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Dick Fickey Member
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Help. I screwed up on both my cars when I installed the fuel tanks. I noticed this when I happened to look at a fuel system diagram. I ran the line from the T to the pressure relief valve and the other line from the tank out to the bottom of the floor. I guess I assumed the pressure relief valve was supposed to allow pressure to build up for the carbs and then allow the fuel to circulate back to the tank. I did not have an anti-siphon valve in the line. I have been running the cars with no problem. With the lines hooked up as the diagram shows and no anti-siphon valve is there enough pressure for the carbs? It appears that the fuel will just continue to circulate back to the tank and not build up any pressure. It looks like I should pull the tanks and correct my screw up. |
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Tim Murphy Member 
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As I could never figure out that entangle mess of fuel lines by the tank and pump, I have no idea of what you said. I just hooked mine up so it made sense and worked. If you have been running the cars with no problem everything should be OK. If it ain't broke, don't fix it. |
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Esprit2 Member
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The simplest way to know if enough fuel is reaching the carbs is to check the fuel pressure at the carb inlet. The pump's outlet pressure is ~3.8 psi, but with flow losses and the carb's higher elevation above the pump, the carb inlet pressure should be 1.5 - 2.5 psi. The carbs can probably handle 3.0 psi, but excessively high pressure can result in an overall rich condition that cannot be adjusted out. Get the pressure right before attempting any carb tuning. Back at the tank, the fuel return line should include an inline flow restrictor that has a (?) 0.046" / 1.17mm diameter through hole. That works like a 'dumb' pressure regulator. The hole size is selected specifically for the J-H, it's stock fuel plumbing, and the carbs height above the fuel pump's output. Any changes to the system, or crud that partially plugs the bleed orifice, and the pressure at the carbs will be too high. If the bleed orifice is omitted, then there is an un-restricted, large diameter fuel bypass back to the tank, and the resulting fuel pressure at the carbs might be too low. You can drive yourself nuts trying to diagnose the fuel system, or just check the fuel pressure at the car inlets. If it's right, relax. If it's too low or too high, then jump back into the diagnosis. Without an anti-drainback valve, fuel in the lines will drain back to the tank when the pump is switched off. On re-start, it will take longer for the pump to re-fill and pressurize the fuel lines, but once done, the engine should run the same with or without the anti-drainback valve. The valve is more of a convenience item than a 'must have to work' item. It's best to have one, located back close to the tank end of the plumbing Regards, Tim Engel |
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Esprit2 Member
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The simplest way to know if enough fuel is reaching the carbs is to check the fuel pressure at the carb inlet. The pump's outlet pressure is ~3.8 psi, but with flow losses and the carb's higher elevation above the pump, the carb inlet pressure should be 1.5 - 2.5 psi. The carbs can probably handle 3.0 psi, but excessively high pressure can result in an overall rich condition that cannot be adjusted out. Get the pressure right before attempting any carb tuning. Back at the tank, the fuel return line should include an inline flow restrictor that has a (?) 0.046" / 1.17mm diameter through hole. That works like a 'dumb' pressure regulator. The hole size is selected specifically for the J-H, it's stock fuel plumbing, and the carbs height above the fuel pump's output. Any changes to the system, or crud that partially plugs the bleed orifice, and the pressure at the carbs will be too high. If the bleed orifice is omitted, then there is an un-restricted, large diameter fuel bypass back to the tank, and the resulting fuel pressure at the carbs might be too low. You can drive yourself nuts trying to diagnose the fuel system, or just check the fuel pressure at the car inlets. If it's right, relax. If it's too low or too high, then jump back into the diagnosis. Without an anti-drainback valve, fuel in the lines will drain back to the tank when the pump is switched off. On re-start, it will take longer for the pump to re-fill and pressurize the fuel lines, but once done, the engine should run the same with or without the anti-drainback valve. The valve is more of a convenience item than a 'must have to work' item. It's best to have one, located back close to the tank end of the plumbing Regards, Tim Engel |
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Esprit2 Member
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Sorry about the double post. I'm not sure how that happened. I can't figure out how to delete this one. Would Greg or a moderator please delete this message. Thanks, Tim Last edited on 10-07-2018 03:37 pm by Esprit2 |
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Dick Fickey Member
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Thanks for the reply's. Now to figure out how to make an in line restrictor. I assume they are not available. Any ideas? |
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Dick Fickey Member
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The more research I do on the tank pluming the more confused I get. Does the restrictor go in the return line from the pump to the tank or the line from the tank to the charcoal canister? Trying to understand this system before I put the tank back in one car and tear the tank out of the other one. |
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Tom Bradley Member
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The restrictor goes in the line from the output of the fuel pump going back to the top of the fuel tank. You can make one by getting an aluminum rod which will fit tightly inside the fuel line and drilling a 3/64" hole through it. The rod only needs to be about 1/2" long, if that. Personally I replaced it with an anti-backflow valve that was good for gasoline from the my local auto parts store. It has been working OK. Keep in mind that the fuel system needs to do 2 things. First it needs to have enough flow rate to keep up with the gas flow at maximum engine speed. Next it needs to limit the maximum pressure so that it does not force fuel past the shutoff valves in the carbs when they are closed. I have used both Facet and Mr Gasket pumps with 4PSI maximum pressure rating with no problems. I think the modern pumps and carb shut-off valves are probably better behaved than the ones that went in originally back in the 70's. |
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Esprit2 Member
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The attached photo shows the fuel restrictor... for what that's worth. In effect, the restrictor is just a 'dumb', non-adjustable pressure regulator. So an alternative is to install an inline, adjustable fuel pressure regulator. Just delete the T-fitting in the fuel pump outlet hose to the carbs, and its return hose to the tank. That return line is where the restrictor would normally be. Then install the adjustable pressure regulator as close to the carb inlets as practical, and set it to 2.0 to 2.5 psi. That's what I run in my Britcars that have electric pumps. The fuel pump is usually mounted lower than the carbs, and way in the back of the car. With 4 psi at the pump outlet, flow loss plus some drop due to elevaton change will normally result is something less than 4 psi at the carb inlets. When diagnosing fuel system problems, measure the pressure at the carb inlets. That's what counts. As long as the carb's floats and inlet shut-off valve are working well, a slightly high pressure shouldn't be a problem. However, as they age and become less effective, excess pressure over 2.5 psi can result in an overall rich condition. Regards, Tim Engel Attachment: Fuel System - Return Line Restrictor 0.046 Inch ID.jpg (Downloaded 124 times) Last edited on 10-07-2018 03:58 pm by Esprit2 |
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Dick Fickey Member
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Tim Thanks for the reply and Picture. I looked and I actually found one in my parts. Did not know what is was before. spoke to Mike at Delta and he gave me a great lesson on JH fuel systems. Difference between years and between US and UK. He recommended deleting the T fitting as you said and run straight to the carbs with no fuel pressure regulator. will see if this works out. If not simple to install a regulator without tearing anything apart. He also said to plug the return hose to the tank. Packed with all of this knowledge I will put the tank back in and change the lines in the other car. |
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Tim Murphy Member
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Dick Fickey said in his first post: "It looks like I should pull the tanks and correct my screw up." Tim Murphy: I am glad you found a solution that was easy and practical. Sometime we JH owners overcomplicate things. The engineers at Jensen made many mistakes for many reasons. There is no sense for us today to duplicate those mistakes. Take care and enjoy your JH. |
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