Posted by Alan Bowes on March 04, 1998 at 02:28:25:
In Reply to: M37 carb troubles posted by Joel Foley on March 03, 1998 at 18:09:20:
Okay, as to the valves:
The vent line from the oil filler and the crankcase ventilation line both may have shut-off valves installed. These were designed to be shut off during fording operations. Since I doubt that you plan on submerging your vehicle, you can leave them open.
The crankcase ventilation line has a metering valve which must operate correctly, not only for proper mixing of crankcase gases with the intake air for pollution control, but also to prevent too large of a vacuum leak. This valve can be easily disassembled and cleaned.
The M37 uses a closed crankcase ventilation system. The metering valve allows more crankcase gases to be drawn into the intake manifold at high manifold vacuum (at idle, light loading, coasting, or deceleration) than at low vacuum (high load, acceleration). Under high load conditions, the combination of low manifold vacuum and extra blow-by gases create a positive-pressure condition in the crankcase and these gases are vented out of the oil filler breather tube into the air intake above the carburetor. At low load conditions, the combination of less blow-by and higher manifold vacuum creates a low-pressure condition in the crankcase and air is drawn in the opposite direction from the carburetor air intake through the oil filler vent tube and down into the crankcase to replace the gases that are being sucked up into the intake manifold.
Anyway, if the metering valve is stuck open, it could create an excess vacuum leak which could cause a slightly lean mixture at idle. On the other hand, if it is stuck closed, it could cause a slightly rich mixture at idle. However, it generally would not be a drastic effect.
If you have a severely over-rich mixture, there could be other causes. The most common is a float level setting that is too high. A leaky float valve could also cause it. If the ball check valve for the accelerator pump discharge is stuck open, it could allow more fuel to be drawn into the carb by venturi vacuum, though this effect would be most pronounced in high-load conditions and should not affect idle mixture significantly. If the step-up piston and/or rod is hanging up, it could create an overly rich mixture under lighter load conditions, but not so much at high load or at idle, since it is open anyway at high load and at idle there would generally not be enough venturi vacuum to be significant. Rich idle mixture can also result from a damaged idle mixture screw or orifice, from a damaged gasket between the main carburetor body and the throttle body. A loose main jet or step-up jet could create an overly rich mixture under light or heavy load conditions. A badly clogged air cleaner can have some effect on mixture, although this is mitigated to some extent by the fact that the fuel bowl vent is vented to an enclosed area between the air cleaner and the carb throat, which means that the higher vacuum in the air intake affects both the venturi pressure and the fuel bowl pressure about equally. A choke has a much greater effect on mixture, because it is between the fuel bowl vent and the venturi, so it increases the pressure differential between the venturi and the carburetor bowl, which allows more fuel to be drawn in by the nozzle in the venturi. Which brings us to the choke. Is it opening up all the way?
Let's see, what else? Some replacement step-up pistons didn't have a vacuum break passage drilled from the top of the piston to a groove near the bottom of the piston. Without this hole, there is a tendency for more fuel to be sucked down around the outside of the step up piston and into the intake manifold when at idle or a low-load condition with high manifold vacuum. However, this usually only happens when the float level is too high or when driving on very steep or rough roads that slosh the fuel around in the bowl.
All that said, it is most likely due to a float level misadjustment or a float valve problem. I suppose that you could have a hole in the brass float, allowing it to sink and raising the fuel level in the bowl, but that doesn't happen too often.
That should at least give you a few ideas.
Have fun,
Alan