[71merc]

Good evening,
I need some advice. 1971 250C w/M130 engine and Zenith 35/40s. Rear carb idle air screw adjustment will not change mixture and can remove from carb and still no difference. I have rebuilt both carbs and front carb idle mixture adjustment works fine. I bought another carb and installed tonight. Same exact issue as before, idle mixture will not adjust. Both carbs are sync'd properly, vehicle has a "decent" idle and starts right up. Idle jets are open and clear. Can't figure why both carbs acts the same. 40 plus years working on cars and cut my teeth on carbs but I'm stumped.

 

[rumb]

The carb must be set up in the precise order listed here.

Zenith Carb Manual TOC

Check for vacuum leaks under the carb. is the hard plastic spacer un broken? 4 gaskets used under each carb? carb/spacer/metal plate/spacer/manfold?

Is the big diagphram in good shape - the one on the backside of carb, pull the vac line to it and make sure the linkages move coming from it.

 

[71merc]

The carb must be set up in the precise order listed here.

Zenith Carb Manual TOCa
1) Carbs are set by the manual AND sync'd

Check for vacuum leaks under the carb. is the hard plastic spacer un broken? 4 gaskets used under each carb? carb/spacer/metal plate/spacer
2) Checked for vacuum leaks and found none, spacers don't appear to have cracks and the gaskets have crossed my mind. Ordering 4 new ones. Car will stall when block carb openings.

Is the big diagphram in good shape - the one on the backside of carb, pull the vac line to it and make sure the linkages move coming from it.

3)checked vacuum to diaphragm and it held for 5 minutes plus works well on vehicle.

Still stumped as the idle mixture screw is the only issue. Car runs great just gas a rough idle. Also both carburetors are doing exactly the same thing so I' m pretty sure it isn't a carb issue....:frown:frown:frown:frown:frown