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Old 05-26-2003, 10:07 AM
stevebfl stevebfl is offline
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Float level was a major factor in those carbs. The level of the fuel should be 5mm below the parting surface of the top cover. To measure, disable igition, remove bowl cover, hold float pivot retaining sping down with ones finger (not imparing float rotation), sop up enough gas that it is significantly below the level, and have someone crank the car till the bowl fills. Do not measure the miniscus of the fuel go by the surface level.

Be sure your main jets measure the 100 that they should be. Take them to 105 if you wish.

And to fix the problem one must deal with the secondary air valve. If one looks it can be seen that the secondaries open whenever the throttle goes beyond a certain point. There is a point at the secondary throttle shaft that one can wire the secondaries closed (there is a small lever and a startegically place whole for the wire) so that they never open. This can occur as the linkage from the primaries is through a spring and it just stretches when the secondaries don't open. I would advise that for diagnostic purposes you wire the secondaries shut and take it for a drive. You will probably be amazed at how well it works on 2 barrels. I have wired a number shut where the secondary airvalve was beyond repair.

Once, sure that this gets rid of the bog, one can attempt a repair. The concept behind the secondary air valve is that the secondaries are so big that on openning the venturi velocity is so small that no fuel flows and that is where the bog comes from: two big holes adding air with no fuel. The airvalve acts like a choke and allows a certain negative pressure to be developed under the plate (which then opens the plate). This pressure replaces the venturi pressure differential to move fuel untill wide open and of sufficient velocity. The act of openning the secondary airvalve also lifts the secondary metering rods. Even with the rods all the way up, if the valve goes wide open there is no pressure difference to move fuel and the thing just goes slower.

MB made a set of weights and a fixture to place them on the airvalve trying to get quantitative with their adjustment; won't happen. The balance is dynamic to other variables and has to be set by feel.

The first thing is to make sure the choke is off and place one finger on the airvalve. does it spring open and closed against the finger? Does it bind? The warping that affects this problem happens in the top and causes the three pivot points of the airvalve shaft to be non colinear. This causes the most serious binding. I have reamed the holes and sanded the shafts strategically to make a smooth rotation. It doesn't need to be smooth for more than 90 degrees of rotation. Slightly bending the shaft can help (when done at precisely the right point.)

Once the shaft moves freely the spring must be adjusted. At one end of the airvalve shaft there is a small allen set screw that tightens on a small shaft that has a small screwdriver slot in the end. Turning this shaft winds up the spring. *Before loosening the set screw be very sure that you are in the screw slot positively* as it will unwind in a hurry when the set screw is loosened.

Loosen the set screw and tighten the spring to taste (bouncing the plate with ones finger for flavor). The real test is on the road. One should start loose and drive. There will be a bog as the secondaries are engaged with the throttle (by the way remove the wire holding the secondaries closed for this part). Tighten some more. Theoretically when one passes the optimum spring tention there will be a small puff of black smoke from too rich a mixture and it may feel similar to the original bog. It will bog from too little or too much fuel.

Mostly you won't get it too tight; for one nasty reason. As you reach too tight the spring will come off the little adjuster we have been fooling with. This will require removing the top to reset the spring, so be carefull not to go that far.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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