Vacuum secondary or accelerator pump problem?
Thread Starter
Drifting
Joined: Sep 2006
Posts: 1,986
Likes: 21
From: Columbus MN
Vacuum secondary or accelerator pump problem?
I have a 600 CFM Holley Street Avenger carb (0-80670) on my GMPP 350 HO Deluxe crate engine (w/ Super T-10 4-spd trans). It works great at any steady speed but bogs down under medium to hard acceleration from any speed, including from a stop.
What do you think is causing my '80 to bog down under moderate-hard acceleration in any gear at any speed?
At first I thought the stock vacuum secondary spring (silver) could be too heavy for a 3000 lb car. Maybe I need a lighter spring (Holley yellow) to let it open sooner (just like I needed 2 light timing advance springs to advance sooner and faster).
Then I thought the accelerator pump isn't squirting enough gas when I step on the gas. I could swap the stock 30cc accelerator pump nowfor a 500cc pump. (Adjusting the accel pump nut didn't make any noticeable difference.)
I have a clear fuel filter under the hood. The filter looks clean and full of gas, but that might not be true in driving conditions.
BTW, the ignition timing seems to be excellent after installing a MSD advance kit. After trying 5 spring combos, I settled on 2 copper (light) springs, 12* @ 600 rpm, 32* mechanical advance at 4000 rpm, and 52* total advance. It used to take a noticeably LONG time to advance.
What do you think is causing my '80 to bog down under moderate-hard acceleration in any gear at any speed?
At first I thought the stock vacuum secondary spring (silver) could be too heavy for a 3000 lb car. Maybe I need a lighter spring (Holley yellow) to let it open sooner (just like I needed 2 light timing advance springs to advance sooner and faster).
Then I thought the accelerator pump isn't squirting enough gas when I step on the gas. I could swap the stock 30cc accelerator pump nowfor a 500cc pump. (Adjusting the accel pump nut didn't make any noticeable difference.)
I have a clear fuel filter under the hood. The filter looks clean and full of gas, but that might not be true in driving conditions.
BTW, the ignition timing seems to be excellent after installing a MSD advance kit. After trying 5 spring combos, I settled on 2 copper (light) springs, 12* @ 600 rpm, 32* mechanical advance at 4000 rpm, and 52* total advance. It used to take a noticeably LONG time to advance.
Melting Slicks
Joined: Nov 2000
Posts: 2,738
Likes: 53
From: Bogart GA
check your fuel levels in each bowl, make sure it is just dribbling out the holes. Next, remove your accel pump cam so you get no shot when accel.
If it stops bogging then most likely you have a to rich issue. If it gets worse then you prob need more gas. Your power valve rating should also be about 2 "hg lower than your lowest vacuum reading. Make sure you note how the cam is mounted before you remove. You may also need to change you jets. It would help if you could find someone to measue your
O2 values. I have an LM-1 wide band and love it. Takes the guess work out of it. Does it just bog or do you get lean pops?
If it stops bogging then most likely you have a to rich issue. If it gets worse then you prob need more gas. Your power valve rating should also be about 2 "hg lower than your lowest vacuum reading. Make sure you note how the cam is mounted before you remove. You may also need to change you jets. It would help if you could find someone to measue your
O2 values. I have an LM-1 wide band and love it. Takes the guess work out of it. Does it just bog or do you get lean pops?
Thread Starter
Drifting
Joined: Sep 2006
Posts: 1,986
Likes: 21
From: Columbus MN
No accel cam made the problem worse
Originally Posted by KJL
Does it just bog or do you get lean pops?
So it appears that I have a too-little problem. Our son has a couple of accelerator cams left over from his 50 cc accelerator pump swap. I'll try the most agressive cam first to see if that makes a difference.
A lighter secondary spring, 50 cc accelerator pump, and bigger jets are on my list of options.
BTW, I did not have this problem until I recently switched to full length ceramic headers and a H-crossover exhaust. My thinking is that the old carb setup can't keep up to the new exhaust.
Thread Starter
Drifting
Joined: Sep 2006
Posts: 1,986
Likes: 21
From: Columbus MN
Originally Posted by KJL
You may also want to increase your initail timing to 16 deg so your total initail plus mech is 36 deg. You should also try to get all 36 in before 2500 rpm or so. 4000 is way to late.
I tried setting the initial timing to 16*, but the starter had trouble turning the engine over until it fired.
Melting Slicks
Joined: Nov 2000
Posts: 2,738
Likes: 53
From: Bogart GA
Sounds like you have a plan. I had to switch over to a larger squiter as well. I eventually went with the 50 cc pump which fixed the lean spike issue. Lean spikes are not always noticeable. I believe most off the shelf carbs have mild spikes that lend themselves to better fuel economy. Put some headers on and increase the demand at acceleration and you cross that line. I found power I never new I could get until I mapped my O2 curve wth the LM-1. My 427 small block wanted lots of fuel when accelerating. I ended up with 72 primary jets, 50cc pump with an aggressive cam and large squirters. I also enlarged my powervalve orifice's. The result is a much flatter O2 curve and lots of power. Fuel economy be damned!! Your secondary can really only be tuned running at WOT. Best to do it on a dyno. Also, a 50cc pump may require a carb spacer to clear the intake.
Race Director
Joined: Sep 2005
Posts: 18,640
Likes: 121
From: Farragut,TN
Would think that with the change to full length headers caused the engine to lean out think I would go up 2 jet sizes on the primary side of the carb and go from there. in any event you will have to re tune the carb.
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Thread Starter
Drifting
Joined: Sep 2006
Posts: 1,986
Likes: 21
From: Columbus MN
Originally Posted by KJL
I eventually went with the 50 cc pump which fixed the lean spike issue. Lean spikes are not always noticeable. I believe most off the shelf carbs have mild spikes that lend themselves to better fuel economy. Put some headers on and increase the demand at acceleration and you cross that line. I found power I never new I could get until I mapped my O2 curve wth the LM-1. My 427 small block wanted lots of fuel when accelerating. I ended up with 72 primary jets, 50cc pump with an aggressive cam and large squirters. I also enlarged my powervalve orifice's. The result is a much flatter O2 curve and lots of power. Fuel economy be damned!!
Originally Posted by KJL
Your secondary can really only be tuned running at WOT. Best to do it on a dyno. Also, a 50cc pump may require a carb spacer to clear the intake.
Thread Starter
Drifting
Joined: Sep 2006
Posts: 1,986
Likes: 21
From: Columbus MN
Originally Posted by 694speed350
Would think that with the change to full length headers caused the engine to lean out think I would go up 2 jet sizes on the primary side of the carb and go from there. in any event you will have to re tune the carb.
Tech Contributor
Joined: Aug 1999
Posts: 14,376
Likes: 6,375
From: At my Bar drinking and wrenching in Lafayette Colorado
The Street Avenger carbs run extremely lean. Bump primary jet size up 3 to 4 sizes, and then make the secondary jet size 6 sizes larger than the primary. Finally, install the short white spring from the Holley Secondary Spring kit in your secondary diaphragm. This will fix your problem.
