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Quadrajet Idle Mixture Adjustment

25K views 7 replies 5 participants last post by  SWHEATON 
#1 ·
Hello,

I am working on my fathers boat which has a 454 with a performer manifold and quadrajet carb. It runs great but when you try to adjust the idle mixture screws you get no reaction at all. I am wondering if it could have the wrong carb base gasket? I have already checked for vacuum leaks btw.

Thanks.
 
#3 ·
if the idle screws have no effect, you need to start out by trying to get the throttle blades closed farther by backing off the idle speed screw. if it won't idle, then put more initial timing in it or hook the vacuum advance to a full manifold source.
 
#4 ·
Might want to look at this link:

http://cliffshighperformance.com/simplemachinesforum/index.php?topic=45.0

Cliff Ruggles says that the typical "marine" carb doesn't have the idle air bypass feature. Therefore the throttle blades would have to be opened farther to get an appropriate idle speed. And therefore the idle transfer slots would tend to be uncovered.

Might be a good idea to invest in Ruggles' book:

Amazon.com: How to Rebuild and Modify Rochester Quadrajet Carburetors (S-a Design): Cliff Ruggles: Books

and see if you can add some idle bypass air to your carb--assuming that everything else is working properly, and you don't have something way out of adjustment, or dirt in the idle air or fuel passages.
 
#5 ·
Thanks for all the great feedback. It does in fact have a marine carb on it so the idle bypass not being drilling could be a possibility. I will not dig in any further since it runs so good.

Thanks again to all.
 
#6 ·
As stated,its very likely a retarded base timing issue causing the idle screw to be set higher then normal to obtain an idle which pushed the prim throttle blades past /out of the idle circut which will make the odle mix screws inactive.

Many marine motor have mild perfcams from factory so try 16-18 deg base timing and i bet the motor well be much happier there.

Adv the base timing will also make you have to decrease the idle enough to drop the prim throttle blades back down into the idle circut so the ildle mix screws will become active again.

If the dist has a vac adv also try hooking it to full intake vacuum all te time too.

Sobefore going nuts digging into the carb take a few mins to try doing the above suggested items which very well may fix the problem.

If not then go deeper into carb issues.

Let us know how you make out .

scott
 
#7 ·
As stated,its very likely a retarded base timing issue causing the idle screw to be set higher then normal to obtain an idle which pushed the prim throttle blades past /out of the idle circut which will make the odle mix screws inactive.

Many marine motor have mild perfcams from factory so try 16-18 deg base timing and i bet the motor well be much happier there.

Adv the base timing will also make you have to decrease the idle enough to drop the prim throttle blades back down into the idle circut so the ildle mix screws will become active again.

If the dist has a vac adv also try hooking it to full intake vacuum all te time too.

Sobefore going nuts digging into the carb take a few mins to try doing the above suggested items which very well may fix the problem.

If not then go deeper into carb issues.

Let us know how you make out .

scott

I don't think I have many options at this point without swapping out the Mercruiser Thunderbolt Ignition. Basically there is no vacuum advance or mechcanical advance to modify. Esentially the distributer is just a trigger that is controlled by a seperate control module. It runs quite well but I would like to be able to adjust the mixture. I will try to lower the idle to see if I can gain some adjustability.
 
#8 ·
Again,try adv base timing 1st before doing anything else because some of the Mercruiser motors have mild perf cams that can use more base timing to see if the motor will tollerate it which i bet it will and if it helps the problem with idle mix screws being inactive.

Scott
 
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