Before winterizing this year I decided to dial in the air fuel idle mixture on our 350 Marine Power quadrajet. I used a vacuum gauge and got no change from either screw, no better, no worse. I even closed the screws all the way and it made no change. It was rebuilt last year so I'm not sure where to go here. Any ideas?

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It sounds like you are getting extra fuel from somewhere, if the carb was a Holley I would say it was a bad power valve. A long time ago Quadrajets had a problem with the base plate cracking and leaking fuel into the intake manifold. A cracked baseplate usually showed up as a starting problem when the motor was hot. You may also have a baseplate gasket problem. Either way, the carb has to come off and be disassembled and carefully inspected for the source of the extra fuel. All this is assuming your idle isn't set to high, the motor should be idling between 5 and 6 hundred RPM while adjusting the carb. 

It always starts as expected and idles around 700RPM, but the reason I wanted to adjust the air fuel screws is that I noticed that it surges or seems a bit unstable, although it never stalls. I will make a visit to the barn and remove it for closer inspection. Thanks for the valuable information. 

Dan Stokely said:

It sounds like you are getting extra fuel from somewhere, if the carb was a Holley I would say it was a bad power valve. A long time ago Quadrajets had a problem with the base plate cracking and leaking fuel into the intake manifold. A cracked baseplate usually showed up as a starting problem when the motor was hot. You may also have a baseplate gasket problem. Either way, the carb has to come off and be disassembled and carefully inspected for the source of the extra fuel. All this is assuming your idle isn't set to high, the motor should be idling between 5 and 6 hundred RPM while adjusting the carb. 

Captain Wise,

I would remove your Qjet and send it to Cliff’s High Performance as you will be pleasantly surprised by his work. Qjets MUST be completely dismantled ,cleaned and pressure tested to get them to work properly. Look in the library heading and click on the links page as he is there. I’ve used his parts for 15 years , nothing but top notch products and knowledge. Also he runs your carburetor on a engine to tune it before he returns it.

Don’t wait as he gets extremely busy in winter.

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