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A friend of mine asked me to have a look at his car the other day. It has a slight hesitation off the line. It has a 1975 Quadrajet carb and when I cracked the throttle fuel is only spraying into one of the primary bores. In a couple of days we will get together again to tear down the carb. My knowledge of Rochester carbs is somewhat rudimentary. I am more of a Holley guy. Any suggestions on where to look for the blockage would be most appreciated.
From: At my Bar drinking and wrenching in Lafayette Colorado
Bill -
Here is some more info for you:
To clean out the blockage, you need to removed the airhorn and flip it upside-down.
The accel pump passageway runs from the upper venturi-area of the carb, and consists of 2 holes: The first hole is from the venturi area into a "key-shaped" plenum. From the "key plenum" the hole continues in a straight line into the actual round accel pump passage channel. The hole is small: I use a 0.020" safety wire to "stake" it out. It's a little tricky getting the wire fed through both holes and all the way into the round feed passage, but you have to make sure you get the wire all the way through both holes:
To verify you got it, use a squirt bottle with solvent (I use IPA) to test and verify that you have a clear and strong discharge from the round passage all the way through. If you don't get a solid stream of fluid shooting out the discharge hole, you don't have it unplugged:
It's seldom that the hole from the key plenum to the venturi is plugged. The hole that gets plugged is the tiny hole from the round feed passage to the key plenum.
Thanks, Lars. Pulled the carb off tonight and cleaned out the accelerator pump passages as you suggested. You were 100% correct. Not sure what was plugging up the passage but once it was cleaned out we got two good streams of fuel. If it weren't for your advice, we might have figured it out eventually but it would have taken us all night. The car runs great, the problem is solved, and the owner is happy. To my ear the engine could use a little more pump shot though. Is there some way to increase the pump shot volume on a Q-jet?
From: At my Bar drinking and wrenching in Lafayette Colorado
Bill -
Good job on the cleanout of the discharge orifice. Just a lucky guess on my part about that...
There is a "long" pump and a "short" pump for the Q-Jet. If you use the "long" pump, the pump is positioned much lower in the pump bore, resulting in an abbreviated pump shot. Using the "short" pump will place the pump diaphragm at the top of the pump bore, near the fill slot, resulting in a much longer and more significant pump shot. Use the short pump, or grind a little off the tip of your long pump to bring the pump higher up in the bore.
Lars
Q-Jet accelerator pump
