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Old 01-19-2014, 02:01 PM   #1
justpassinthru
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Default Why Shimming A Fuel Pump Changes Fuel Pressure

I have seen a lot written about this but no explanation of how it accomplishes it.

I am confused with how adding or removing shims to the fuel pump increases or decreases fuel pressure.

It would seem to me that by doing this, it would just move the stroke into a different position in the pump. How would that change the fuel pressure?

For example: Lets say there is 0 clearance or even compression between the push rod and the pump lever when the pump shaft is on the base circle of the cam lobe.
If I were to add a shim between the pump shaft and lever, I cant see how that would increase fuel pressure. Just the diaphragm position would be moved and the stroke would stay the same.

I could see that creating clearance between the pump shaft and pump lever would lower fuel pressure due to reducing the stroke of the pump along with making a noisy pump.

I just donít know how, by doing this you could increase fuel pressure without increasing the stroke.

Please enlighten me.
Bill
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Old 01-19-2014, 02:14 PM   #2
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

Adding a shim would relieve a preload on the diaphram, thereby reducing the volume of the stroke. Volume doesn't readily convert to pressure, however, spring tension does. A spring does not offer consistent pressure throughout its compression, and no two springs are identical in this respect. Original pump springs had better quality control than those available today, and more success may be achieved by switching springs than by adding shims.
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Old 01-19-2014, 02:19 PM   #3
hardtimes
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

Hey Bill,
I do not think that 'shim' is added 'between pump lever/pump shaft.
If you add gasket/spacer to pump base , that could/would reduce pressure. Similarly, if you put a longer or shorter push rod/shaft, it would affect pressure, i.e.- short stroke on pump diaphragm..less pressure; longer stroke (within limits) should increase pressure..no ? With proper push rod (factory) on mech pump, you would get what pressure was designed for. Do not see reason to change that..maybe bandaid for faulty pump spring/ diaphragm,eh ? It want more pressure than stock..put in appropriate elect pump and maybe regulator !

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Old 01-19-2014, 02:59 PM   #4
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

The shim will take the free play out of the rocker arm which is lost motion and with free play you lose pump action. I can fine tune the pump output pressure with an adjustable push rod. I like 4 lbs cold down here where it's warm all year. As the pump gets hot the pressure drops. With this new gas if you keep the pressure up it reduces the chance of vapor lock. In 95 degrees plus the pressure will drop down below 2 lbs. When it get to 1- 1/2 the pressure starts to jump up and down and that is the edge of the engine starting to run ruff. When the engine is shut down the pump will be in the 135 degree range and this causes a full blown vapor lock as the engine and pump temperature rises after shut down and the engine won't start until the pump cools or is cooled with cold water or a plastic bag full of ice. I wouldn't just install shims as to thick will ruin the diaphragm. I know when I adjusted the pushrod I/2 turn it was making a noticeable change. Rebuild the pump to get rid of slop in the rocker arm. This way the pump starts to pump as the cam turns. Warn the rocker arm is moving but not moving the diaphragm so you may only get 1/2 the action of the pump. G.M.
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Old 01-19-2014, 04:03 PM   #5
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

To answer your specific question----the fuel pressure is controlled by the pressure of the diaphragm spring. By compressing the spring to a shorter height it will return the diaphragm with slightly more force. So, the pushrod moves the rocker arm which pulls the diaphragm downward against the spring. With a shim between the pushrod and rocker arm cup the spring will operate at a more compressed height.
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Old 01-19-2014, 04:23 PM   #6
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

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I can the see the logic in that JWL but since the spring is moving only just over 1/4 " the increase in pressure would be so meniscal that it could hardly be measured . A unfettered spring can be compressed up to 2" .The pressure difference from fully expanded to full compression in that case would be a lot more and could be measured .
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Old 01-19-2014, 07:47 PM   #7
Charlie ny
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

Ted,
Miniscule as it may be adding a shim to lengthen the p/r or changing the
fuel pump stand gasket will make a difference. Depending on the components used
to build the pump .010-.015 can quite easily change the pressure by 1 psi or more.
I'm extremely happy for anybody who can build a f/p and the thing tests 'on
the money' the first time.
I know I'm dragging this out but the thickness of the diaphragm, the
machining accuracy of the f/p stem, washer thickness as well as the quality of
the spring and rocker arm can make building a good f/p time consuming.
I caution guys to not think just because pump sucks and pushes when
manually actuated that it is in fact good, not in my shop anyway.
Charlie ny
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Old 01-20-2014, 04:25 AM   #8
flatheadfan
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

I am not sure if this is what you are referring to but, during WWII when gasoline was highly rationed, the practice of "shimming" the fuel pump was somewhat common. In an effort to reduce fuel pump output and thereby limiting gasoline usage extra gaskets were paced between the fuel pump mounting stand and the intake manifold. This reduced the amount of fuel being sent from the fuel pump to the carburetor by reducing the effective push rod travel. Primitive, but it must have worked. I guess, in many respects this was a "backwoods governor." Times were tough......

