KJL

I have a rich idle condition that I cannot seem to fix. I am bleeding air through the PCV line and that seems to help some. I have read on the forum where it is a common problem with Q-Jets and many people drill small holes in the primaries to fix this..

Xracer

I did that with my 750 DP Holley to get it to run better at low speeds and idle. But I am not sure about a q-jet. Better ask Lars.

clem zahrobsky

some Q jets came stock with 3/32' holes in the pri butterflys so give it a try as you can always epoxy them up if it does not work. :chevy

KJL

Thanks Clem, I think you are the one that said to try this the first time!! I have this part throttle stumble the is kicking my butt. I disconnected the accel pump rod and the stumble went away. So I figured I had a rich condition. I moved the rod to the outer hole and that helped some, but if I turn the idle screws in any further, the idle gets rough. I picked up the idle speed and noticed that I was starting to pull fuel in from the the main circuit no doubt adding the the rich condition. If I understand correctly the idle transfer slot should be expose about .030" past the blade and drill a 3/32" hole into each pri blade.

clem zahrobsky

.030/.040 should be OK :chevy

BeaterShark

What timing for this post. I was just about to ask about drilling holes in the primary throttle blades.

My blades are competely over the transfer slot. The kicker is that I don't have any off-idle hesitation, but I figured that I should set it up correctly anyway. I also have a few parts carbs I could use if it doesn't work for me.

My question is: does it matter where on the throttle blade you drill the hole? I know that it is supposed to be on the same side as the transfer slot, but is there an ideal location? Clem, do you have any pics of one of those factory Q-jets with the holes drilled in them?

clem zahrobsky

try doug roe QJ book as it may have pictures. they should be drilled in the front half if the butterfly. :chevy

RMS73

I drilled the holes in the primary plates and it did not seem to help to por idle and transition range. Replaced the spring under the power piston and the problem disappeared.
Roger

BeaterShark

There is a complete page explaining the problem (with pics), but only one sentance telling to drill holes (no pics).

I figured it would be in the center, but wanted to get an experienced opinion before I go putting holes in my carb.

Thanks

KJL

I have seen pictures where the holes are drilled in the front edge. I prob is not "that" critical. The funny thing with my problem is the transition off idle from a stand still is ok, it is when I am at cruise rpm remove my foot from accel and closing the thottle blades thus dropping the carb into the idle circuit while coasting at cruise rpm. When I apply the gas there is a brief hesitation which I believe to be caused by a rich condition at that point. I have reduced it but can't seem to get rid of it. Lars suggested that larger metering rods might help...finding parts for these old carbs is kind of a PITA. Where did you get the new power piston spring?

oger

If you disonected the acc. pump and the stumble went away you don't need holes you need to file the edge of the throttle blades so the more of the transfer slot is showing. The problem you are having usually only happens on engines that make a lot of vacuum at idle and you have to close the throttle blades too far at idle. I have seen some Qjets where the sec. were opened too far at idle that caused your problem make sure the linkage isn't bent going to the sec.


[Modified by oger, 6:04 PM 8/5/2002]

KJL

I think I may have discovered something, after thinking back as to when the rich condition started I realized it happened after I disconnected the vapor canister!! I am sure the engineers at GM had that all figured along with the associated PCV valve size. When I removed the lines I am sure I reduced the amount of air flow goinging into the idle curcuit as well as increasing manifold vacuum a bit. I have been experimenting with bleeding air through the PCV line. I found at idle, as I increased the air flow the Idle rpm increased. With no additional fuel dribbling out of the boosters and the idle fuel adjustment needles set as well as possible one can assume with the increased rpm that the condition is rich. Right now I running an approx 3/16" orifice. This would be equal to about two 1/8" hole in the primary blades. The off idle hesitation is much better as well now that I have increased the orifice size. When I think I got the right sizes I will drill the primary blades and check manifold vacuum. Hopefully this will give me more adjustablilty with the idle feed screws. Up to this point they basically have been on-off valves. :crazy: