irratic idle problem - The 1947 - Present Chevrolet & GMC Truck Message Board Network

project72lwb · 06-02-2002, 08:05 PM

I have a stock 350 in truck and has developed an irratic idle after driving for a few minutes, particularly noticable when running air conditioner. Gave truck a tune up(cap, rotor--hei, plugs, wires, fuel filter) wonder if carb is problem. I'm running 8btdc on timing with an idle in gear of 700 but even with the strain the ac puts on the motor I would'nt think I should have to re-tune the engine for air use. Carb is edelbrock 1406/elec choke. Basically feels like the truck is flooding when using the air. Help??

Longhorn Man · 06-02-2002, 08:51 PM

Is this a new problem?
Does it only do this when the engine has been running a while?
I ask becouse these AFB carbs are very sensitive to vapor lock.
Also sounds like your floats may be too highallowing raw fuel to be dumped into the carb.

Mike C · 06-02-2002, 09:38 PM

GM uses a fast idle solenoid to bump up the idle when the AC is engage on the Q-jet. Lowers rpm by about 200 in my truck, but it idles fine at 600. The compressor is a significant load on the motor. I put a 50% underdrive crank pulley on my Camaro and had to lower the idle 400 rpm!

project72lwb · 06-04-2002, 08:50 PM

Where would the fast idle solenoid be if my truck were to have one. I have been running this carb for about a year and until recently I can't recall it having this problem before. At first start it idles fine but after a warming up it starts to resemble a flooding situation. You thoughts are appreciated---

tom hand · 06-04-2002, 09:02 PM

How much cam are you running? If running a larger than stock cam the A/C may be dropping the vacuum enough to open the power circuit on the carb causeing a flooding condition. If this is the case you will need different springs on the metering rods. Just a thought.

project72lwb · 06-04-2002, 09:18 PM

motor is bone stock 350, factory air setup, dual exhaust--starting to wonder if carb needs rebuild kit, maybe floats are filling with fuel and it takes a while to do so. might be whats happening

tom hand · 06-04-2002, 10:33 PM

OK that was my first guess but here are a couple more. First check with a vacuum gauge and make sure the motor will hold 17 - 18 inches of vacuum in gear with the A/C on. If not the power circuit in the carb could still be the problem. If it will not hold that much vacuum, first use some carb cleaner and check for vacuum leaks. Spary the carb cleaner anywhere you suspect a leak and if the idle changes you found it. If that doesn't turn out to be the problem, try using the vacuum gauge to adjust the timing for more vacuum, I have seen many small blocks that the timing marks were not correct. In fact I would run as much timing as the motor will tolarate anyway because you will get better gas milage. Advance the timing till the motor "pings" under full throttle, then retard the timing till it just stops. This is usally the best setting for a sbc if the starter will tolarate it. Many times you can run 10 - 12 degrees btdc with premeium fuel. Last I have seen old vehicles have rust in the fuel lines that will go right thru a fuel filter. If this is the problem you will have stuff in the carb that looks like mud. The only solution for this is to replace the rusted component. I have run as many as three filters and still it will come thru. Good Luck!

bigjimzlll · 06-04-2002, 11:25 PM

You might check fuel pressure too....the 1406 cant handle anymore than five lbs of pressure. If all else fails a carb spacer might help.

06-02-2002, 08:05 PM	#1
project72lwb Registered User Join Date: May 2002 Location: owasso, ok Posts: 30	irratic idle problem I have a stock 350 in truck and has developed an irratic idle after driving for a few minutes, particularly noticable when running air conditioner. Gave truck a tune up(cap, rotor--hei, plugs, wires, fuel filter) wonder if carb is problem. I'm running 8btdc on timing with an idle in gear of 700 but even with the strain the ac puts on the motor I would'nt think I should have to re-tune the engine for air use. Carb is edelbrock 1406/elec choke. Basically feels like the truck is flooding when using the air. Help?? __________________ 68 chevelle(garage queen) 72 cheyenne super(driver) Owasso, Oklahoma

