on my '74 34dmsa carb, it over fuels. I can take off the spring and screw the screw in until it stops. But it seems to have little effect on the engine at any point. It does not shut off when screwed into the end.
What does it screw into and can it be fixed?
Ron
idle mixture screw
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burgundyspider
- Posts: 44
- Joined: Sun Dec 20, 2015 7:33 am
- Your car is a: 1974 Spider
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So Cal Mark
Re: idle mixture screw
you're getting fuel from another circuit in the carb. Could be due to incorrect float level, or an idle speed that is too high
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carl
Re: idle mixture screw
Agree with Mark, if idle mixture screw has no effect, you are probably idling on the main circuit which means the idle speed screw is set too high.
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squealingtires
Re: idle mixture screw
I had the same issue on the 34DMSA recently. Things I learned:
1. Vacuum leaks are evil and like Satan must be eliminated. Cap everything, disconnect the brake booster and check the idle jet holder screw seals for leaks (these are external and on opposing sides of the carb.
2. The engine should idle with the idle mixture screw set at 2 turns out (from the seat) and idle position set at one turn in (from contact). If you've capped everything and the engine does not run then you have a blockage in the idle circuit. The only way you can get the engine to run is to turn in the idle stop which begins to draw fuel from other circuits as Carl and Mark pointed out. If this is the case then disassemble the carb, soak in a carb solvent, blast some compressed air through all the passages and retry. Before reassembling, verify inspect the carb closely and inspect each orifice is clean and clear. I verify by spraying an aerosol carb cleaner through the ports until the pattern coming out seemed to be clear of any obstructions (wear goggles for this because you may not know where the spray will come out and you don't want that stuff in your eyes.)
It took me 3 times of doing this before I had the issue solved. So incredibly frustrating
but all the kind folks on the the board were more than willing to walk me through the process. 
Once you get it running well then start to attach your vacuum hoses and monitor the engine. Any change in RPM could indicate a vacuum leak. I also highly recommend an analog vacuum gauge and digital RPM guage (automotive multimeter) to provide a visual cues for engine performance and will allow you to tune for maximum vacuum at idle.
Like I said, I just went through all these steps and they are fresh in my mind so let me know if you have any trouble.
1. Vacuum leaks are evil and like Satan must be eliminated. Cap everything, disconnect the brake booster and check the idle jet holder screw seals for leaks (these are external and on opposing sides of the carb.
2. The engine should idle with the idle mixture screw set at 2 turns out (from the seat) and idle position set at one turn in (from contact). If you've capped everything and the engine does not run then you have a blockage in the idle circuit. The only way you can get the engine to run is to turn in the idle stop which begins to draw fuel from other circuits as Carl and Mark pointed out. If this is the case then disassemble the carb, soak in a carb solvent, blast some compressed air through all the passages and retry. Before reassembling, verify inspect the carb closely and inspect each orifice is clean and clear. I verify by spraying an aerosol carb cleaner through the ports until the pattern coming out seemed to be clear of any obstructions (wear goggles for this because you may not know where the spray will come out and you don't want that stuff in your eyes.)
It took me 3 times of doing this before I had the issue solved. So incredibly frustrating
but all the kind folks on the the board were more than willing to walk me through the process. Once you get it running well then start to attach your vacuum hoses and monitor the engine. Any change in RPM could indicate a vacuum leak. I also highly recommend an analog vacuum gauge and digital RPM guage (automotive multimeter) to provide a visual cues for engine performance and will allow you to tune for maximum vacuum at idle.
Like I said, I just went through all these steps and they are fresh in my mind so let me know if you have any trouble.
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baltobernie
- Patron 2020
- Posts: 3466
- Joined: Sun Nov 25, 2007 6:00 pm
- Your car is a: 1973 Spider [sold]
- Location: Baltimore, MD
Re: idle mixture screw
Squealingtires, does this carb have a nipple for ported vacuum? If so, he could attach the gauge there, and if it reads anything above zero, he is into the main circuit.squealingtires wrote: I also highly recommend an analog vacuum gauge ...