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Old 07-24-2000, 09:12 AM
stevebfl stevebfl is offline
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Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Larry,

The early pumps were all vac controlled mixture (pneumatic control). Even the first gas pumps were that way. I have a 58 300d ch189 that uses basically the same pump as did the Gull Wing ch198. The throttle housing has jets in it to modify the application of vacuum to the pump.

In the 621,615, and 616.916 the pumps were pneumatic controlled. They had very low manifold vacuum (I think I measured it once at about 6in at idle. It allowed a diaphram against a spring in the back of the pump to move the rack rich or lean.

Many mistakenly think that the linkage to the back of the pump controls mixture; it does NOT. Its sole purpose was a dampening action similar to the rack dampeners used at idle (on 617.95x) to prevent the ruummp...ruummp...ruummp that occured at constant speed.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
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