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#501975 - 04/13/08 04:12 PM
'56 4-bbl Carb bogs down on takeoff after extended idle
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RTMartens
Member
Registered: 10/08/07
Posts: 30
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1956 Buick Century w/original 4-bbl, cheap electronic fuel pump. The car is a daily driver. Starts well, runs well, idles well, takes off well--generally. It's happened before, but I'm only now recognizing a pattern.
It seems that if the ambient temp is 80 degrees or so, and if I have to sit at a stop light idling (in Drive) for 3-5 minutes or so (haven't timed it yet), after getting on the throttle I get up to 25-30 mph or so, then the car starts to bog down--like it's flooded. It happened 3X on Friday.
I try pumping the gas pedal to clear it, but that probably only makes it worse. Friday it died completely but restarted fine each time. With the electronic fuel pump it shouldn't be a vapor lock issue, and once I get things cleared out and good airflow moving again, no problems.
Engine is not overheating, exhaust manifold valve is stuck open (at least I assume so, as volume and pressure out of dual exhausts is identical.) Oil pressure is fine, fuel capacity is fine.
Any ideas?
Rick
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#502011 - 04/13/08 07:45 PM
Re: '56 4-bbl Carb bogs down on takeoff after extended idle
[Re: RTMartens]
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Jim_Cannon
Member
Registered: 08/25/06
Posts: 235
Loc: Spring, TX
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Check fuel pump pressure. Many electric pumps put out too high pressure for old carbs.
What do you know about needle valves in carb? They might be hard rubber tips now and not sealing like they used to.
Don't assume anything about exhaust valve. If you can't figure out that it is open by looking at shop manual and comparing to car, remove exhaust pipe and look up in manifold to see what you have.
_________________________
Jim '29 Ford Model A Phaeton (MARC #71406) '63 Riviera, Fawn Mist, Saddle Leather (ROA #9774) Jim's 1963 Riviera
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#502040 - 04/13/08 09:23 PM
Re: '56 4-bbl Carb bogs down on takeoff after extended idle
[Re: RTMartens]
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JohnD1956
Member
Registered: 03/29/03
Posts: 2221
Loc: New York
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Check the base gasket on the carb. I had a stumble in my 56 and discovered tightening the carb down eliminated it. Which mostly goes to say it could be any of several things.
One other thing I'd recommend is checking the fuel line where it runs next to the intake. It does get pushed down in there sometimes and pulling it out so it's not resting on anything ought to assist.
_________________________
John C. De Fiore BCA # 3757 56 Super 56R 69 GS 400 Convertible 69 Electra Limited 2 dr.( well, no longer limited although still unique) 95 Riviera Supercharged (sold) 06 Lacrosse CXL Doing my part to stimulate the economy! Member of the UPSTATE NY CHAPTER check it out at http://unybca.skyphix.com/
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#502050 - 04/13/08 09:45 PM
Re: '56 4-bbl Carb bogs down on takeoff after extended idle
[Re: RTMartens]
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old-tank
Member
Registered: 12/29/99
Posts: 969
Loc: Seguin, TX, USA
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Rick
You have described a vapor lock problem and/or inadequate fuel delivery. At 20-30mph the carb runs dry and if no more fuel is delivered it dies or trys to die. Here is an old thread that mentions my troubles with vapor lock . I can usually deal with the vapor lock after idling in hot weather, but the high altitude plus heat is harder to fix: I have now wrapped my fuel line from the pump to the carb with insulation.
Willie
_________________________
55 Centurys 63; 63; 66C People who use tools bleed a lot! Keep your mouth open so it doesn't hit you in the face! BuickRestorer
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#502655 - 04/16/08 12:54 AM
Re: '56 4-bbl Carb bogs down on takeoff after extended idle
[Re: serb]
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NTX5467
Long Time Member
Registered: 12/09/99
Posts: 2721
Loc: DFW, TX
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An electric fuel pump is not necessarily a "fix" for vapor lock, but it might decrease the tendency somewhat. Modern fuels can tend to be more prone to vapor lock than the fuels available when the vehicle was designed.
Insulating the fuel line can help with the underhood heat situation, but the fuel must be cooler BEFORE it gets that far in the system!
