so i have a 1985 300d with a loud knock to it and it has really low compression (tested cold) but has no signs of low compression other than its hard to start sometimes. The car doesn't shake when driving and the knock cant be heard very well over the turbo when moving faster than 45. i already rebuilt the injectors and did a valve adjustment. so i was wondering if the timing could be off and would cause the knock low compression. btw the car does not smoke at all and idk if it uses much oil because it already has a large oil leak.

I am not sure what you mean? Do you mean the Knock sounds like a Diesel Fuel Nailing type Knock or do you think it sounds like a Mechanical Knock (Rod Bearing Failure).

What is your Mileage and did something bad like overheating or loss of Oil pressure happen to the Engine?

A valve adjustment might help the compression issue.

If the Camshaft Timing is off due to Timing Gear Wear/Chain Stretch stretch you could install an Offset Woodruff Key to correct it (done before re-timing the Fuel Injection Pump).
If you already have the Drip Tube re-timing the Fuel Injection Pump would not cost anything except for a new Crush Washer. (If the Timing is late it can cause Nailing and I found out if it is too advanced it will also nail at idle.)

If you think there is a Mechanical Knock you might want to inspect the bottom of the Oil Pan for Metal Fragments to see if a Bearing is going out.

In the last 3 Months or so a few people have had their Crankshaft Dampers go bad. Some said they had a knocking noise. If possible a Mechanics Stethoscope to try to locate exctly where the sound is coming from.

Then there is that darn Air Filter Bracket. When mine was loose it sounded like something was going to fall off of the Engine.
I also did that trick I read on this forum of putting a piece of Heater Hose between the Air Filter Housing and the Intake Manifold to keep the Air Filter Housing from hammering on the Manifold.

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84 300D, 82 Volvo 244Gl Diesel