Need some carb tuning advice
08-12-2011, 10:53 AM
#1
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Need some carb tuning advice I am running a 670 Street Avenger with a vac secondary. My LQ9's only modifications are long tube headers and an X pipe.
I have a vacuum gauge, I set the curb idle, idle mixture, and float levels.
I got some help the other day here and problem is getting better.
I changed to the stiffest spring I had for the vac secondary diaphragm. The bog I have at initial WOT is not as noticeable now , but I do have a little popping (not backfiring) and the bog is still there.
This carb has no secondary power valve. My choices now are primary jet, secondary jet, pump discharge nozzle and power valve. In the stock configuration they are respectively 65, 68, .031, and 65
I have a vacuum gauge, I set the curb idle, idle mixture, and float levels.
I got some help the other day here and problem is getting better.
I changed to the stiffest spring I had for the vac secondary diaphragm. The bog I have at initial WOT is not as noticeable now , but I do have a little popping (not backfiring) and the bog is still there.
This carb has no secondary power valve. My choices now are primary jet, secondary jet, pump discharge nozzle and power valve. In the stock configuration they are respectively 65, 68, .031, and 65
08-12-2011, 11:02 AM
#2
cam size? converter? rear gear? are they all matched? My next thought is your jetting seems off, the power valve is worth 6-8 jet sizes, so if fronts are 65, the rears should be 73. Next you can play with squirter size on the front and rears, with a 30cc shot, the largest you can do is "35" squirters. After that a 50cc shot is needed. Typically you'd swap a 31 squirter to 33 then 35 and see how tuning it goes. Really sounds to me like the converter is too small for the cam and that isn't helping by loading the motor up too soon at too low an rpm. If you roll the throttle taking off from a light over 2 seconds or so, is the bog gone and the motor revs up more quickly?
08-12-2011, 11:35 AM
#3
I am running a 670 Street Avenger with a vac secondary. My LQ9's only modifications are long tube headers and an X pipe.
I have a vacuum gauge, I set the curb idle, idle mixture, and float levels.
I got some help the other day here and problem is getting better.
I changed to the stiffest spring I had for the vac secondary diaphragm. The bog I have at initial WOT is not as noticeable now , but I do have a little popping (not backfiring) and the bog is still there.
This carb has no secondary power valve. My choices now are primary jet, secondary jet, pump discharge nozzle and power valve. In the stock configuration they are respectively 65, 68, .031, and 65
I have a vacuum gauge, I set the curb idle, idle mixture, and float levels.
I got some help the other day here and problem is getting better.
I changed to the stiffest spring I had for the vac secondary diaphragm. The bog I have at initial WOT is not as noticeable now , but I do have a little popping (not backfiring) and the bog is still there.
This carb has no secondary power valve. My choices now are primary jet, secondary jet, pump discharge nozzle and power valve. In the stock configuration they are respectively 65, 68, .031, and 65
08-13-2011, 07:51 AM
#5
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Street Avenger Carbs are rated with Dry flow of air...when you add the fuel to the package that carb only flows about 590 CFM... In essence, your engine needs more than that...and actually, you need a lot more...like an 870, at the minimum..
That being said, it can be made to run OK, but you're going to need to step up the jet package probably 2 - 4 steps depending on your climate and air temp.
Another thing that's pretty important is to match your power valve to your vacuum at Idle..they come with a .65 PV from the factory which works in the range of most stock Gin 1 engines, however, if you have significantly more vacuum in gear than 13...you need a stronger power valve. the rule of thumb is to take your vacuum reading (manifold..not ported) , under a load after the engine is achieved operating temp, then divide the number in half to determine the power valve needed.
The problem you're describing is the tip in stumble that occurs while the carb is transitioning from the Idle, to the main circuit...the other thing you may end up adjusting is the Cam used to actuate the Accelerator pump, to one that's more aggressive...you're feeling a lean condition, and getting all of the things I described into account and adjusted, will get you closer to success.
Hope my rambling helped a little, Carbs are kinda my specialty, as I'm a engine builder/dealer for Pro-Systems....and unfortunately, the street avengers, are not a very good performance option.
That being said, it can be made to run OK, but you're going to need to step up the jet package probably 2 - 4 steps depending on your climate and air temp.
Another thing that's pretty important is to match your power valve to your vacuum at Idle..they come with a .65 PV from the factory which works in the range of most stock Gin 1 engines, however, if you have significantly more vacuum in gear than 13...you need a stronger power valve. the rule of thumb is to take your vacuum reading (manifold..not ported) , under a load after the engine is achieved operating temp, then divide the number in half to determine the power valve needed.
The problem you're describing is the tip in stumble that occurs while the carb is transitioning from the Idle, to the main circuit...the other thing you may end up adjusting is the Cam used to actuate the Accelerator pump, to one that's more aggressive...you're feeling a lean condition, and getting all of the things I described into account and adjusted, will get you closer to success.
