Question for Lars... Carb Flat Spot/Bog under
Thread Starter
Le Mans Master
Joined: Feb 1999
Posts: 8,351
Likes: 75
From: Tampa Bay FL
Question for Lars... Carb Flat Spot/Bog under
Lars,
As my sig indicates, my carb is set up with the following jets/metering rods: '74' main jet, '42' main metering, 'DA' secondary.
I cleaned up and properly oiling the K&N air filter after replacing the intake gaskets (I had over-oiled previously in a major way). I took the car out for several trips to ensure all was normal. On the third outing yesterday, I rounded a corner and hit the pedal and experienced a bog/flat spot. I let off of the accelerator and then kept on driving. That was the first time I had hit the accelerator hard since before the repair. I'm going to keep an eye on the car for the next several outings in event the bog occurs again.
If the bog returns, should I consider any changes in the fuel metering? If so, what would you propose?
As my sig indicates, my carb is set up with the following jets/metering rods: '74' main jet, '42' main metering, 'DA' secondary.
I cleaned up and properly oiling the K&N air filter after replacing the intake gaskets (I had over-oiled previously in a major way). I took the car out for several trips to ensure all was normal. On the third outing yesterday, I rounded a corner and hit the pedal and experienced a bog/flat spot. I let off of the accelerator and then kept on driving. That was the first time I had hit the accelerator hard since before the repair. I'm going to keep an eye on the car for the next several outings in event the bog occurs again.
If the bog returns, should I consider any changes in the fuel metering? If so, what would you propose?
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,392
From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Question for Lars... Carb Flat Spot/Bog under (TedH)
Ted -
The most common cause of the symptom you describe is a bad accel pump. I have had a bunch of cases over the last 2 years of accel pumps that swell up and partially seize in the pump bore due to the modern oxygenated fuels. This either reduces or eliminates the pump shot. Take a look at your carb with the engine shut off: Play with the throttle and verify that you are getting an immediate accel pump fuel shot out of the discharge nozzles with the slightest movement of the throttle, and verify that this shot runs smoothly and continuously for the majority of the primary throttle travel range. If not, replace the accel pump.
Your 74/42 combo sounds good, and should perform very well. Just make sure your power piston is not adjusted to go too deep into the jets: With the piston fully depressed, you should have about .020" - .030" from the top of the plastic retaining collar lip to the top of the piston cylinder. If the piston is set too deep you can get an off-idle lean-out condition in spite of a good jetting combo.
The most common cause of the symptom you describe is a bad accel pump. I have had a bunch of cases over the last 2 years of accel pumps that swell up and partially seize in the pump bore due to the modern oxygenated fuels. This either reduces or eliminates the pump shot. Take a look at your carb with the engine shut off: Play with the throttle and verify that you are getting an immediate accel pump fuel shot out of the discharge nozzles with the slightest movement of the throttle, and verify that this shot runs smoothly and continuously for the majority of the primary throttle travel range. If not, replace the accel pump.
Your 74/42 combo sounds good, and should perform very well. Just make sure your power piston is not adjusted to go too deep into the jets: With the piston fully depressed, you should have about .020" - .030" from the top of the plastic retaining collar lip to the top of the piston cylinder. If the piston is set too deep you can get an off-idle lean-out condition in spite of a good jetting combo.
Thread Starter
Le Mans Master
Joined: Feb 1999
Posts: 8,351
Likes: 75
From: Tampa Bay FL
Re: Question for Lars... Carb Flat Spot/Bog under (lars)
Lars,
If accelerator pump is the problem, what make/brand would you recommend given that I probably use the new (cleaner) oxygenated fuels. NOTE: I tend to run only fuel from the better establishments: 76, BP/Amoco, Chevron.
If accelerator pump is the problem, what make/brand would you recommend given that I probably use the new (cleaner) oxygenated fuels. NOTE: I tend to run only fuel from the better establishments: 76, BP/Amoco, Chevron.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,392
From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Question for Lars... Carb Flat Spot/Bog under (TedH)
If you give me your carb number I can give you the part number for a pump that is compatible with the fuels. Don't have the info here at the office, but I can get it at home tonight.
