Re: Why does my diesel not rev when in gear?
If you could provide some more information on your exhaust color, it would be very helpful. Blue exhaust means that you are burning oil (bad compression), black means you aren't getting enough air (plugged air filter), white is water or timing and grey is poor combustion (a good sign of bad injectors)
As mentioned, the older volvos can get very hard to start with a lack of compression, air in the fuel lines will make it extremely difficult as well. For starters, you need to get the engine cranking at the appropriate speed and it sounds like your batteries are old and worn out. A slow cranking engine will not build as much pressure due to a longer time for the air in the cylinder to leak out and a longer time to loose heat to the cylinder walls. To rule out air in the fuel lines, try pumping the hand primer for a long time before starting the engine on a cold start sometime. If it fires up like it used to, then you have an air leak but if it doesn't show any improvement, then you probably don't have one. Many people insist that they don't have an air leak when they actually do. The air leak can be either on the send or return side and they can be very difficult to find because they are often invisible. If batteries and looking for an air leak don't solve your problem, then I would look into running a compression test and hooking up a fuel pressure gauge.
When an engine fails to rev up in gear, it either isn't making enough power or there is too much load being put on it. The easiest things to check as to whether you are making sufficient power is to make sure that your shutdown cable is properly adjusted and your throttle cable is properly adjusted. You should be getting full travel on the arms of both of these, if you don't, you need to fix it. The next place to look would be fuel pressure if you have a fuel pressure gauge. Fuel pressure can be inadequate due to an air leak in the suction side of your fuel line (this could be related to starting), a bad lift pump, a plugged fuel filter or a bad overflow valve. A pressure gauge at the inlet to the injection pump is really the easiest way to go through this but you can trouble shoot it without it. Finding an air leak would be easiest to find when trying to sort out your starting issue. A bad lift pump will be very difficult to diagnose without a pressure gauge. The fuel filter is cheap and your should change it and cut the old one open if you haven't done so already. To check your overflow valve, try pinching off the fuel return line with a pair of vice grips for a very quick test and see whether things improve. Also, if you haven't looked at your air filter, it would be worth doing.
It is also possible that you are putting too much load on the engine although this seems unlikely if it revs sometimes and it doesn't rev others. Were you overpropped to start with? Is your prop or hull dirty? These are the most obvious culprits but there are plenty of others such as a worn out cutlass bearing.
If I had to take a wild guess based on your symptoms, I would guess that you have an air leak in a fuel line on the suction side of your lift pump which makes the engine hard to start and means that the injectors are not getting a good supply of fuel. Someone who really knows engines can often tell this by sound and exhaust color alone.