I have a 283 sbc with 3 Holley 94s. To reduce fuel pressure from the new mechanical fuel pump, currently 9 lbs, (should be 6lbs according to specs), can I shorten the fuel pump push rod and hence reduce the fuel pump push rod travel to reduce the pressure to suit the 94s. Has any one tried this? How much travel does the push rod have? Or as an alternative can I use a bypass regulator and run a return line not back to tank but to a tee fitting into delivery line from tank, placed before the fuel pump. This second idea was suggested to me by some one but I don't believe that it will work as the pump will effectively be working against itself. Many thanks for any replies.
i don't think that shortening the push rod will work if you want 6 PSI why not use one that puts out that pressure? try one for a `66 Chevy 283....5 to 6-1/2 PSI . you will have to change some of your plumbing http://www.napaonline.com/Catalog/C...p-OE-Mechanical-Type-/_/R-NFPM3120_0417965217
or you could try this one , the plumbing may not need to change http://www.napaonline.com/Catalog/C...p-OE-Mechanical-Type-/_/R-AFP40725_0252760767
Thanks, that is the pump I have . It should do 6 lb but is doing 9lb, I need to get down to 2-3 lbs for. Holley 94 carbs
well yes, if you really want 2-3 lbs you need a regulator. get a good one , not one of those cheap dial ones Mr Gasket sells fyi.... i have the second one i posted on both of my hot rods and they put out around 6 lbs
Trying not to make my questions too long as I am a one finger typist. But a little further info. I have tried a Holley regulator and was ok with the old stuffed fuel pump but unfortunately that was putting fuel into the sump. So a new fuel pump and the Holley regulator played up - to much fuel pressure perhaps. So installed a new $300 fuelab regulator which was supposed to be a blocking regulator but was a bypass one so that needs a return line to tank to make it work properly but as car is all but finished it is impractical to do a return line. Can't get a low enough psi blocking regulator here so hence the original questions. If using a regulator can I run a return line to delivery line from tank and plumb to a tee fitting just before pump or could I do with out regulator and shorten the fuel pump push rod travel to bring pressure down, which would be trial and error to get it right. Thanks. Clemens.
Yes I had thought of that, would the angles create a problem, as push rod is about 45 degrees and shimming it would be at .horizontal if you get what I mean. Would it bind? Does any one know the typical amount of travel, although I will measure mine if I go down this path. I know that the flatheads used thicker gaskets to adjust pressure so that is where this idea came from. Thanks guys.
If my research and understanding are correct. A shorter push rod will only reduce the amount of fuel drawn in from the tank, lowering your flow rate. It will not change the discharge pressure. Like Hoop98 said, discharge pressure is controlled by the diaphragm return spring, that is what pushes the fuel out to the carb. You would need a weaker return spring to lower the pressure.
I am using the Holley low pressure regulator with my 3x2 step up and have no fuel problems. I did have to readjust the float level on the front carb as it was weeping gas. I keep the pressure set at a constant six pounds with no problems.
Sounds like it would be easier to adapt a flathead pump to my sbc . I'm determined to stick with my 94s. - actually 3 x 1&1/16 as I've spent at least 40 hours reading and learning about them here on the hamb, and are now confident to pull them apart and tweak or modify them as necessary.
I think the bypass back to the pump inlet would work fine. Sounds like you have everything to do that already. If the pump inlet check valve is closed, fuel will just back feed toward the tank. The pressure in the bypass line is lower than the pressure exerted by the diaphragm spring so that pressure should hold the inlet check closed until the push rod pulls the diaphragm again overcoming the return spring pressure.
Stone bird, A big thanks, I have discussed this with a few guys over here and not got definitive answers, what you say is what I'm trying to figure out.
