Nonresponsive air screw on carb

Bill Goddard

Bill Goddard

Well-known member
I started my newly overhauled engine today for the first time and let it run until it warmed up to 160*. It smoothed out after a while but still is not the smooth quiet chug a lug we like to hear. That I'm sure will come with time,however; the air adjustment screw shows no affect on the engine when it is turned in and out. It is a new screw. I started the engine with the screw 1 1/2 turn out from bottom. I turned it in tight and there was still no change. I am sure there is no other source of air besides the main intake of the carb and the GAV was set at 1/4 turn open. Any ideas on what's wrong? Bill G
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The engine must be set at as slow of idle as possible, then you adjust the screw out until it runs rough, then in until it runs rough, then set 1/2 way in between. the engine must be warm.
 
If it has a limiter cap on the mixture screw, take it off, to allow a little more range of adjustment toward the richer side. (Which is screwed IN) (The Dog sed it's clockwise!)
Bill W.
 
You say it's a new screw, but is it an old carb? The lower part of the threaded hole for the screw may be corroded so it feels like the screw bottoms out but it doesn't actually fully seal the passage. You can buy a special jet tap from model A suppliers to chase the thread so the needle can fully seat.
 
We have had reports of springs that won't compress enough to close the port, but the car should stall if the engine is running slow enough to use the idle circuit and the screw is out too much.

You set the idle on the butterfly to run the engine slow. Screw in the the mixture screw until the engine stumbles, turn the screw out counting the number of turns until it stumbles again. Once again turn the screw in 1/2 the number you counted.
 
Thanks to all who replied. I think you are all right. We did have the idle high to keep it running, the carb threads for the air screw might be at fault.

Bill G
 
I had the same problem a few weeks ago and the spring was to long and would not compress enough to bottom out.
 
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