Idle issues with carb LS
Idle issues with carb LS Posted this in Drag Tech, figured I put it up here in hopes that someone may chime in.
Carb'd 346" LS1, cam-only internally.
239/247 @ .050 comp cam
QFT 750 Mech carb
-4.5Hg PV
-jetted 74's fr and 84's rr
MSD 6LS
-MAP reads 14.7 psi dead, 10.5 psi idling
Air/Fuel while idling is 12.0-12.2
Vac modulated 4l60 MVB
Vig 4400 converter
Idling at 900 rpm
When I put it in gear(1st or rev) the mtr stumbles and tries to die. You can give it some throttle to keep it from dying, or if you do nothing> it dies.
Other things that happen going into gear:
-A/F goes from 12.0 to 10.0(bottoms out)
-MAP goes from 10.5 psi to 12.5 psi(needle jump around)
-RPMs fall from 900 to 500.
I did some fooling around with timing and idle adjustment on the carb. It seems to do best with 22deg of idle timing and 2 turns out on the idle screws, all four corners.
Things I've already done:
-Checked for vac leaks>none
-Checked power valve>good
-Cleaned metering blocks, jets, the hole carb>no trash found
-Fuel PSI set at 6.5psi as per QFT tech
-Fuel bowls set 1/2 way up on sight glasses per QFT tech
-Endless adjusting of idle screws
I took the car out on the road and it pulls good, runs out without issue. Just have this idling problem when going into gear... I can increase the idle rpm to 2000 and it won't go dead but it sounds like a 'tater truck.
I'm at a loss of what to do next. I'm tempted to fool around with the air bleeds to lean the idle mixture up a bit as it seems like it's loading up. I've had some locals tell me to plug the front PV and square up the jets.
Any suggestions??
Carb'd 346" LS1, cam-only internally.
239/247 @ .050 comp cam
QFT 750 Mech carb
-4.5Hg PV
-jetted 74's fr and 84's rr
MSD 6LS
-MAP reads 14.7 psi dead, 10.5 psi idling
Air/Fuel while idling is 12.0-12.2
Vac modulated 4l60 MVB
Vig 4400 converter
Idling at 900 rpm
When I put it in gear(1st or rev) the mtr stumbles and tries to die. You can give it some throttle to keep it from dying, or if you do nothing> it dies.
Other things that happen going into gear:
-A/F goes from 12.0 to 10.0(bottoms out)
-MAP goes from 10.5 psi to 12.5 psi(needle jump around)
-RPMs fall from 900 to 500.
I did some fooling around with timing and idle adjustment on the carb. It seems to do best with 22deg of idle timing and 2 turns out on the idle screws, all four corners.
Things I've already done:
-Checked for vac leaks>none
-Checked power valve>good
-Cleaned metering blocks, jets, the hole carb>no trash found
-Fuel PSI set at 6.5psi as per QFT tech
-Fuel bowls set 1/2 way up on sight glasses per QFT tech
-Endless adjusting of idle screws
I took the car out on the road and it pulls good, runs out without issue. Just have this idling problem when going into gear... I can increase the idle rpm to 2000 and it won't go dead but it sounds like a 'tater truck.
I'm at a loss of what to do next. I'm tempted to fool around with the air bleeds to lean the idle mixture up a bit as it seems like it's loading up. I've had some locals tell me to plug the front PV and square up the jets.
Any suggestions??
Does your power valve match your vacuum? Do you have a wideband on the car? If you find that you're having to open the butterfly too much to get the car to idle, you may be off the idle circuit. The fix is to drill holes in the butterfly to keep the blade closed and stay on the carbs' idle circuit. Since you're idling at 900rpm and it's dropping to 500, I am wondering if your converter is too tight. I read that it's a 4400 stall. Is it a lock up? Any chance it's trying to lock up at idle? How did you test your power valve to verify it's working properly?
