1978 Mark V Accelerator Pump issues

[svroman58]

Has anyone ever experience a problem with an accelerator pump? My car runs great, but when I start to go up a hill or an incline at a speed over 65 mph it acts as it’s not getting enough fuel and then it loses power. It had a new Motorcraft 2 barrel carburetor put on about 3 months ago.

 

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A 1978 Mark should have a carburetor.
A Motorcraft 4350 if a 4 barrell carburetor.
A Motorcraft 2150 is a 2 barrell carburetor.

In any case there is an accelerator pump in the carb that shoots additional fuel even at the slightest movement of the accelerator pedal.
Either the pump is leaking or the pump arm needs to be adjusted.

Attached is a pic of a 2150 pump arm adjustment. This is but one way to adjust the pump.
If the pump is leaking, the diaphagm needs to be replaced.
You can tell if the diaphagm needs to be replaced by removing the external pump cover. There are 4 screws on the outside. If removing the covers there is any fuel on the outside of the cover, the diaphragm needs to be replaced.

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[Action]

If you have brand X carb or there have been other modifications to the engine, please state what mods have been done.

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[svroman58]

A 1978 Mark should have a carburetor.
A Motorcraft 4350 if a 4 barrell carburetor.
A Motorcraft 2150 is a 2 barrell carburetor.

In any case there is an accelerator pump in the carb that shoots additional fuel even at the slightest movement of the accelerator pedal.
Either the pump is leaking or the pump arm needs to be adjusted.

Attached is a pic of a 2150 pump arm adjustment. This is but one way to adjust the pump.
If the pump is leaking, the diaphagm needs to be replaced.
You can tell if the diaphagm needs to be replaced by removing the external pump cover. There are 4 screws on the outside. If removing the covers there is any fuel on the outside of the cover, the diaphragm needs to be replaced.

Action
View attachment 20445

Thanks for the info

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[svroman58]

If you have brand X carb or there have been other modifications to the engine, please state what mods have been done.

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It had a new motorcraft 2 barrel carburetor installed about 3 months ago. No modifications have been done.

 

[Action]

Wow, a brand new 3 month old carb should be good.

Rereading your post, if the only issue is over 65mph, that may be an issue with the fuel level in the carb.
That is a float adjustment or a weak fuel pump not putting enough fuel into the carb bowl.

Confirm the issue does not show up at speeds below say 45mph.
Thanks

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[svroman58]

Wow, a brand new 3 month old carb should be good.

Rereading your post, if the only issue is over 65mph, that may be an issue with the fuel level in the carb.
That is a float adjustment or a weak fuel pump not putting enough fuel into the carb bowl.

Confirm the issue does not show up at speeds below say 45mph.
Thanks

Action

It usually starts around 45 mph. If I’m driving around town and go up a hill, I have no problem and my town has some big hills. If I’m driving on a flat highway or interstate, I can put it to the floor and it doesn’t hesitate.

 

[Action]

OK thanks for the clarity.

If I might clarify, it is a part throttle stumble or lean misfire at highway speeds.
Does not occur during Wide Open Throttle. (WOT)

If that is the case, a lean mis-fire as you describe above was rather common when the vehicle was new.
The design was to create as lean a mixture at all times to meet Fed regulations.
The issue is a carburetor is just not that accurate to give a precise air fuel mixture during all of the modes.

The example might be traveling on a highway and there is a slight grade of hill approaching. Part the way up the hill the engine runs out of power to get over the hill with a little throttle movement. Either a far greater acceleration is needed or the vehicle has to slow down.
If this is the scenario, it was called a high speed lean mis-fire. There was an insufficient rich mixture to support complete combustion during a certain engine loading at speed.

If that is the issue, the fix is to enrichen the mixture. Possible ways to do that:
Very important that EGR is fully working. EGR is only supposed to be working at vehicle speeds of 40 or higher and on a warmed up engine. EGR enrichens the air fuel mixture ever so slightly.
Equally as important that there are zero vacuum leaks. Any even slight vacuum leak will create a leaner mixture. There are a lot of hoses that can be approaching 50 years of age. Any of these plus vacuum devises can leak ever so slightly causing an issue.
Increase float level very slightly. This may richen up all modes. Like using a shot gun to take down a specific target.
Adjust acelerator pump one hole richer. Some of these are adjustable as in the picture above. Some are not.
Adjust both mixture screw 1/4 turn rich. This circuit is not supposed to be active at these speeds. However I have had some success.

