What causes quadrajet off-idle stumble/throttle lag?

Recently put a brand new quadrajet carb on my starboard side Crusader. I am having this problem where when I increase the throttle, the engine will "stumble" and the RPMs will go down for a bit then jump back up to normal. It does not happen when I smoothly increase the throttle, only when giving it a more sudden push. I tried adjusting the idle mixture screws on the base, but it doesn't seem to have much effect. What usually causes this type of problem? Is it the idle mixture?

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This may not apply to all scenarios.

Typically when leaving the low speed fuel metering while going into the high speed fuel metering, there is a lag or dead spot during the transition (regarding fuel/air delivery).
The carburetor's accellerator pump is to supply an enriched mixture during this transition.

If the accellerator pump and/or nozzles are not doing their job..... the engine will appear to hesitate or stumble as you describe.

See if the accellerator pump is working correctly.
Make sure that the low speed air screws are adjusted correctly.
Perhaps see if the fuel bowl level is correct.


With twins... we can always swap carburetors as to pin point that it is indeed a carburetor issue, or is something else.
If the issue follows the carburetor..... there's your answer.




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If there is an adjustment for the size of the accelerator pump shot, be sure it's on the richest setting.

Jeff

The accelerator pump should squirt fuel immediately and simultaneously with each throttle change from low speed to high speed metering.

Adjustment options for accelerator pump ratio change/piston travel.

Hate to break the news to you but you didn't get a brand new marine quadrajet...they have been out of production for a very long time. Depending upon who "refurbushed" it, the concern could be the carb or the rest of the engine. also of benefit would be more details - how much sudden throttle opening causes the issue, etc.

The idle screws are mixture screws and won't make an difference once the throttle plates uncover much of the transition slots.

I'd be inclined to go gentle on the throttles anyhow....

makomark said:

1.... Hate to break the news to you but you didn't get a brand new marine quadrajet...they have been out of production for a very long time.

Depending upon who "refurbushed" it, the concern could be the carb or the rest of the engine. also of benefit would be more details - how much sudden throttle opening causes the issue, etc.

2... The idle screws are mixture screws and won't make an difference once the throttle plates uncover much of the transition slots.
I'd be inclined to go gentle on the throttles anyhow....

Click to expand...

1.... Just this morning I was talking to a man who thought that he had a NEW Q-jet. I explained the same thing to him.

2.... I agree, unless the low speed system is extremely lean now. But all-in-all, I agree with Mark.





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Just to circle back on this one (thanks for all the replies).

It seems like the problem was having the idle mixture needles out too far. If I screwed both of them in, then backed them out just a hair (1/2 turn), that seems to have mostly fixed it. Previously I had them backed out like 2-3 full turns. A contributing problem was also I think I had a small vacuum leak where the fuel overflow line connects to the carb -- they just pounded a metal tube in there that was not sealing well. Fixed that with some high-temp silicone around the tube.

That tells me the carb has been messed with...the idle mixture screws should be out more like 2 full turns and then go from there..

The fitting you describe is for the sight tube's connection....since it is above the throttle plates, it can't contribute to a vacuum leak. that said, its a good thing to get fixed. when done properly, it is a threaded connection that should draw tight...no pounding required.