A common sign of a faulty valve, is the carb’s severe inability to hold an adjustment. No matter what you do, you’ll always have to go back and adjust it again. Other signs are that it’ll idle erratically, accelerate poorly or just outright stall.
Of course you’ll have to do some diagnostics to rule out other issues first- like an air leak or some other carb issue.
But if it still has trouble even after all things have been run through and the needles are (re)adjusted, the valve will more than likely
be the issue.
The Power Mac 6 service manual has a pretty good rundown on the MDC. But most checklists tend to skimp on the details on when a check valve is failing.
(zama comes to mind)
Here is a different and a bit more detailed amalgamation checklist. You can see the valve come up as a cause for several problems.
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