I would NOT recommend doing all of these at the same time. The first two yes. Followed by a test drive if you made any changes.
After that, anything that is done, follow it with a test drive to see if there is a difference.
This way you can be sure of what made a difference.

Then post back.

Action

______________________________

Help support this site so it can continue supporting you!

 

[svroman58]

OK thanks for the clarity.

If I might clarify, it is a part throttle stumble or lean misfire at highway speeds.
Does not occur during Wide Open Throttle. (WOT)

If that is the case, a lean mis-fire as you describe above was rather common when the vehicle was new.
The design was to create as lean a mixture at all times to meet Fed regulations.
The issue is a carburetor is just not that accurate to give a precise air fuel mixture during all of the modes.

The example might be traveling on a highway and there is a slight grade of hill approaching. Part the way up the hill the engine runs out of power to get over the hill with a little throttle movement. Either a far greater acceleration is needed or the vehicle has to slow down.
If this is the scenario, it was called a high speed lean mis-fire. There was an insufficient rich mixture to support complete combustion during a certain engine loading at speed.

If that is the issue, the fix is to enrichen the mixture. Possible ways to do that:
Very important that EGR is fully working. EGR is only supposed to be working at vehicle speeds of 40 or higher and on a warmed up engine. EGR enrichens the air fuel mixture ever so slightly.
Equally as important that there are zero vacuum leaks. Any even slight vacuum leak will create a leaner mixture. There are a lot of hoses that can be approaching 50 years of age. Any of these plus vacuum devises can leak ever so slightly causing an issue.
Increase float level very slightly. This may richen up all modes. Like using a shot gun to take down a specific target.
Adjust acelerator pump one hole richer. Some of these are adjustable as in the picture above. Some are not.
Adjust both mixture screw 1/4 turn rich. This circuit is not supposed to be active at these speeds. However I have had some success.

I would NOT recommend doing all of these at the same time. The first two yes. Followed by a test drive if you made any changes.
After that, anything that is done, follow it with a test drive to see if there is a difference.
This way you can be sure of what made a difference.

Then post back.

Action

Thanks for the info. I just dropped it off at the sho that has a carb mechanic.

 

[svroman58]

OK thanks for the clarity.

If I might clarify, it is a part throttle stumble or lean misfire at highway speeds.
Does not occur during Wide Open Throttle. (WOT)

If that is the case, a lean mis-fire as you describe above was rather common when the vehicle was new.
The design was to create as lean a mixture at all times to meet Fed regulations.
The issue is a carburetor is just not that accurate to give a precise air fuel mixture during all of the modes.

The example might be traveling on a highway and there is a slight grade of hill approaching. Part the way up the hill the engine runs out of power to get over the hill with a little throttle movement. Either a far greater acceleration is needed or the vehicle has to slow down.
If this is the scenario, it was called a high speed lean mis-fire. There was an insufficient rich mixture to support complete combustion during a certain engine loading at speed.

If that is the issue, the fix is to enrichen the mixture. Possible ways to do that:
Very important that EGR is fully working. EGR is only supposed to be working at vehicle speeds of 40 or higher and on a warmed up engine. EGR enrichens the air fuel mixture ever so slightly.
Equally as important that there are zero vacuum leaks. Any even slight vacuum leak will create a leaner mixture. There are a lot of hoses that can be approaching 50 years of age. Any of these plus vacuum devises can leak ever so slightly causing an issue.
Increase float level very slightly. This may richen up all modes. Like using a shot gun to take down a specific target.
Adjust acelerator pump one hole richer. Some of these are adjustable as in the picture above. Some are not.
Adjust both mixture screw 1/4 turn rich. This circuit is not supposed to be active at these speeds. However I have had some success.

I would NOT recommend doing all of these at the same time. The first two yes. Followed by a test drive if you made any changes.
After that, anything that is done, follow it with a test drive to see if there is a difference.
This way you can be sure of what made a difference.

Then post back.

Action

It seems to happen when it’s 50 degrees or colder outside

 

[Action]

That is meaning full too.
There are a couple of engine vacuum systems that are temperature controlled.
EGR would be one
The blend air door at the end of the air cleaner is another
If the distributor has two vacuum lines at the advance or a PVS valve in the thermostat. Again engine coolant temp sensitive.

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