I have a HD5 that I've had some time but haven't used. Tinker on it once in a while. Wanted to use it recently and found a few problems that need resolving. Starts easy but under the slightest load blows black smoke and losses power. Removed the air cleaner and it was a mess, dirt clogged the mesh. Took a while but got it cleaned up nice. Still the same with it back in. Thought maybe it was clogged worse than I thought so tried running for a short bit with it disconnected, no difference, hooked back up. Gauges were all inop so I replaced them. I found with the new gauges that the thermostat is missing or stuck open, temp doesn't raise until it is shut down. Fuel pressure is 6-8 psi at idle and only 20 psi at full throttle. Where the restrictor should be on the block is a straight fitting. Is there a source for the restrictor or a way for making one?

I have the service and owners manuals but don't see what size the hole should be. I took a look and found that there is a thermostat in the dozer and it is a 180 deg as it should be, will replace it. I found that the gasket that had been used covered the bypass hole, should it be covered or open? I would think open.

there is no restrictor in the fuel line per-say as the flow from pump also is the cooling for the injectors - (why there are 2 lines ) as fuel passes through injectors it returns to tank and the only restrictor or anything is the diaphragm on back of stock fuel pressure gauge as the gauge would read pulsing caused when injectors stop flow when they are being operated by cam and plunger in injector is depressed . 
 Fuel pressure is created at pump and will vary as speed of engine changes - that's normal Also if oil bath air cleaner is not kept clean in both the oil in bowl and the element itself they need to be washed thoroughly and kept clean . 
 Remove element , soak in diesel or gas or a solvent to make sure it's clean and flows air without restriction - use proper weight oil (filled to line in cup) as it's required .   

Did you check the air box drain to see if its plugged with dirt/ mud wasps? 

as an example.  on my HD20 the restrictor orifice is .058-.063.    of course this is a whole different animal but you can be sure yours will be smaller.

Edited by CAL(KS) - 19 May 2021 at 9:00am

Ha, I'd say that needed to be cleaned! Worn tracks are an issue with a lot of the AC machines since parts are not so readily available. There's an HD-11 a few miles down the road that I've had my eye on, but I know I'd have a ton time and of money in it between engine parts and undercarriage. I hope your HD5 holds up long enough for your purposes.