Hi,
The 4.3 that you replaced is also a vortec engine, but it doesn't usually say it on the eng. and if you replaced it with an automotive eng. then you are going to have to tune it differently as the automotive engine requires different ign. timing, thermostat, jetting,etc... this could be your problem if you are using the automotive eng.

If it's a marine engine, then the following may apply....

The screws you are referring to are the idle air screws and will only affect the idle mixture. A good nominal adjustment for them would be approx. 2 1/4 turns out from their seat. The hesitation you are experiencing is most likely either a bad accelerator pump or the metering jets need to be switched out for the correct size. I would bet that it is your accelerator pump though as this causes the exact symptom that your are experiencing. When you push the throttle down, it squirts a shot of fuel into the primaries to compensate for the increased demand.


I hope this helps..you can reply to this post if you need further assistance. Otherwise, please click on the accept button to accept my answer.

Best regards,

Michael

Ok, then it's probalby the accelrator pump then.. the pump is inside the carb and if you're going to get into the carb, I highly recomend having it completly gone thru by a professional carb rebuilder! Lots of shops will say they can rebuild these Quadra-jets but very few actually knowwhat they are doing and can in fact make them worse. The Quadra jet is a great carb but is also the most complicated and difficult to propperly set up!
I nevver rebuild carbs anymore and send them to my trusted carb outfit called ReCarbco. It only runs a couple hundred dollars but I get it back looking and working like new with a warranty. They also test run them on a similar engine before shipping it out.

Michael

Don't forget to click accept if you found my answer to be helpful as this is the only way we get credit for out time.

you can only adjust the idle mixture screws... Turn them all the way in until you feel them seat, then back them out between two and two and a half turns. Then start the engine and set the throttle to 1000 rpm. Now, very slowly turn the screws one at a time in or out trying to increase the rpm's. when you get both screws right, the rpm should be higher then what you started with.
But, remember, this only affect the way it runs at idle and nothing else. None of the other jets in that carb are adjustable.
You can chech the accellerator pump by removing the spark arrestor and the throttle cable, then hold the choke plate open so you can see down the bore and pump the throttle a couple of times... if it's working you will see a stream of fuel sqiurt into each of the two primary bores.

MIchael

Edited by Michael on 5/5/2009 at 7:56 PM