Roch Quadrajet Tip
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From: Naperville IL
Roch Quadrajet Tip
I thought I would pass on a tip I learned the hard way rebuilding my Rochester Quadrajet. This is one I did not see documented elsewhere, so here goes.
Background:
My '73 BB has the original Rochester Quadrajet and I verified it still has the stock jets and metering rods. The engine does have an aftermarket cam and does not pull as much vacuum as stock (~14 in). I purchased the car from a fellow that is a certified mechanic (aircraft as well) and I felt pretty certain that he had rebuilt it properly before me. In any case, I have been having some driveability and gas mileage problems that I wanted to resolve. So, I rebuilt the carb again. After my rebuild, it was better, but still not quite right.
I thought perhaps my problem was with the power piston spring since it was stock and designed for more manifold vacuum. After an email exchange with Lars, I decided to take his advice and check the operation of the power piston with the engine idling. To those who are not familiar with this procedure, you just slip a small screwdriver into the front vent tube and depress the hanger arm on the power piston. With the engine off, you should see about 3/16" of "spring" in the movement of the power piston. With the engine at idle, the piston should be bottomed and you should see no movement at all when you press on the hanger arm.
Before I mounted the carb on the engine I tried the test on my workbench. Lo and behold, after I assembled the top onto the carb, I had NO movement in the power piston. How could this be???
The tip:
There is a springy piece of wire that wraps around the hanger arm of the power piston and hooks around each primary metering rod. This wire holds the metering rods in the hanger arm and, I presume eliminates any chance of them falling out (though I'm not sure how that could happen once assembled). This springy piece of wire was simply looped around the hanger in my case with the loop making a diagonal spiral across the hanger arm as it wrapped around it. You can imagine it being like one loop of a coiled spring. It turns out that the edge of this loop was getting caught in the slot that fits down over the power piston to keep it aligned as it moved up and down. This caused the power piston to be fully depressed when the cover is screwed down. The fix is to overlap the ends of the springy wire so the loop is now perpendicular to the hanger harm instead of wrapping around it in a spiral.
After fixing this subtle little problem everything seems to work great. Throttle response is much better and torque coming off idle is much better. We will have to wait for spring to check the gas mileage as there is now snow and salt on the streets in Chicago.
I hope this is useful to someone. It sure took me weeks of investigation to track it down. I guess if I had been smart enough to check the power piston operation on the car in the first place, I would have at least had a clue.
Happy Holidays!
Background:
My '73 BB has the original Rochester Quadrajet and I verified it still has the stock jets and metering rods. The engine does have an aftermarket cam and does not pull as much vacuum as stock (~14 in). I purchased the car from a fellow that is a certified mechanic (aircraft as well) and I felt pretty certain that he had rebuilt it properly before me. In any case, I have been having some driveability and gas mileage problems that I wanted to resolve. So, I rebuilt the carb again. After my rebuild, it was better, but still not quite right.
I thought perhaps my problem was with the power piston spring since it was stock and designed for more manifold vacuum. After an email exchange with Lars, I decided to take his advice and check the operation of the power piston with the engine idling. To those who are not familiar with this procedure, you just slip a small screwdriver into the front vent tube and depress the hanger arm on the power piston. With the engine off, you should see about 3/16" of "spring" in the movement of the power piston. With the engine at idle, the piston should be bottomed and you should see no movement at all when you press on the hanger arm.
Before I mounted the carb on the engine I tried the test on my workbench. Lo and behold, after I assembled the top onto the carb, I had NO movement in the power piston. How could this be???
The tip:
There is a springy piece of wire that wraps around the hanger arm of the power piston and hooks around each primary metering rod. This wire holds the metering rods in the hanger arm and, I presume eliminates any chance of them falling out (though I'm not sure how that could happen once assembled). This springy piece of wire was simply looped around the hanger in my case with the loop making a diagonal spiral across the hanger arm as it wrapped around it. You can imagine it being like one loop of a coiled spring. It turns out that the edge of this loop was getting caught in the slot that fits down over the power piston to keep it aligned as it moved up and down. This caused the power piston to be fully depressed when the cover is screwed down. The fix is to overlap the ends of the springy wire so the loop is now perpendicular to the hanger harm instead of wrapping around it in a spiral.
After fixing this subtle little problem everything seems to work great. Throttle response is much better and torque coming off idle is much better. We will have to wait for spring to check the gas mileage as there is now snow and salt on the streets in Chicago.
I hope this is useful to someone. It sure took me weeks of investigation to track it down. I guess if I had been smart enough to check the power piston operation on the car in the first place, I would have at least had a clue.
Happy Holidays!
