Zenith air screw adjustment

marc silva

marc silva

Senior Member
On a Zenith carb. when you open the air screw(counter clockwise)from a full seat, does that let in more air or fuel? Making it richer or leaner at idle? When I come to a stop it smells rich. It idles good with the GAV closed and the air screw is 1 turn open. Float is set correctly.I am thinking it needs to be a little leaner at an idle.

Thanks as always, Marc
 
I just read how to adjust after a rebuild on Renners site. With a warm engine, gav closed, spark retarded, idle air turned out 1.5 turns, idle speed down to almost will die. Adjust idle air screw in until idle changes, then out until idle changes, some where in between you will find the sweat spot. Adjust idle speed and have a nice drive :-)
 
I know everyone says the GAV position shouldn't change the idle mixture, but if my GAV is open much more than a quarter of a turn beyond my normal driving position, it gets a rich mixture at idle. As far as driving goes, it works great so I haven't messed with it.
 
Art Newland said:
I know everyone says the GAV position shouldn't change the idle mixture, but if my GAV is open much more than a quarter of a turn beyond my normal driving position, it gets a rich mixture at idle. As far as driving goes, it works great so I haven't messed with it.
Click to expand...

Art, there talking about the idle screw, not the gav. My car has a su1 cam and is .125" with a b carb and head. I really can't get an idle low enough to only run on the idle jet alone. ( I think it's 350/ 500 rpm) so mine idles using the gav as well.
 
Art Newland said:
I know everyone says the GAV position shouldn't change the idle mixture, but if my GAV is open much more than a quarter of a turn beyond my normal driving position, it gets a rich mixture at idle. As far as driving goes, it works great so I haven't messed with it.
Click to expand...

My GAV does the same thing. I have to open it about 1 1/2 turns for the first minute after starting a cold engine, then I can close it after a couple minutes and usually leave it closed while driving.

I never have flow tested any of my jets, but it did give me 22 MPG at 45 MPH on a long trip a several years ago. If it ain't broke, I don't fix it. Just keep things lubed and do preventative maintenance.
 
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