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Old 01-20-2014, 09:38 AM   #9
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

Quote:
Originally Posted by Charlie ny View Post
Ted,
Miniscule as it may be adding a shim to lengthen the p/r or changing the
fuel pump stand gasket will make a difference. Depending on the components used
to build the pump .010-.015 can quite easily change the pressure by 1 psi or more.
I'm extremely happy for anybody who can build a f/p and the thing tests 'on
the money' the first time.
I know I'm dragging this out but the thickness of the diaphragm, the
machining accuracy of the f/p stem, washer thickness as well as the quality of
the spring and rocker arm can make building a good f/p time consuming.
I caution guys to not think just because pump sucks and pushes when
manually actuated that it is in fact good, not in my shop anyway.
Charlie ny
Charlie, This confirms my above post where I stated 1/2 turn on my adjustable push rod made a noticeable change of pressure. This was a few years ago and I didn't remember or want to be specific on how much I changed length or how much the pressure changed and it seems it may have been a 1/4 turn on the threaded push rod that made a change in pressure. But anyway this shows the effect of "shims". Putting random thickness of shims in the rocker cup is just a WAG (wildassedguess) and with the pressure change on just .015 would be a miracle to get even close to the pressure you wanted. I have seen nuts in the cups as shims. Of course some of these extreme measures are taken in an effort to over come road problems. I have a spring testing machine and will test a few pump springs as soon as I have time. I want to put a slight load on the spring like the pre load when hooked on the diaphragm then compress the spring 1/4" then 3/8" and see if the spring pressures change. The machine has spring height and pressure measurements. The field problem with the variables, stand height, gasket, rocker arm play and push rod height all come into play when the pump gets installed. Did you know Ford had a fuel pump test machine? There is a picture in one of the service bulletins. This would be the thing to have or build one to assure all rebuilt pumps are the same. G.M.
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Old 01-20-2014, 01:33 PM   #10
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

I wasn't going to post this but since the subject has been refreshed and am working on fuel pumps now. I did this test yesterday .The pump spring on the scales was compressed to 1/2 " to get 2.1/2 Lbs ,so if your push rod was say 32" or in Charlie's example .010-.015 short that means if would affect your pump pressure by 0.078125 of a Lb ,and that as I said is miniscule ,I have the A/C manual in front of me and they do not sight the push rod as something that is used to affect pressure .The push rod controls volume only as in .fords38v8 post . I am aware of how the gaskets can affect pump travel ,I had posted extensively on the old Ford Barn of the merits of welding up your push rod or tailor making it to suit your manifold pump stand and because of after market gaskets being thicker etc. .(not sure were the old posts are ).
Also here are some shots of the diaphragms you can see the black one there that came out of a car from the states that as been eaten away by the fuel ,I am having to make these up in Fuel resistant material .(the orange one ) The flex line there has been affected (hardened) also .
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File Type: jpg 01Jan2005_2543.jpg (33.7 KB, 43 views)
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Old 01-20-2014, 02:49 PM   #11
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

The Ford spec for the spring is 6.12 to 7.40 pounds at .625 inches.
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Old 01-20-2014, 03:48 PM   #12
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

Quote:
Originally Posted by FlatheadTed View Post
...The pump spring on the scales was compressed to 1/2 " to get 2.1/2 Lbs....
Ted, With your scale, and with the Ford Spec in mind as posted by JWL (The Ford spec for the spring is 6.12 to 7.40 pounds at .625 inches.), See what you get through the range of a normal pushrod throw. With your "miniscule" and Charlies "one pound or more", it would be good to know which is more accurate.

I can see how a misunderstanding of goals can skew the answers of such a question, being that the throw of the pushrod would be shortened or extended at either the beginning or the end of the spring compression, the former being zero pressure, the latter being full pressure. How to reconcile for a clear answer is the burning question!
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Old 01-20-2014, 05:20 PM   #13
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

To work out your question you would need the height of the spring in the pump under compression divide that by 625 then work out the variation over the stroke range of the pump just over 1/4 or 3/10 "travel ,What I am saying is if a stock pumps pumps at 2.1/2 Ib and it takes 2min to fill the carburettor, then you shorten the rod by say 1/8"It then might take 4 Min to fill the carb but at that point it should reach the 2.1/2 Ib
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Old 01-20-2014, 08:19 PM   #14
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