In reading the first comments in the post, it made me wonder if the carb idle was adjusted too rich and then had to "clean out" once the vehicle started to accelerate and cruise. When it was mentioned that it was after getting "to low cruise speed" and then falling flat, that could mean a "no fuel supply" situation.
In further diagnosing the situation, you can simulate the situation on a deserted road or parking lot. When it dies out after getting up to 25-30mph, then you can coast to a safe stop, remove the air cleaner, work the accelerator linkage, and look for accelerator pump shot IMMEDIATELY after you coast to the safe stopping place AND turn off the engine (and electric fuel pump).
In this scenario, once you've checked for accel pump discharge (the inlet to the accel pump circuit is at the bottom of the float bowl), then you can turn on the ignition and electric fuel pump and see if the electric pump is working and how soon it will be before the accel pump works again and the engine will restart.
Although conventional wisdom would lead us to a fuel-related issue, don't also neglect to see if the ignition system is also in good condition and base timing set "to spec". Making sure that all carb baseplate bolts are snugged-down to spec is a good suggestion, too.
Just some thoughts,
NTX5467
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#502842 - 04/16/08 08:11 PM
Re: '56 4-bbl Carb bogs down on takeoff after extended idle
[Re: NTX5467]
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RTMartens
Member
Registered: 10/08/07
Posts: 30
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Thanks for all of the suggestions!
I'm leaning away from the vapor lock theory for the following reasons: I'm using the steel line from the fuel tank that leads up to the original fuel pump location, from there it switches to 5/16" rubber fuel line and runs to the passenger fender where the electronic fuel pump is located. From there it runs directly to a plastic in-line fuel filter and into the carb. It's not mounted to the engine anywhere, so I can't see that much heat being transferred to it.
I am thinking there might be some leakage past the float-operated needle valve due to excessive fuel pressure at extended idle. I need to test my fuel pressure just ahead of the carb and watch to see if the pressure climbs over a period of extended idle. I may need to buy a fuel pressure regulator to tone it down to match the original mechanical pump specs.
There are no apparent visible fuel leaks on the outside of the carb, and the car runs fine under all conditions other than those described in the original post.
Thanks,
Rick
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#502895 - 04/16/08 10:52 PM
Re: '56 4-bbl Carb bogs down on takeoff after extended idle
[Re: RTMartens]
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old-tank
Member
Registered: 12/29/99
Posts: 969
Loc: Seguin, TX, USA
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Rick
Electric fuel pumps work best mounted near the tank...they push fuel better than pulling it. Also if there are any leaks between the tank and the pump, air could be pulled into line.
Willie
_________________________
55 Centurys 63; 63; 66C People who use tools bleed a lot! Keep your mouth open so it doesn't hit you in the face! BuickRestorer
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#502935 - 04/17/08 07:54 AM
Re: '56 4-bbl Carb bogs down on takeoff after extended idle
[Re: old-tank]
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carbking
Member
Registered: 11/26/04
Posts: 527
Loc: MO
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I would concur with Willie. For good results, electric fuel pumps MUST be mounted lower than the fuel tank.
An electric pump will push liquid around the world, but won't pull it 6 inches!
We chased a similiar problem via telephone with a customer for weeks, until he finally mentioned an electric pump mounted on the frame where it went over the rear axle. I suggested lowering the pump. It said there was no place to put it lower. I suggested just trying the original pump and removing the electric. He didn't like the idea, but did try it - end of issue!
Jon.
_________________________
Good carburetion is fuelish hot air!
Owner - The Carburetor Shop LLC (of Missouri)
The most expensive carburetor you will ever buy....is the wrong one for the application that you attempt to modify!
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#503120 - 04/17/08 07:06 PM
Re: '56 4-bbl Carb bogs down on takeoff after extended idle
[Re: brh]
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RTMartens
Member
Registered: 10/08/07
Posts: 30
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Heat affecting the pump wasn't something I had considered. It's getting warmer under the hood due to the extended idle, and the pump is working against an all but closed float valve for a long time, so it's probably heating up internally as well.
It's an electronic pump I installed as a "temporary" solution 8 years ago. (I think the brand is Facet?) Not sure exactly what excess heat might do to the electronic pump (or how it actually moves fuel), but for an electric motor-driven pump, I can imagine some cavitation may occur.
