Losing power at highway speeds

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danmulligan
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Losing power at highway speeds

Unread post by danmulligan » Sat May 06, 2017 8:18 am

Help, I have a 1965 Corsa Conv with the 140, 4 carb of course. Lately if I'm on highway going 60-65 I lose power and it slows to about 55. I can put the pedal to the floor and it just bogs like something is clogged. Then after about a minute it jumps back and has full power.Coming home I was on highway about 20 minutes and it would happen every 5-7 minutes, 3-4 times in 20 minutes. I am not mechanical at all. It doesn't happen around town at slower speeds. As an amateur, it's like I lose my secondary carbs, then they kick back on, maybe that's silly, don't know. HELP
Thanks for any suggestions
Dan Mulligan
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64powerglide
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Re: Losing power at highway speeds

Unread post by 64powerglide » Sat May 06, 2017 9:32 am

You should check the fuel pump first. If that's OK check the electronics, coil,points ect.. Sounds like it's starving for fuel at high speeds.
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danmulligan
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Re: Losing power at highway speeds

Unread post by danmulligan » Sat May 06, 2017 9:59 am

fuel pumps a good idea, I'll check. I do have electronic ignition. Is it possible the secondary carbs could not be kicking in or does that not make any sense? again, amateur here, or is it likely not a carb issue. The motor was rebuilt and all new carbs were installed less than a 1000 miles ago.
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Re: Losing power at highway speeds

Unread post by 64powerglide » Sat May 06, 2017 10:42 am

Amateur here too, just using some common sense ideas. My first thought was fuel as you say it bogs down when you accelerate, there could also be some foreign matter in a carb float bowl that clogs up the fuel supply then settles back down but when you are using more fuel at higher speed it gets picked up again. We'll just have to see if anyone have other ideas. Check all the wiring around the coil for any loose connections or getting grounded out.
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bbodie52
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Re: Losing power at highway speeds

Unread post by bbodie52 » Sat May 06, 2017 4:45 pm

The mechanical fuel pump must create a vacuum in the long fuel line between the tank and the pump to draw the fuel to the pump, where it is then pressurized and fed to the carburetors.

If there is a leak anywhere between the fuel tank and the pump inlet, the pump will not be able to create a good vacuum to draw the fuel from the tank and into the mechanical fuel pump. (This type of leak may not be readily apparent, since the feed line from the tank is not under pressure). Such a leak can make the pump appear to be defective, when the problem is actually a faulty fuel line between the pump and the tank. The pump may be fine and may be able to create adequate fuel pressure to the carburetors, but fuel starvation would be likely to occur, as the pump is starved for fuel delivery from the tank, caused by a vacuum leak. The problem may not be obvious when driving at slow speeds or around town. But at highway speeds or when fuel consumption is greater, the engine may consume the small amount of reserve in each carburetor float bowl, and the pump may not be able to replenish the fuel fast enough if delivery volume is below the standard of 1 pint in 40 seconds or less.

The illustration below shows the location of the two flexible fuel hoses in the line between the tank and the fuel pump. Check the condition of both hoses. If they are rotted or cracked they should be replaced and securely clamped with good hose clamps to ensure that there are no vacuum leaks that would prevent the pump from drawing a steady flow of gasoline from the tank.

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There is a small fuel filter "stone" inside each carburetor body inlet. These seldom get clogged to the point where they would restrict fuel to the point of fuel starvation, but it is possible. If the rest of the system checks good, you might check these fuel filters.

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skipvair
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Re: Losing power at highway speeds

Unread post by skipvair » Sat May 06, 2017 9:16 pm

Check fuel filters too. But as 64 pg says, sounds like fuel starvation. First check pressure at pump. Could possibly be vapor lock, but that will often shut it down


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Re: Losing power at highway speeds

Unread post by skipvair » Wed May 10, 2017 8:57 am

I have had filters clog. Put it to the floor and see if fuel starves even at low speed as it revs. Would not run at highway speeds.


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Re: Losing power at highway speeds

Unread post by bbodie52 » Wed May 10, 2017 11:15 am

skipvair wrote:Wed May 10, 2017 10:57 am

...Put it to the floor and see if fuel starves even at low speed as it revs. Would not run at highway speeds.
Putting your foot to the floor at low speed does not really prove anything. The carburetor float bowl is similar in function to the tank and float valve on the common toilet. There are several ounces of gasoline stored in the float bowl, and whenever the fluid in the float bowl begins to be depleted the float is lowered and the needle and seat valve opens to allow the fuel pump to replenish the fuel supply in the float bowl. This "pre-positioned" supply of gasoline in the carburetor serves as a "storage buffer" to try to ensure that the carburetor is never starved for fuel as the demand for gasoline changes. Cruising at low speeds uses a small amount of gasoline, and the fuel pump would normally have no problems with keeping up with the demand and maintaining the upper level of gasoline established by the float height. While opening the throttle fully at low speeds would increase the demand for gasoline momentarily, it is not enough to drain the carburetor's float bowl. However, consistent high-speed driving at highway speeds does increase the demand for fuel replenishment to the point where the fuel pump needs to work harder and more frequently to keep up with the consistent higher demand.

If your loss of power is only occurring at highway speeds, this may be an indication that the fuel pump volume capacity is unable to keep up with the higher demand for more fuel at long-term highway speeds — to the point where the float bowl may, at times, become emptied. If the main jet at the bottom of the float bowl becomes uncovered and is exposed to air, the results would be intermittent power loss as a fuel/air mixture becomes more air and less fuel.

You can test a fuel pump for both output pressure and output volume using the procedures that I posted earlier from the factory shop manual. By disconnecting the fuel line and cranking the engine you can measure the fuel volume output to see if it meets the standard of 1 pint in 40 seconds at cranking speed. If the pump fails this test you should check for a cracked hose or air leak somewhere along the feedline between the fuel tank and the fuel pump. Even a small air leak can cause a failure in the fuel pump's attempt to draw fuel from the fuel tank. Fuel starvation at this point will prevent the pump from being able to pressurize the gasoline and deliver it in adequate quantities to each carburetor.
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gnrand
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Re: Losing power at highway speeds

Unread post by gnrand » Wed May 10, 2017 11:27 am

Is it possible to install a fuel pressure gauge that you could read at highway speeds.

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skipvair
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Re: Losing power at highway speeds

Unread post by skipvair » Sun May 14, 2017 7:25 am

Please post what you HAVE done to check things such as fuel filters etc.

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