Quote:
Originally Posted by FlatheadTed View Post
To work out your question you would need the height of the spring in the pump under compression divide that by 625 then work out the variation over the stroke range of the pump just over 1/4 or 3/10 "travel ,What I am saying is if a stock pumps pumps at 2.1/2 Ib and it takes 2min to fill the carburettor, then you shorten the rod by say 1/8"It then might take 4 Min to fill the carb but at that point it should reach the 2.1/2 Ib
And at this point, as the pressure of the fuel held in the line and above the diaphragm is at 2.1/2 lb and the diaphragm is squatted down on the spring, the pushrod and rocker arm are still moving full stroke and following the cam eccentric but now they are moving against the rocker arm spring and the diaphragm is not moving, only holding that pocket of fuel against the float valve until the float drops a bit. Under normal driving conditions, the diaphragm only moves a few thousandths, under a hard pull it will stroke perhaps 1/8". The only time it moves at full stroke is when priming, and should prime within 20 seconds. At least, so says the Victor Page book. ..B.
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Old 01-20-2014, 08:53 PM   #15
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

Barlea, That makes a lot of sense, and serves to illustrate why pressure is not dependent on volume and vice versa.

Another thought on Ted's spring on a scale... I wondered how JWL's "Ford Spec" could be so different, and realized that the 2 1/2 lbs is PSI, while Ted's scale measures Dry Weight, so apparently it takes 6 to 7 lbs pressure on the spring to produce 2 1/2 lbs PSI on the face of the diaphram.
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Old 01-20-2014, 10:42 PM   #16
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

Barlea has nailed it .the push rod really only delivers the fuel to the pump ,the rebound of the spring delivers it to the float chamber, Correct me if I am wrong but JWLs specs show a compressed spring down to ,625 almost its maximum therefor at maximum pressure !.My 1/2 is only a partial compression ,but as you say Alan its against the diaphragm so it needs to be converted to liquid pressure .You would also need to take into consideration the small spring working against the push rod .GM gets a variation with his adjustable rod ? ,
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Old 01-21-2014, 10:34 AM   #17
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

This is my approach to the question, While all you are discussing the pressure developed from the spring you forget about the condition of the diaphragm.This is a displacement issue problem created by a deteriorated pump diaphragm has nothing to do with the spring or pump stroke distance.

The reason you have to add additional length to the fuel pump push rod to make it function correctly is due to the stretch of the diaphragm itself. As the diaphragm deteriorates with age and fuel exposure it stretches and produces less displacement of fuel. By increasing the stroke of the pushrod you are able to flex the diaphragm an additional amount thus increasing the fuel pumped volume and upping the pump push pressure. But this doesnot cure the defective diaphragm issue. You need a diaphragm that is flat not looking like a toadstool.

Adding additional length to the push rod or shimming the spring is a stop gap measure when the problem is something else.
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Old 01-21-2014, 10:46 AM   #18
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

Quote:
Originally Posted by FlatheadTed View Post
Barlea has nailed it .the push rod really only delivers the fuel to the pump ,the rebound of the spring delivers it to the float chamber, Correct me if I am wrong but JWLs specs show a compressed spring down to ,625 almost its maximum therefor at maximum pressure !.My 1/2 is only a partial compression ,but as you say Alan its against the diaphragm so it needs to be converted to liquid pressure .You would also need to take into consideration the small spring working against the push rod .GM gets a variation with his adjustable rod ? ,
Don't forget with a Stromberg the needle valve has a restriction hole of .098. Only so much fuel can flow through that hole. To vary the amount of fuel flow you have to increase or decrease the pressure. The amount of fuel to the carb fuel bowl is determined by intermittent opening and closings of he needle valve. The more fuel used the faster the needle valve moves to try and maintain the fuel level. Needle valves have a little delayed action and the float level can vary 1/2". This can be observed running a 97 carb with the top off or with the Ford KR Wilson carb sight glass The double ball type needle valve supplied by Uncle Max reacts so fast you can hardly see any variation in fuel level. This is where a slight increase in fuel pressure will increase the flow through the .098 hole if more fuel is required to maintain a constant level of fuel in the bowl at HIGH speeds. Too high of fuel pressure will cause the fuel pressure to over come the upward pressure from the float and allow gas to push through the needle valve. A measurement of the upward float pressure on the needle valve would be good to determine the REAL fuel pressure required. It could be float size may need to be a little larger?? G.M.
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Old 01-22-2014, 03:55 AM   #19
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Default Re: Why Shimming A Fuel Pump Changes Fuel Pressure

Its been done to death this subject but one of the things I have found over the years of doing many of these pumps is the ware at this point,(arrow ) there is just a small contact point ,this is one of the causes why you shim the rod .You can see the shim on the left from the push rod .
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