Does your power valve match your vacuum? Do you have a wideband on the car? If you find that you're having to open the butterfly too much to get the car to idle, you may be off the idle circuit. The fix is to drill holes in the butterfly to keep the blade closed and stay on the carbs' idle circuit. Since you're idling at 900rpm and it's dropping to 500, I am wondering if your converter is too tight. I read that it's a 4400 stall. Is it a lock up? Any chance it's trying to lock up at idle? How did you test your power valve to verify it's working properly?
Question is, does the power valve crack on at your
idle vacuum level. It mustn't. 8.5"Hg vacuum is kind
of iffy, check your power valve / spring and make
sure you leave some room fur surge not to crack
it on every dip (making surge cycle forever).
I think you want a much leaner idle too, that rich
is only going to depress vacuum / elevate MAP.
Now with a ****-poor MAP (vacuum) level comes
some low idle timing and maybe part of the problem
is a difference between idle advance and main
advance; does MSD6AL "know" about idle vs open?
Stock PCM jacks timing around modally, big time.
Check the idle advance and see if you can get it
some additional (but I'd try leaning out the fuel
first, until you stop seeing vacuum improve -
old school).
idle vacuum level. It mustn't. 8.5"Hg vacuum is kind
of iffy, check your power valve / spring and make
sure you leave some room fur surge not to crack
it on every dip (making surge cycle forever).
I think you want a much leaner idle too, that rich
is only going to depress vacuum / elevate MAP.
Now with a ****-poor MAP (vacuum) level comes
some low idle timing and maybe part of the problem
is a difference between idle advance and main
advance; does MSD6AL "know" about idle vs open?
Stock PCM jacks timing around modally, big time.
Check the idle advance and see if you can get it
some additional (but I'd try leaning out the fuel
first, until you stop seeing vacuum improve -
old school).
MSD will do vac advance/retard based on what you set it at... it will also adjust for best idle quality if used in conjunction with a PCM, which my car does not have. Regrettably, I'm starting to think that I might have a converter/trans issues. Starting to think that the converter may be locking up. It's supposed to be a 4400(Vig) but will start power braking at around 1300 rpm. My old TCI SF was advertised 3000 and wouldn't start to push hard until about 1800 rpm.
As far as idle mixture and timing, I've went up and down on both. A/F for idle I've adjusted anywhere from 11.8 up to 14.2 with the same results. Timing anywhere from 10 deg to 30. Guess I'll pull the tranny and send the converter off for a re-do... I bought it used.
As far as idle mixture and timing, I've went up and down on both. A/F for idle I've adjusted anywhere from 11.8 up to 14.2 with the same results. Timing anywhere from 10 deg to 30. Guess I'll pull the tranny and send the converter off for a re-do... I bought it used.
Where do you have your map connected to? It should be connected to the vac advance line on the carb and not the hole in the intake under the carb baseplate. try tuning it without vac advance to see if the stumble goes away slightly.
With my carb and carb seutp i know it tried to die on me when i start slipping in the clutch and i must admit i have stalled it out a few times. With a large cam, that intake and running it carbed it greatly lowers the power at idle. Maybe try to up the idle to around 1200 or so and see if that helps too.
Now when you put it in gear you say the vacuum drops to 12.5 psi? thats like 4.4HG which is where your power valve opens up right? Try a 6.5 power valve and see if it helps if you have one just laying around.
Also you might get a good bit of help in the "Conversions and Hybrids" forum with tuning your setup.
With my carb and carb seutp i know it tried to die on me when i start slipping in the clutch and i must admit i have stalled it out a few times. With a large cam, that intake and running it carbed it greatly lowers the power at idle. Maybe try to up the idle to around 1200 or so and see if that helps too.
Now when you put it in gear you say the vacuum drops to 12.5 psi? thats like 4.4HG which is where your power valve opens up right? Try a 6.5 power valve and see if it helps if you have one just laying around.
Also you might get a good bit of help in the "Conversions and Hybrids" forum with tuning your setup.