Eyeballing the picture in my shop manual, it looks like the exhaust manifold valve is open when the largest end of the counterweight it down. The counterweight appears to be down on my car, so I assume the valve is open. I'd have to pull a significant portion of my exhaust system to be sure.
Rick
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#503641 - 04/20/08 02:51 AM
Re: '56 4-bbl Carb bogs down on takeoff after extended idle
[Re: RTMartens]
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NTX5467
Long Time Member
Registered: 12/09/99
Posts: 2721
Loc: DFW, TX
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I believe there would be no reason for any electric fuel pump (other than a specific-for-fuel injection pump) to be factory-calibrated for more than about 6psi, even a "high flow" model as 5.5psi is generally the fuel pressure spec of the mechanical pumps they replace. These same pumps are designed to work against a cycling closed carb float needle and seat sitaution at low-demand times, so one getting overheated during extended idle periods is not generally an issue. Generally what causes problems with them is sucking air (even on the late model fuel injection models!).
I suspect that if the needle and seat were allowing some leakage due to a slight high fuel pressure at idle, it would eventually cause fuel to come out of the vent tubes and spill over from the venturi cluster and possible leak from any other orifice below the "new" fuel level . . . plus causing a resultant flooding situation rather than what you describe.
Is the electric fuel pump an "impeller" model or one of the smaller "square" pumps? Just curious . . .
Is the "bogging down" on brisk acceleration or on more leisurely acceleration? What happens, in the same scenario, if you torque the engine against the foot brake in "Drive"?
Have you checked the fuel/air ratio on a shop's engine analyzer as it idles? Plus slowly torqueing the engine against the foot brake (checks the vacuum level at which the "power valve" starts to open)? Not to forget to check the ignition pattern during these tests! Are the spark plugs getting sooty during this idle period? Are they the correct heat range and such?
Might the situation be diminished after a long freeway drive?
Just some curiousities . . .
NTX5467
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#508722 - 05/10/08 01:49 PM
Re: '56 4-bbl Carb bogs down on takeoff after extended idle
[Re: NTX5467]
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RTMartens
Member
Registered: 10/08/07
Posts: 30
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Update!
Gentlemen:
Thanks for all of the advice! My previous posts indicate what I thought the problem was. I was wrong.
First I replaced the plastic see-through fuel filter--it was very small, looked only "partly" dirty--but when I cut it open I noted it was very dirty. I replace it with another clear one with about 5 times the volume and sized for the 5/16" fuel line instead of the previous one that stepped down to 3/16".
As I watched the new filter fill, I could see the fuel stream was very weak and inconsistent, and the fuel pump (Napa-branded Facet "square" electronic pump) was almost too hot to touch.
I replaced the pump with a small Mr. Gasket unit rated for the correct PSI, and mounted it on the frame rail as close as prudently possible/accessible to the tank. It's noisier than heck--even worse than a Chevy Suburban.
Performance-wise, I haven't had a problem. Yesterday I made the same trip under the same conditions as one of my previous problem runs, and it worked flawlessly--even when I drove it hard. It's also completely eliminated a small hiccup that used to occur under heavy acceleration.
I think the problem resulted from trying to use the pump to pull fuel, and the pump was probably 14" above the bottom of the tank, and it was hot under the hood, and my filter was too restrictive.
Again, thanks for the help!
Rick
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#508738 - 05/10/08 03:27 PM
Re: '56 4-bbl Carb bogs down on takeoff after extended idle
[Re: RTMartens]
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carbking
Member
Registered: 11/26/04
Posts: 527
Loc: MO
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Rick - you can often quiet a noisy electric pump by cutting rubber spacers from rubber tubing. Slide these over the mounting bolts between the pump and the frame.
Jon.
_________________________
Good carburetion is fuelish hot air!
Owner - The Carburetor Shop LLC (of Missouri)
The most expensive carburetor you will ever buy....is the wrong one for the application that you attempt to modify!
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#508850 - 05/11/08 12:22 PM
Re: '56 4-bbl Carb bogs down on takeoff after extended idle
[Re: carbking]
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NTX5467
Long Time Member
Registered: 12/09/99
Posts: 2721
Loc: DFW, TX
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I'm glad everything's back to normal.
Enjoy!
NTX5467
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