Hope my rambling helped a little, Carbs are kinda my specialty, as I'm a engine builder/dealer for Pro-Systems....and unfortunately, the street avengers, are not a very good performance option.
Last edited by 72sscamaro; 08-15-2011 at 07:27 AM.
08-13-2011, 09:17 AM
#6
Sounds lean, which cct or how many are the question's 
I've been trying to make EFI transition for the past few years, been longer since I tuned a carb.....I was one those that could make the Carb run correctly with the nasty cams.
Carbs are so much easier than EFI, no nannies masking the real problem. In the end EFI is better for all the drive ability issue's.
I might be able to help if you wish & you can tolerate my rusty Carb tuning abilities, just let me know.
My first step was always to get the idle right first, the idle air screws have to have range both ways. Then you have that Vac reading, then make sure the Power Valve is a couple of inches below the Idle Vac. You do not want the power valve interfering with tuning of the idle cct & main cct.
Once you have that then try some light cruising with very light throttle opening. Check for surge, if it surges trying going up in the mains two sizes at a time until its gone. If it does not surge try going down two sizes at a time until it does surge then up two sizes. It may not surge it might just demostrate flat or lean spots.
I found if I did not get these first two cct's correct first the chasing your tail syndrome kicked in.
If I were tuning a Carb today the vary thing first I would do is invest a good WB, that will save you hours & days of time.
I've been trying to make EFI transition for the past few years, been longer since I tuned a carb.....I was one those that could make the Carb run correctly with the nasty cams.
Carbs are so much easier than EFI, no nannies masking the real problem. In the end EFI is better for all the drive ability issue's.
I might be able to help if you wish & you can tolerate my rusty Carb tuning abilities, just let me know.
My first step was always to get the idle right first, the idle air screws have to have range both ways. Then you have that Vac reading, then make sure the Power Valve is a couple of inches below the Idle Vac. You do not want the power valve interfering with tuning of the idle cct & main cct.
Once you have that then try some light cruising with very light throttle opening. Check for surge, if it surges trying going up in the mains two sizes at a time until its gone. If it does not surge try going down two sizes at a time until it does surge then up two sizes. It may not surge it might just demostrate flat or lean spots.
I found if I did not get these first two cct's correct first the chasing your tail syndrome kicked in.
If I were tuning a Carb today the vary thing first I would do is invest a good WB, that will save you hours & days of time.
08-13-2011, 10:57 AM
#7
Street Avenger Carbs are rated with Dry flow of air...when you add the fuel to the package that carb only flows about 590 CFM... In essence, your engine needs more than that...and actually, you need a lot more...like an 870, at the minimum..
That being said, it can be made to run OK, but you're going to need to step up the jet package probably 2 - 4 steps depending on your climate and air temp.
Another thing that's pretty important is to match your power valve to your vacuum at Idle..they come with a .65 PV from the factory which works in the range of most stock Gin 1 engines, however, if you have significantly more vacuum in gear than 13...you need a stronger power valve. the rule of thumb is to take your vacuum reading (manifold..not ported) , under a load after the engine is achieved operating temp, then divide the number in half to determine the power valve needed.
The problem you're describing is the tip in stumble that occurs while the carb is transitioning from the Idle, to the main circuit...the other thing you may end up adjusting is the Cam used to actuate the Accelerator pump, to one that's more aggressive...you're feeling a lean condition, and getting all of the things I described into account and adjusted, will get you closer to success.
Hope my rambling helped a little, Carbs are kinda my specialty, as I'm a builder for Pro-Systems....and unfortunately, the street avengers, are not a very good performance option.
That being said, it can be made to run OK, but you're going to need to step up the jet package probably 2 - 4 steps depending on your climate and air temp.
Another thing that's pretty important is to match your power valve to your vacuum at Idle..they come with a .65 PV from the factory which works in the range of most stock Gin 1 engines, however, if you have significantly more vacuum in gear than 13...you need a stronger power valve. the rule of thumb is to take your vacuum reading (manifold..not ported) , under a load after the engine is achieved operating temp, then divide the number in half to determine the power valve needed.
The problem you're describing is the tip in stumble that occurs while the carb is transitioning from the Idle, to the main circuit...the other thing you may end up adjusting is the Cam used to actuate the Accelerator pump, to one that's more aggressive...you're feeling a lean condition, and getting all of the things I described into account and adjusted, will get you closer to success.
Hope my rambling helped a little, Carbs are kinda my specialty, as I'm a builder for Pro-Systems....and unfortunately, the street avengers, are not a very good performance option.
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08-14-2011, 03:00 PM
#10
I don't know what yours will like, but my motor has a "smallish" cam (220@.050") and it needed to be jetted up in the primaries 2 jet sizes and switched to a 7.5 power valve. We also richened up the idle a little with the idle mixture screws and the problem was solved. PS: make sure you check your idle vacuum with the car in gear with an automatic.
