes is insane (or insanely low, I should say).
The cockpit is not fuelproofed, but no fuel is ever going to make it there, so that's fine. The engine bay is completely covered in epoxy. One part of the model that isn't fuelproofed, but should be is the bottom - a lot of spent oil is being blown there from underneath the engine cowling. At some point I will recover that part with fresh tissue and secure it with thinned down epoxy.

With regards to the RPM, the engine manual says it should be "no less than 15500" (on a 8x4 prop). This to me sounds like pure fantasy, in no report or video I've seen does it ever exceed 14.5k. My example is doing 12.5k in best case scenario Fair enough, I use more oil and less kerosene in the mixture than specified in the manual, so that's definitely a factor contributing to less-than-stellar performance. As these engines are hard to come by I prefer longevity over squeezing the last watt of power or the last gram of thrust out of it.

An 8x4 is a rather small prop for a 2.5cc diesel. I would use a 9x4-9x5 prop. That will keep the revs down and thus lower the fuel consumption too.

That would be true for a typical diesel, but the MARZ is really atypical. It has a short stroke, making it more suitable for lower diameter props. Also, the crankpin is a weak spot, therefore 9'' props aren't recommended, other than for breaking in, where the engine runs at low compression levels.
The engine manual calls for a 8x4 prop, so that's what I'm running it on.

A larger prop is actually a lower load on the engine and the conrod, as with on a diesel engine you will reduce the compression ratio accordingly. That is why one should run-in a diesel engine on a "larger" prop.

I come across the Marz with regular frequency. I personally steer clear due to the manufacturing of these engines. The Marz, Meteor, Ucktam, Polet and others were basically teaching implements. You either had one that worked, worked ok or didn't work at all. While you have the supposed listed possible rpm's, it wouldn't surprise me to find the rpm range are  all over the place for these engines. I  agree with Kris that the 8x4 is too light. The other problem is that the manufacturers that decided the 8x4. What 8x4 prop was it? The glass and nylon  props that were typically available  are all over the place in terms of pitch and the end result is generally lighter then claimed. Therefore I can see the rpm's being higher due to a lighter pitch. I have 4 .15 sized diesels and none of them utilize a 4 pitch prop. That would be the MVVS .15, the Enya .15 Mk II, Silver Swallow ( Yin Yan), and the CS copy of the Yin Yan. I use a 8x6 for combat and a 9x4 for sport flying.

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This is my MARZ.

It powers my well-used SIG AkroMaster on 52" lines with authority.

It took a while to run in but is a ready starter on running settings with no more than a choke prime.

To me they are the best value 2.5 c.c. sport diesel. Sure, they are a bit rough in presentation but they go like a cut snake. Look at the specs:

1. Twin Ball Raced
2. Reverse Drum Induction
3. Sprinkler Venturi and a decent "ST" type NVA
4. Webra Mach 1 2.5 copy porting

Nearly ten years since I first lifted it out of its box and it just keeps getting better.

Here it is fresh out of the box on the stand back in 2013. It has a "Macheast" exhaust collector ring.

https://www.youtube.com/watch?v=kde17U2ZlNA

I’ve heard that if you get a good one, it’s a good engine. Average engines run well, but may quit abruptly due to a piston or bearing failure. A fair amount of them are duds from the beginning. Generally it was recommended to dismantle the engine first before attempting to run it to get rid of shavings and to smooth the edges a little if necessary. Just like any piece of Soviet machinery really.

I have one that I got with a bunch of engines, it’s frozen solid, not sure if it has seized mid-air or if it is just stuck with castor goo. There’s relatively large amount of old Soviet diesels here in Finland, as they were a popular souvenir to bring home back in the days of the bus tours to Leningrad. Mostly MK-17s, but a lot of MARZ’s too.

These were made at a helicopter repair facility during times when there were no choppers to overhaul or to make/repair parts for.

There’s an old joke that went around in Finland since the ’60s regarding the Soviet manufacturing quality:

”Big country, big tolerances”