Accelerator pump tuning
Thread Starter
Racer
Joined: Sep 2004
Posts: 473
Likes: 25
From: La Quinta CA
Accelerator pump tuning
I'm trying to eliminate an off-idle hesitation on my '67 327/300HP engine - it idles smoothly at 600 to 700RPM, runs great at speed, but has just a bit of stumble when accelerating from a stop. The engine has been rebuilt to stock specs (as close as possible), runs cool, lots of power, etc, so I don't think I have a timing issue. I've adjusted the accelerator pump per the Holley procedure, and checked the .015 clearance at WOT.
I think I need to change the accelerator pump cam to eliminate this problem, and it would be nice to know what cam would have been used for this engine? The carb is a list 3814A which is correct for a 300HP engine with K19.
I have the same problem in my '66 350HP K19 car, so I'm hoping to kill two birds with one stone.
Does anyone know what pump cams were used in what engines?
Bob
I think I need to change the accelerator pump cam to eliminate this problem, and it would be nice to know what cam would have been used for this engine? The carb is a list 3814A which is correct for a 300HP engine with K19.
I have the same problem in my '66 350HP K19 car, so I'm hoping to kill two birds with one stone.
Does anyone know what pump cams were used in what engines?
Bob
Melting Slicks
Joined: Sep 2003
Posts: 3,050
Likes: 2
From: Orange Ct
I have the same problem. It has followed me though 2 carbs, 2 different type electronic ignitions, new heads, with or without vacuum advance or PCV, numerous timing setups and every possible setup of the accelerator pump. The momentary hesitation at throttle tip in remains. I wonder if it's the current gas or some other odd thing that we are missing.
Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh
I have the same problem. It has followed me though 2 carbs, 2 different type electronic ignitions, new heads, with or without vacuum advance or PCV, numerous timing setups and every possible setup of the accelerator pump. The momentary hesitation at throttle tip in remains. I wonder if it's the current gas or some other odd thing that we are missing.
Melting Slicks
Joined: May 2007
Posts: 3,461
Likes: 18
From: cooper city florida
I have had the same problem for a year on my 2818 Holley. I worked on it for 2 days straight. Here's what I did to finally get rid of it. I had a .025 squirter,and I upped it to a .028. Helped some,but it did not get rid of it all. Next I bought the Holley cam kit. The green cam,in the #1 hole got rid of it completely. When I changed to that cam,I noticed that the spring was no longer touching the accelerator arm,which of course would cause a huge dead spot. Extended the spring down until it was solid against the lever,and made sure I had the .010 to .015 clearance,between the bottom of the spring and the pump lever,at wide open throttle. If you go to You Tube and do a search on" Holley Accelerator Pump",there is a very good video on the subject.
Thread Starter
Racer
Joined: Sep 2004
Posts: 473
Likes: 25
From: La Quinta CA
Well, I have tried a red cam in the #1 hole - it is better, but the hesitation is still there. I ordered a complete cam kit - then I can try some other options. I've viewed the Holley video - it's very helpful. The squirter in the carb is a .028 which should work for this engine.
Bob
Bob
Melting Slicks
Joined: Sep 2003
Posts: 3,050
Likes: 2
From: Orange Ct
Try this. Turn the idle needle screws out (CCW) one (or more) turn from where you have it. (should be around 1.5 turns out normally) The engine may run a little rough and see if the hesitation goes away. If it does, adjust the idle screws for a compromise of smooth running and no hesitation.
The other thing I found was the that if the secondary throttle stop screw holds the secondary plates open too far, it will cause the problem. No combination of cams was able to solve the problem till I hit on this. The normal adjustment for the throttle stop screw is 1 to 1.5 turns from seated. Carb needs to off the car to adjust unless the screw is replaced with an allen head set screw.
The other thing I found was the that if the secondary throttle stop screw holds the secondary plates open too far, it will cause the problem. No combination of cams was able to solve the problem till I hit on this. The normal adjustment for the throttle stop screw is 1 to 1.5 turns from seated. Carb needs to off the car to adjust unless the screw is replaced with an allen head set screw.
