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NicksSticks, ASE Certified Technician
Category: Car
Satisfied Customers: 1424
Experience:  Ford Drivetrain and Chassis Master, ASE Master Technician
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Popping and backfires on a 77 911 2.7l with Zenith Tin..adjustment

Customer Question

I'm getting lots of popping and backfires on a 77 911 2.7l with Zenith Tin 40's.. Idle adjustment screws are out almost 7 turns. This helps but I don't think it is right.

I've read it might be the Idle Circuit - I have no idea how to adjust these?
Also, what is the correct fuel preasure for these carbs.

Submitted: 7 years ago.
Category: Car
Expert:  danny replied 7 years ago.
This truly sounds like an air flow meter. Do you do your own work?
Customer: replied 7 years ago.


I do my own work.


Give me more details... the air flow meter? There are at least 16-18 adjustment screws on these two carbs. Idle, air, acc pump.. etc.


When I turn the idle screws out 7 or 8 turns - I get popping. Not as bad as just the recomended 3 or 4 turns out, but I still do. The exhaust smells rich though. One of the books I have mentions an Auxiliary enrichment device. However, mine is disconnected to my knowledge. There is supposed to be power going to a solenoid, however I don't see the wires or the connections on the carbs. This or the Idle Circuit are supposed to help the car from leaning out? That is what I am reading.

Expert:  danny replied 7 years ago.
I am going to opt out so another mech may asst you.
Expert:  NicksSticks replied 7 years ago.

No air flow meter, unless updated on your vehicle, Sounds like more of a valve open problem, Valves are most likely not seating correctly, good way to check, take a playing card, or paper business card, put on a exhaust port, and see if the exhaust attempts to suck it back at any time, if so, cylinders must be removed. The 3-4 turns back should get the carbs in a ball park to run correctly, and the fact that extensive tuning of the carbs did not get it running correctly proves my theory.

Sorry about the blunt answer and if not happy let me know.

Thanks so much,


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