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Thread: Air/fuel mixture on a Rochester Quadrajet carburetor?

  1. #1
    Eliminator 207 Ski
    Does anyone know how to adjust the air/fuel mixture on a Rochester Quadrajet 7045229 Z8 1576 carburetor? I have a 1995 Eliminator 207 jet boat with a 454 Marine Power engine. There are two needle adjustments screws in front of the carburetor, but I'm not sure which it is and which one does what. I just don't want to run too lean and blow the engine. Any help would be greatly appreciated. Thanks!!!

  2. #2
    those are the air/fuel idle mixture screws, they only affect idle mixture. Once the engine is running above idle, the carb starts pulling from the boosters and the idle circuit is not used, so blowing up an engine because idle mixture is wrong is not gonna happen, if the idle mixture is wrong the engine will just die when you pull back to idle, either from wet plugs(too rich), or too lean and not enough fuel to keep the engine running.

  3. #3
    How does it run wide open? Do you notice any surging? I wasn't getting enough fuel to mine. Make sure you have a good fuel pump and a regulator if your pressure is above 6 or 7 lbs.

  4. #4
    Eliminator 207 Ski
    Thanks SandWinder and Malcolm. Sorry it took so long to reply. I thought that those mixture adjustment screws gives the carb gas when it needs it at high speeds. I did play with the mixture screws when I first put it on the engine, but it didn't give me any problems with the idling. I was having a hard time starting the engine is the reason why I play around with it and I know I didn't put it back in the same location. Why is there two mixture screws though? Do you know how many turns out from the closed position? Or should I just leave it alone since it's idling fine?

  5. #5
    Eliminator 207 Ski
    It runs strong wide open. It doesn't bog out or anything like that. The fuel pump and regulator is fine also. So once I full throttle, the engine should get enough gas and no need for any adjustments? I'm used to the RC gas engines. Not real engines. Just a rookie on real engines.

  6. #6
    Quadrajets are one of the more difficult carbs to tune, they have a lot of flexibility as far as adjustment so they are more fuel efficient if tuned properly, but have more things to tweak.
    As was said, the 2 screws on the bottom front are the idle mixture. There are 2 mixture screws because the engine idles on the front 2 barrels of the carb. The normal starting point to adjust them is 1.5 turns out from the closed position, but as long as the motor idles well it doesn`t really matter how many turns out they are. They should both end up the same number of turns out.
    For midrange there is a set of metering rods and jets on the front two barrels of the carb, to see or change them you have to remove the entire top portion of the carb.
    For WOT the secondaries open (back two barrels). The metering rod for those are easily changed, all you have to do is pull one tiny little screw and the metering rods and hanger lift out.
    To change the midrange or WOT mixture you would need new metering rods. There are not a lot of places left to get them. You can either try junkyards and swap meets, or I think the one line of edelbrock carbs uses the same ones and you can order replacements through them.
    If you are serious about tuning a Q-jet you will want to read "Rochester carburators" by Doug Roe. It has lots of tips and tricks and the charts that you will need to decode the stampings on the metering rods.

  7. #7
    Eliminator 207 Ski
    Thanks G-Body. This helps a lot. Thanks again to everyone that helped me on this.

  8. #8

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