06-02-2002, 09:38 PM	#3
Mike C Registered User Join Date: Dec 1999 Location: Austin, TX, USA Posts: 7,723	GM uses a fast idle solenoid to bump up the idle when the AC is engage on the Q-jet. Lowers rpm by about 200 in my truck, but it idles fine at 600. The compressor is a significant load on the motor. I put a 50% underdrive crank pulley on my Camaro and had to lower the idle 400 rpm! __________________ 44 Willys MB 52 M38A1 64 Corvette Coupe 68 Camaro 'vert LT1 & TH700 69 Z/28 355 12.6's @110 69 Chevy Short Step 4 1/2"/7" drop 72 Jimmy 4WD 4spd 4" & 35's 02 GMC 2500HD 4x4 Duramax

06-04-2002, 08:50 PM	#4
project72lwb Registered User Join Date: May 2002 Location: owasso, ok Posts: 30	Where would the fast idle solenoid be if my truck were to have one. I have been running this carb for about a year and until recently I can't recall it having this problem before. At first start it idles fine but after a warming up it starts to resemble a flooding situation. You thoughts are appreciated--- __________________ 68 chevelle(garage queen) 72 cheyenne super(driver) Owasso, Oklahoma

06-04-2002, 09:02 PM	#5
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	How much cam are you running? If running a larger than stock cam the A/C may be dropping the vacuum enough to open the power circuit on the carb causeing a flooding condition. If this is the case you will need different springs on the metering rods. Just a thought. __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

06-04-2002, 09:18 PM	#6
project72lwb Registered User Join Date: May 2002 Location: owasso, ok Posts: 30	motor is bone stock 350, factory air setup, dual exhaust--starting to wonder if carb needs rebuild kit, maybe floats are filling with fuel and it takes a while to do so. might be whats happening __________________ 68 chevelle(garage queen) 72 cheyenne super(driver) Owasso, Oklahoma

06-02-2002, 08:51 PM	#2
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,693	Is this a new problem? Does it only do this when the engine has been running a while? I ask becouse these AFB carbs are very sensitive to vapor lock. Also sounds like your floats may be too highallowing raw fuel to be dumped into the carb.

06-04-2002, 10:33 PM	#7
tom hand CCRider Join Date: Feb 2000 Location: Olive Branch,MS,USA Posts: 2,232	OK that was my first guess but here are a couple more. First check with a vacuum gauge and make sure the motor will hold 17 - 18 inches of vacuum in gear with the A/C on. If not the power circuit in the carb could still be the problem. If it will not hold that much vacuum, first use some carb cleaner and check for vacuum leaks. Spary the carb cleaner anywhere you suspect a leak and if the idle changes you found it. If that doesn't turn out to be the problem, try using the vacuum gauge to adjust the timing for more vacuum, I have seen many small blocks that the timing marks were not correct. In fact I would run as much timing as the motor will tolarate anyway because you will get better gas milage. Advance the timing till the motor "pings" under full throttle, then retard the timing till it just stops. This is usally the best setting for a sbc if the starter will tolarate it. Many times you can run 10 - 12 degrees btdc with premeium fuel. Last I have seen old vehicles have rust in the fuel lines that will go right thru a fuel filter. If this is the problem you will have stuff in the carb that looks like mud. The only solution for this is to replace the rusted component. I have run as many as three filters and still it will come thru. Good Luck! __________________ 72 GMC Sierra SWB almost finished---- 84 Softail Olive Branch MS

06-04-2002, 11:25 PM	#8
bigjimzlll Senior Member Join Date: Dec 2001 Location: Redding,CA...USA Posts: 4,736	You might check fuel pressure too....the 1406 cant handle anymore than five lbs of pressure. If all else fails a carb spacer might help. __________________ It's called "drag racing" if they called it "tic..tic..WHAM!..BANG! F*&K!!!", they'd have to keep the magazines under the counter with the other men's publications click the clicky to join the site.... http://67-72chevytrucks.com/vboard/payments.php 67 lwb..first hotrod in 25 years..540 best ET is 9.45 @ 141.44 Anderson,CA



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