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Le Mans Master
Joined: Jan 2002
Posts: 8,913
Likes: 283
Sometimes the cam lever can get bent when bubba adjusted it too tight. Select a cam, white or pink (good start) and make sure the cam lever is riding on the cam. Next adjust the spring bolt. You should have a shot/pee at the slightest throttle movement. If the cam lever is bent (will only be a slight bend to cause a problem) you can try and bend it back, or get a new one.
Mark
Mark
Team Owner
Joined: Oct 2000
Posts: 38,897
Likes: 1,926
From: Washington Michigan
Thread Starter
Racer
Joined: Sep 2004
Posts: 473
Likes: 25
From: La Quinta CA
Thanks, guys -
I hadn't thought about the secondary throttle plate adjustment, but that could well be part of the problem. But I would think that idle would be affected if the secondary plates were to far open?
Bob
I hadn't thought about the secondary throttle plate adjustment, but that could well be part of the problem. But I would think that idle would be affected if the secondary plates were to far open?
Bob
Melting Slicks
Joined: Sep 2003
Posts: 3,050
Likes: 2
From: Orange Ct
Your right JohnZ, 1/2 turn from closed.
Thread Starter
Racer
Joined: Sep 2004
Posts: 473
Likes: 25
From: La Quinta CA
Team Owner
Joined: Oct 2000
Posts: 38,897
Likes: 1,926
From: Washington Michigan
Thread Starter
Racer
Joined: Sep 2004
Posts: 473
Likes: 25
From: La Quinta CA
When these cars were new, was there a difference in driveability between the cars equipped with K19 and those that weren't? I remember in later years the California cars had some real issues, but since I did not own a new Corvette in 1967, I have no reference point. Of course then there's the memory issue. . .
Bob
Drifting
Joined: Apr 2008
Posts: 1,540
Likes: 51
From: Lexington Park Maryland
Make sure the dwell on your points is within spec (28-32). Simple 2 minute task. My dwell setting turned out to be my "carb fuel" issue when stumbling from start was happening to me. Remember...... 90% of fuel problems are electrical !!
Racer
Joined: Sep 2001
Posts: 374
Likes: 0
From: Rochester NY --- FL (Oct to May FL.)
Try this. Turn the idle needle screws out (CCW) one (or more) turn from where you have it. (should be around 1.5 turns out normally) The engine may run a little rough and see if the hesitation goes away. If it does, adjust the idle screws for a compromise of smooth running and no hesitation.
The other thing I found was the that if the secondary throttle stop screw holds the secondary plates open too far, it will cause the problem. No combination of cams was able to solve the problem till I hit on this. The normal adjustment for the throttle stop screw is 1 to 1.5 turns from seated. Carb needs to off the car to adjust unless the screw is replaced with an allen head set screw.
The other thing I found was the that if the secondary throttle stop screw holds the secondary plates open too far, it will cause the problem. No combination of cams was able to solve the problem till I hit on this. The normal adjustment for the throttle stop screw is 1 to 1.5 turns from seated. Carb needs to off the car to adjust unless the screw is replaced with an allen head set screw.
I had a severe stumble on acceleration and found it was in the idle circuit as listed above but i had to turn the sv=crews in and acheive the highest possible vacumn reading (advice from the forum crew) It worked and the stumble is gone, the 64 goes like stink now.
Good Luck
Instructor
Joined: Apr 2006
Posts: 150
Likes: 2
From: Taylor MI
accel pump
Sometimes the cam lever can get bent when bubba adjusted it too tight. Select a cam, white or pink (good start) and make sure the cam lever is riding on the cam. Next adjust the spring bolt. You should have a shot/pee at the slightest throttle movement. If the cam lever is bent (will only be a slight bend to cause a problem) you can try and bend it back, or get a new one.
Mark
Mark
