My truck seems to bog right off idle. It feels like a miss in the engine, but have narrowed it down to the accelerator pump.
I unhooked the linkage to the pump and then tried the truck, the bog is gone!
Then I just tried pushing the button on the front of the carb (accelerator pump button) and the engine almost dies.

What is going on here? Will a new accelerator pump (the assembly on the front of the carb) fix this?

Is there any harm in leaving it unhooked?
The performance is good with it unhooked, so whats the point of the pump?

Matt
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1978 F250 4x4 - 351M
Don't crush them, restore them!

 

because gasoline is denser and starts moving slower than air when you step on the gas pedal you get a rush of air and the gas is still trying to gain speed. Hence the accelerator pump provides a shot of gas to keep the engine rich enough to not backfire until the fuel can pick up speed to keep the mixture correct.

 

Ok, that makes sense. But why would my accelerator pump cause the sucking air?

Can it be replaced? Will a new one help?
thanks!

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1978 F250 4x4 - 351M
Don't crush them, restore them!

 

I have no idea, I fear how bad my engine would backfire if I ran without one....

 

What kind of carb is it? You can have a situation where too much fuel is added when punching the throttle.

 

Bad diaphragm (small holes develop from age) would cause a vac leak when its actuated. Am surprised that bog is gone without it unless you compensated by enriching the idle circuit when trying to located the problem. Enrichment will lower your gas mileage. Good idea to fix pump diaphragm, might need to accelerate quickly sometime and not be able to. Tom

 

Changing the diaphram is real easy, just 4 screws! If you have the same experience as I had it can get expensive. The parts store I tried told me I had to buy a complete carb kit. I have since found that in the aisle of Canadian Tire is a section of "HELP!" products and they had a diaphram for just a couple of bucks. If we have it here in Canada, I am certain you would have the equivalent there.

Bob

 

It is the ol' motorcraft 2150 carb. The mixture is supposedly set right, just a while ago too!

Well, I live 10 minutes from the local canadian tire store, so maybe I will go check it out!
What part of canada are you from 69cyclone?



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1978 F250 4x4 - 351M
Don't crush them, restore them!

 

[font size="1" color="#FF0000"]LAST EDITED ON 26-Jun-01 AT 11:34 PM (EST)[/font][p]I went to canadian tire today, however, they had no stock on the diaphragm nor could they look it up on the computer, help 69cyclone, where'd you find yours??!

I went to the biggest one I could find!!

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1978 F250 4x4 - 351M
Don't crush them, restore them!

 

I have seen the accelorator pump diaphram in the performance isle by the carbs in checker auto parts. Eric

 

I picked up a diapraghm today at napa, hopefully it fixes the problem, so I can go after other gremlins!

Matt

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1978 F250 4x4 - 351M
Don't crush them, restore them!

 

weldman

I am sorry I did respnd quickly but I was not online for a couple of days. I am from Parksville on Vancouver Island BC.
I have a Holley on my trucks 2 F350s. I know that the items supplied in the HELP section of Canadian Tire seems to change. I will have to look and see if they still stock it in the local store (perhaps they don't). I know that they seem to want to stay with more "modern" equipment and thus are decreasing want they carry for older carb vehicles. Sorry the lead didn't work out for you. I hope that that your part from NAPA helps to fix your problem.

Bob.

 

I'm from near Toronto. The part did help a bit.

I seem to still have the bog sometimes, which is accompanied by a stiff pedal. Is this merely the linkage?
What pieces should I target?

As well, pushing the accelerator pump button by itself still stalls the engine. Is this normal?

Thanks!

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1978 F250 4x4 - 351M
Don't crush them, restore them!

 

A stiff pedal does sound like a linkage problem. As you say that there is also some bog at the same time, I would suspect some binding of the linkage on the carb itself. This means likely either some part of the choke linkage or perhaps the linkage to the accelerator pump itself. As for the the pump stalling out the engine, this doesn't surprise me, but you may have it on a setting that is too rich. Usually there is a couple of holes to hook the linkage into and this will change the amount of fuel added with each pump. When you step on the gas more air enters that carb as the throttle plates open. The fuel lags behind a bit and so to compensate the accelerator pump puts an added squirt of fuel in. If you add the fuel without adding air, ie push the accelerator pump by itself, you are making the mixture very rich and this potentially could cause the engine to stall. The trick is to get the pump set up so that it puts just the correct amount of fuel when you step on it so that it accelerates smoothly. Not enough fuel and the mixture gets lean and it bogs, too much fuel and it will be too rich and it will sputter. Of course, if your distributor is not advancing correctly you can also have poor acceleration as the richer mixture needs a bit more time to burn and thus needs to have the spark advanced. Just a thought on another area to take a look at.

Bob

 

I have the exact same problem. However the "HELP" section diaphram that I found did not work. The gasket was too small.
I have taken the old diaphram and cut out the gasket and am going to put that in before the new one to give me a thicker depth. Right now it's leaking like a crazy.

Any other suggestions would be appreciated.

Eric