This might help, from howacarworks.com How a mechanical pump works A mechanical fuel pump is driven by the camshaft, or by a special shaft driven by the crankshaft. As the shaft turns, a cam passes under a pivoted lever and forces it up at one end. The other end of the lever, which is linked loosely to a rubber diaphragm forming the floor of a chamber in the pump, goes down and pulls the diaphragm with it. When the lever pulls the diaphragm down, it creates suction that draws fuel along the fuel pipe into the pump through a one-way valve. As the revolving cam turns further, so that it no longer presses on the lever, the lever is moved back by a return spring, relaxing its pull on the diaphragm. The loosely linked lever does not push the diaphragm up, but there is a return spring that pushes against it. <figure class="small"> <legend>Types of fuel pumps</legend> </figure> The diaphragm can move up only by expelling petrol from the chamber. The petrol cannot go back through the first one-way valve, so it goes out through another one leading to the carburettor. The carburettor admits petrol only as it needs it, through the needle valve in its float chamber. While the carburettor is full and the needle valve is closed, no petrol leaves the pump. The diaphragm stays down, and the lever idles up and down. When the carburettor accepts more petrol, the return spring pushes the diaphragm up and, by taking up the slack in the loose linkage, brings it back into contact with the lever, which again pulls it down to refill the pump chamber.
Clem, Quick Fuel makes a 1 - 4# adjustable fuel regulator, the part number is #30-804. Here is a link to the regulator: http://www.quickfueltechnology.com/low-pressure-regulator.html If it's not available in your area, can you buy it off Ebay? This should resolve your issues. Glenn
Try what you are talking about (tee into the feed line) doesn't seem like too hard to modify or much money If it works, great,let us know. If not, try a different regulator or pump
I had the same problem on my new fuel pump (9lbs, needed 4-5 max) I ran a bypass like you mentioned. I bronzed a length of 3/16 brake into the supply line just before the carb, following the main line (steel pipe) back down and over to the inlet side of the pump (joined at this end with a short flexible line, to a short piece of brake pipe, spudded into the pipe from the tank. To get the fuel pressure down, I hooked up a gauge to the carby end, ran the motor and at this stage the pressure was low, I squeezed the return line with side cutters till the pressure read 4.5 lbs.. I checked the pressure a couple of years later, and it had dropped slightly (probably due to wear of the pump spring) so just re-adjusted the pipe a bit.. 12 years later and still no problems..
I would do this and dump the excess back to the inlet of the pump, you're not going to bypass much volume to reduce the pressure to the level you need and as the pump wears you'll bypass less. a backpressure type regulator off the fuel block or at the end of the line supplying all 3 carbs will ensure enough volume to all 3 carbs.
Like many have said, bypass the fuel back to the inlet side of the pump. Dial in 2-3 pounds with the regulator, and you'll be good to go.
Ajustable fuel pressure with a gauge after so you can keep as much pressure off the float valve seat. I have one on both of my cars and they work great!
No need to run a return line with the Quick Fuel 30-804. It is not a bypass type regulator. Easy fix to your problem.
WOW - I'm amazed at the response to my questions. Thanks to all that answered. For now I will try the return line before fuel pump, with the fuelab regulator that I already have, even if I just run it with temporary hoses to see if it works. Will let you know how I get on - could take a few weeks though at my slow pace. Can't seem to find a local supplier for the Quick fuel regulator, but I notice that it not only looks the same as the Holley but both versions share relevant part numbers ,Holley 1-4 psi 12-804, Quickfuel 1-4 psi 30-804 : Holley 4&1/2 - 9 psi 12-803, Quickfuel 4&1/2 - 9 psi 30-803. From what I've read on the Hamb about the Holleys 50% of users like them and 50% tend to dislike them. I haven't yet researched the Quickfuel though. Anyway thanks again for all your great help. I will now try to attach a pic of the car that I'm working on. Clemens.
Update - I have run the return line from regulator and tee'd it back into the fuel line from tank, before fuel pump. So far for trial runs it works great. Thanks for all the advice, to all that gave it, - much appreciated !
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