Running a bit rich

yeah they are known for misfires! Not so long ago when having had the privilege to meet MaseratiMike he ran into the same issue when we drove in the Belgian Ardennes.

If you have no vacuum leaks and everything is good on the fuel system, new injectors are certainly a good idea since they wear out, you can tune it more rich or lean with the mixture screw between the airflow meter and the Fuel distributor.

Be aware the if you set it leaner and it is too lean you can cause serious damage to your engine.

Can you show us a pic of all of your spark plugs and order then per cylinder? Reading out spark plugs tells you a lot in these cases.

 

What exactly are the pressures ? Cold control, warm control, and system.

I doubt if you could damage the engine with the mixture screw in the FD assembly. It really controls idle mixture more so than higher RPMs. If you adjust that screw with the motor running, you will probably get a stumble with 1/4 turn rich, or a 1/4 turn lean. The sweet spot is somewhere in between. Without a cat, it's not that critical.

K-Jet problems are known for vacuum leaks, and clogs. If you're not running a fuel system cleaner, you've probably got some clogs in the FD and injectors. Many fine filter screens in the system that can become clogged up. I highly recommend Techron.

 

I don't believe you will get too much of an idle change from adjusting the mixture screw, but you will get a pronounced stumble at the too lean, and too rich points. Usually 1/4 turn in either direction.

For running mixture, hold the throttle open to 3500 RPM, then press slightly on the air vane. If the RPM goes up, you are too lean there. The difference between the system pressure and the warm control pressure is what sets the running mixture.

 

A method for adjusting the mixture screw is to turn counterclockwise until the lean stumble, then turn clockwise for max vacuum. Note the vacuum readings on these motors is about 1/2 that of Detroit iron.

 

The mixture screw is mostly for the idle setting. Pressing on the air vane at idle will likely just bog down the motor. Try revving the motor to 3500, and then press slightly on the air vane. If the RPM goes down, you are either too rich, or at the optimum setting. If the RPM goes up, you are too lean. You can't set the running mixture with the idle mixture screw. You need to adjust the control pressure for this. Before changing the control or system pressure, make sure you have a gauge, and the specs on the pressures, and probably you should rebuild the WUR first. Common faults with the WUR are a clogged filter screen at the input port, and worn "O" ring in the fuel cell. E.g. a worn "O" ring there can raise the control pressure by 5 PSI. This will make a big difference in performance.

12" @ idle is normal. This depends also on the idle speed. I get 14" @ 1100 RPM after warm-up (12 minutes).

 

I think you have a vacuum WUR which means part throttle should apply max vacuum to the WUR and raise the control pressure, leaning out the mixture. So check that out, you could have a leaky vacuum hose, or a leaky diaphragm in the WUR.

The dieseling could be a leaky injector, or a worn "O" ring in the system pressure regulator. This assembly is located inside the FD. Your test gauge should show control pressure holding over 30 PSI for 20 minutes after the motor is shut off. If not, there is a leak in the system.

20 PSI with a warm motor would be stumbling and putting out black smoke. 49 is normal, or slightly to the lean side. 20 could be a correct cold control pressure. Cold control pressure should go up to the warm setting in 2 minutes. Maybe get a new gauge and test the control pressure again.