Thread Starter
Le Mans Master
Joined: Feb 1999
Posts: 8,351
Likes: 75
From: Tampa Bay FL
Re: Question for Lars... Carb Flat Spot/Bog under (lars)
Lars,
Here's the info from the rebuild sheet you provided for my carb rebuild on 5/13/2002:
Carb# 17066563 (GM General Replacement). For that rebuild, the following problems were noted/corrected by you:
Secondary rods ground
Accel pump seized in bore
Secondary airvalve partially jammed
Carb running 4% too lean at WOT
Well & Bowl fillers missing from fuel bowl
choke misadjusted
choke pulloff misadjusted
[Modified by TedH, 12:17 PM 1/26/2004]
Here's the info from the rebuild sheet you provided for my carb rebuild on 5/13/2002:
Carb# 17066563 (GM General Replacement). For that rebuild, the following problems were noted/corrected by you:
Secondary rods ground
Accel pump seized in bore
Secondary airvalve partially jammed
Carb running 4% too lean at WOT
Well & Bowl fillers missing from fuel bowl
choke misadjusted
choke pulloff misadjusted
[Modified by TedH, 12:17 PM 1/26/2004]
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,392
From: At my Bar drinking and wrenching in Lafayette Colorado
Re: Question for Lars... Carb Flat Spot/Bog under (TedH)
If this is a carb that I have rebuilt, and if you're having a problem with it, please feel free to send it back for a complete checkout and repair. Keep in mind that there are other issues that can also cause stumbles, including ignition timing and timing advance.
Thread Starter
Le Mans Master
Joined: Feb 1999
Posts: 8,351
Likes: 75
From: Tampa Bay FL
Re: Question for Lars... Carb Flat Spot/Bog under (lars)
Lars, Good point. I'll verify the timing this evening. Since I had removed/re-installed the distributor for the intake gasket replacement, I'll check it against my prior baseline to ensure that it is where I need it to be. Will let you know what I find.
Advanced
Joined: Sep 2003
Posts: 59
Likes: 0
From: McKinney TX
Re: Question for Lars... Carb Flat Spot/Bog under (TedH)
I,m no carb expert but I did have a similar problem on two vehicles (my 74 vette and a 73 elcamino). In both cases the stumble problem was fixed by replacing the secondary air valve controller. It seems that when I punched the engine at crusing speeds that the secondaries were opening too quickly letting to much air in at low RPM and thus the slight hesitation. After replacing the valve that controls the secondary air plate, the engines responded perfect without hesitation. the vette was a 350 and the ElCamino a 454 but I used the same part number for the controller valve. I picked them up at NAPA.
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Melting Slicks
Joined: Jul 2001
Posts: 2,609
Likes: 12
From: Massapequa Park NY
Re: Question for Lars... Carb Flat Spot/Bog under (TedH)
HEI distributor - yes? Also check the wires on the pick up coil. The coil rotates to advance the timing and over time develop at crack at the housing. What you get is a stumble/flat spot on acceleration. Rebuilt a carb once trying to find that one, and found out after the fact that it is pretty common.
Jim
Jim
Thread Starter
Le Mans Master
Joined: Feb 1999
Posts: 8,351
Likes: 75
From: Tampa Bay FL
Re: Question for Lars... Carb Flat Spot/Bog under (TedH)
I have since advanced the timing about 4 degrees. I suspect this contributed to the bog. It should be right at the timing 'sweet spot' that I had prior to pulling the distributor for the intake gasket replacement (35 degrees total by 2500rpm).
Safety Car
Joined: Aug 2000
Posts: 3,943
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From: Florham Park NJ Port Credit Ontario
Re: Question for Lars... Carb Flat Spot/Bog under (TedH)
Ted, I had the very same symptoms with my re-built Q-Jet. It was a bad accel pump. Got a new pump from Lars, installed myself and now works perfectly.
Thread Starter
Le Mans Master
Joined: Feb 1999
Posts: 8,351
Likes: 75
From: Tampa Bay FL
Re: Question for Lars... Carb Flat Spot/Bog under (Stewart's74)
Stewart,
Having just advanced the timing Mon evening (and with plan to validate the setting this evening with timing light), I'm going to keep an eye on it for any more flat spots. If it occurs again, I'll be checking the accel pump.
Thx! :seeya
Having just advanced the timing Mon evening (and with plan to validate the setting this evening with timing light), I'm going to keep an eye on it for any more flat spots. If it occurs again, I'll be checking the accel pump.
Thx! :seeya
