Could be a couple things.
First, check the seals on the left side. Spray some carb cleaner or brake parts cleaner on the injector seals at idle. You don't need much. If the idle changes, they are leaking, replace. You may want to replace them on principle.
Next, if the K-Jet has the split manifold like the D-jet does, you need to check the seals between the top and bottom halves. Same drill, but aim at the seals. If those leak, pull the manifold and replace them.
Did you put a new gasket on the left manifold? I's assume so.
If that isn't it, check the valve timing on both sides. I don't think you would run with the left side a whole tooth off, but hey, anything is possible, and that would affect all the plugs on that side.
Make sure you don't' have an idle hose leaking, I'm not sure what the setup is on the K-jet, but the D-jet has one for each side. A leak will screw you here.
Make sure all the vac taps are OK, and that nothing is leaking vac on one side -- use a vac pump (MitiVac -- I should charge them for advertizing! -- works well). If you have a vac servo or door locks, etc leaking into one side you can get lean running over there.
The key, I think, is that all four plugs look different than the other side, has to be something that affects all four equally.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!