I'll go with the blocked CC vent too. Pressure comes from compression pressure leaked by the piston rings. It has to get out somewhere. Unless you abused/neglected it, 1300 hrs. doesn't require an overhaul, but if you haven't been using a chemical to remove carbon from the combustion chamber and are sloppy with your oil change timing, then you have your smoking gun.
Black smoke under load is unburned fuel if the above isn't the problem. You either have clogged injectors, not misting...making blobs of diesel that don't ignite in time and get blown out the exhaust valve after the combustion process has completed for that stroke usually, or you may have a bad injector. Have them tested if you have a facility handy. Cheapest, easiest is to try chemicals first.
1. Go get yourself a diesel injector cleaner chemical and follow the directions. Power Services products are readily available, priced right and work. There are others. Give it time to work.
2. If your load drops your rpms more than 5% at PTO rpms, look at dropping a gear or lightening the load. So for you, assume your PTO rpms are 2400. 5% of 2400 is 120 rpms. If you are running along ready to drop the implement at PTO rpm and your rpms drop from 2400 down to say 2250 or less when you drop the implement, start the PTO, whatever the load, you are overloading your engine in my book. I never read a set number in any manual so I'm posting my gut feel number. 5% tain's gonna wear any main/rod bearings out prematurely!
