I have twin 671 detroit diesel jt engine On a 56 ft ocean motor Yacht open cockpit only gets 1500 rpm should be 2550 rpm under load bottom is clean fuel filters changed props are 28×28 should be 31×28

Gasoline Engines

The 1969 C50 pickup truck was available with three different gasoline engines, including a 292 in-line six-cylinder engine that produced 170 horsepower at 4,000 rpm and 275 foot-pounds of torque at 2,400 rpm. Other engine options included the 350 V-8, which output 200 horsepower at 4,000 rpm and 325 foot-pounds of torque at 2,000 rpm, and the 366 V-8, which generated 235 horsepower at 4,000 rpm and 345 foot-pounds of torque at 2,600 rpm.

Diesel Engines

Customers that were interested in a diesel-fueled 1969 C50 had a choice between the Detroit Diesel (4-53N) engine, which offered 130 horsepower at 2,800 rpm and 278 foot-pounds of torque at 2,000 rpm, or the more powerful Toro-Flow Diesel (DH478) engine, which output 165 horsepower at 2,800 rpm and 337 foot-pounds of torque at 2,000 rpm.

if on a V 6 petrol @ 1500 rpm
could be the knock sensor
distributor timing might be out
or serp belt tensioner bearings

if a diesel i would expect a rattle@1500 rpm

compression pressure is read at cranking speed with throttle wide open. For petrol engine it is 155-165 psi hot dry with an allowed variation of no more than 10% between adjacent cylinders and around 175 psi hot wet (OIl injected to seal rings)
For a diesel engine it can be 550 psi -650 psi hot dry at cranking speed with the only engine being test while running being a detroit diesel 2 stroke
There is no compression reading done at rpms as it is meaningless

1st 1000 rpm
2nd 1300 rpm
3rd 1400 rpm
4th 1500
5th 1600
6th 1700
7th 1700
8th 1700
etc

6.2L diesels ... 84-86 time period (mostly 1984) & were powered by 6.2L Detroit Diesel V8 engine. ... Many M1028s were upgraded at the company level to M1028A2 and A3 specs. ... rated at 155 hp (116 kW) and 240 lb·ft (325 N·m), which was 5 hp (3.7 kW) .

Diesel engines smoke when under a heavy load. Unlike a gas engines, a diesel engine uses fuel that is more like an oil.Smoking is a good indication the fuel is not completly burning because of loaded engine spee. White smoke indicates the fuel is not ignitingefficiently. Truck, boats, tanks abd anything else using diesel fuel will smoke when it gets more fuel than it can use.. Easing up on the throttle will lessen the smoke but not eliminate.. Of coarse if the demand is lessened by applying less loadyhe fuel consumption drops.Not to worry as long as the smoke doesn't attract the EPA.

it will be unlikely as a Detroit engine is designed to fit gearboxes behind it . IT is a standard 1 or 2 housing and by knowing this it cam be decided if a certain box used in heavy vehicles will fit behind the engine . Having said that there may be an adaptor plate to do what you want. Talk with Detroit engineers in line for any information that they can give. However the 4 cylinder 2 stroke Detroit diesel was not an engine that met emission standards and thirsty and was a very noiye engine that in a car will make listing to the audio system practically impossible

series 60 have 2, are you referring to a 671 ,2 stroke or a Silverado v6 detroit

thoroughly clean area around injectors,and then blow with compressed air so no bits will fall in the holes when injectors removed.loosen main injector pipe nut,remove return fuel line,dont lose copper washers,put special fuel bolts back in top of injector firm[by hand] undo holding down bolts evenly,undo fully injector pipe nut,.gently pry injector loose,and remove,making sure sealing washer at bottom also comes out..while I dont specifically know this particular engine,this guide applies to most diesel motors.I have rebuilt a 6v72 detroit motor,yrs ago.Assembly is reversal.may need to crank over to bleed diesel at injector,always use new sealing washers.