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carb stationary rpm issue


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Anyone some input on the following:
When I press the accelerator and then release it the engine rpm drops well below the stationary rpm and sometimes the engines cuts out. If the engine keeps running, the rpm returns to the normal setting of 900 rpm.
Previously one of the carb vent holes (top one on the right) were blocked by the gasket fitted between the carb and the air filter. The problem started to occur when I opened the hole.

The carbs are CDSE 150. Maybe I should change them to either european versions or SU´s.  

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It sounds like:
1.  Possibly that carb is now running very lean, and needs to be adjusted richer.

2.  Someone else may recognize it as a less exotic issue, but that sounds to me like the sequence for adjusting the throttle bypass valves on CDSEs from the factory emissions equipment manual.
It the assembly at part number 10 in this picture:

There is a pdf of the emissions booklet at
I have heard that some folks have removed the throttle bypass valve from operating upon the carbs by using a cleverly-cut gasket that blocks all or part of it from the carb body.
If you or whoever had it before you turned the screw in the throttle bypass valve it will almost certainly need to be re-adjusted.  

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Thanks Rotoflex for your comments. I dont have the feeling the car is running very lean. The sparkplugs are a nice brown colour. In any case I will order the tool and have a try.

The bypass valve may be the issue as well, however if I read the manual you send the link to, when faulty it should increase the rpm too much, not cutting out the engine. It never hurts to see if it works correctly.

I have another problem to attend first though.....Yesterday my newly fitted spin-on oil filter adapter blew a hole (in the pre-tapped openening where otherwise the oil cooler hoses are connected). The part seemed to be machined too much, there was very little material left and blew by the oil pressure....... ??) Before I converted the oil pressure was healthy. Despite the advantages of the spin-on filter, I will convert back to the original set-up.....it served my engine well for the last 40 years!!!!

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