This is how the pop off valve should be mounted. Remove the valve and lightly sand the surfaces and epoxy back in place. Even with the pop off valve, a backfire could cause a crack in the air box or leak at one of the intake runner gaskets or rubber boots. Try as suggested earlier and spray a little starting fluid around the intake to see if the rpm changes. The metering plate might also need to be checked to make sure it is set correctly.
The 1800 rpm idle seems to indicate the cis already had an air leak or is was not properly adjusted. If you want to maintain the car yourself, a set of cis fuel gauges is needed.
Porsche 911 & Turbo CIS Fuel Injection - Page 18
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CIS Primer for the Porsche 911
