Carburetor accelerator pump weak?
Thread Starter
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Joined: Nov 2017
Posts: 91
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Carburetor accelerator pump weak?
Hi Gents,
The accelerator pump in my carburetor seems very weak. I barley get a few drops at each pump action. I have seen others produce an nice stream. I rebuilt the carburetor myself and confirmed all passages are squeaky clean. The accelerator pump itself and the little ball valve are new. Was I sold the wrong diameter pump? It looks about right compared to the original I took out.
Any thoughts what could be causing this? this one has me scratching my head.
Cheers!
Agaon
The accelerator pump in my carburetor seems very weak. I barley get a few drops at each pump action. I have seen others produce an nice stream. I rebuilt the carburetor myself and confirmed all passages are squeaky clean. The accelerator pump itself and the little ball valve are new. Was I sold the wrong diameter pump? It looks about right compared to the original I took out.
Any thoughts what could be causing this? this one has me scratching my head.
Cheers!
Agaon
Tech Contributor
Joined: Aug 1999
Posts: 14,379
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
The accel pump should produce a strong, instant discharge stream. There are several things that can be wrong:
Lars
- The pump you installed is not ethanol resistant, which will make it swell up and almost seize within a few hours of operation.
- You did not polish the pump bore before installing the pump. In order for a new pump to operate in these old carbs, the pump bore must be meticulously polished by hand using fine ScotchBrite.
- The accel pump shaft spring is not stiff enough to overcome the return spring. The springs are most often compressed and collapsed, and you have to stretch them back out to their correct length before using them
- The pump is the wrong length pump. If the pump is too long, it will be depressed too far down "the hole" to operate correctly.
- The pump ball retaining pin is incorrect (too long) and is jamming the pump ball
- The tiny accel pump discharge transfer holes in the airhorn are plugged - impossible to see or correct without actually sticking some .020" diameter wire through them. I have seen these holes remain plugged after several hours of soaking in the hot tank, and after blowing everyhthing out with compressed air. They have to be staked out with .020" wire to assure that they're open - you can't see them.
Lars
Last edited by lars; May 23, 2019 at 09:46 PM.
Thread Starter
Advanced
Joined: Nov 2017
Posts: 91
Likes: 3
The accel pump should produce a strong, instant discharge stream. There are several things that can be wrong:
Lars
- The pump you installed is not ethanol resistant, which will make it swell up and almost seize within a few hours of operation.
- You did not polish the pump bore before installing the pump. In order for a new pump to operate in these old carbs, the pump bore must be meticulously polished by hand using fine ScotchBrite.
- The accel pump shaft spring is not stiff enough to overcome the return spring. The springs are most often compressed and collapsed, and you have to stretch them back out to their correct length before using them
- The pump is the wrong length pump. If the pump is too long, it will be depressed too far down "the hole" to operate correctly.
- The pump ball retaining pin is incorrect (too long) and is jamming the pump ball
- The tiny accel pump discharge transfer holes in the airhorn are plugged - impossible to see or correct without actually sticking some .020" diameter wire through them. I have seen these holes remain plugged after several hours of soaking in the hot tank, and after blowing everyhthing out with compressed air. They have to be staked out with .020" wire to assure that they're open - you can't see them.
Lars
Only the following are a possibly. Every thing else checks out or has been account for. What should be the correct length of the pump (measuring from the plastic base to the top of the shaft)?
When you say retaining pin, do you mean the plug screw that holds the ball in? If so what should its length be?
- The pump is the wrong length pump. If the pump is too long, it will be depressed too far down "the hole" to operate correctly.
- The pump ball retaining pin is incorrect (too long) and is jamming the pump ball
Last edited by Agaon; May 24, 2019 at 02:33 PM.
Tech Contributor
Joined: Aug 1999
Posts: 14,379
Likes: 6,390
From: At my Bar drinking and wrenching in Lafayette Colorado
You can see and read about the accel pump ball retaining pin issue in a post I did here:
https://www.corvetteforum.com/forums...pump-shot.html
For accel pump length, you need to mock up the carb and the pump and note the height of the pump shaft with the throttle in the idle position. Once noted, disassemble the carb, put the pump at the noted height, and verify that the pump plunger is just barely below the edge of the pump well fill slot in the float bowl. I have to do this with every carb I build, and at least 50% of the new accel pumps will be depressed way too far into the pump bore, producing an inadequate pump shot. Grind down the pump shaft to obtain the desired plunger height.
Lars
https://www.corvetteforum.com/forums...pump-shot.html
For accel pump length, you need to mock up the carb and the pump and note the height of the pump shaft with the throttle in the idle position. Once noted, disassemble the carb, put the pump at the noted height, and verify that the pump plunger is just barely below the edge of the pump well fill slot in the float bowl. I have to do this with every carb I build, and at least 50% of the new accel pumps will be depressed way too far into the pump bore, producing an inadequate pump shot. Grind down the pump shaft to obtain the desired plunger height.
Lars
Last edited by lars; May 24, 2019 at 08:41 PM.
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