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Fuel Pressure Regulator...??? Help |
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BrotherBamc 
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Joined: Jan/22/2009 Location: Lewiston Status: Offline Points: 2248 |
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 Topic: Fuel Pressure Regulator...??? HelpPosted: Sep/11/2021 at 6:52pm |
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I need one, (I think).
This is what has been happening. Went to a small show and shine about 22 miles away. When at cruising speeds, the motor feels hesitant at cruising speeds. Feels as if there is a miss, but I don't think that it's a miss. feels more like carb issues. When you put your foot into it, it goes like stink. But, as soon as you let off the gas pedal a little bit to cruise the hesitation starts. If I cruise at about 60 mph and keep the rpms at about 2,000 there's no hesitation. Motor purrs like it should. I've also noticed that the fuel pressure gauge, when cold pumps at about 8-9 psi. Which is a little bit too much for the Holley 4150. Holley states that fuel pressure should be around 6-7 psi. I've checked float level with the car running and with the Comp 280H cam, it makes the motor bounce around so much that fuel does spill out of the site plug. Shut the motor down and fuel level is right at the bottom of the site plug. Rock the car a little and fuel does spill out as it should. I've always thought that the motor does run a little rich. Plugs don't have the light grey color but they aren't black either. Fuel pump is a RobbMc 550 HP with 3/8" or 6an lines. Carb is a Holley 750 dp 4150. I've checked power valve and it's the proper 6.5 and the jets are the proper jets 72's. Primary side. Secondary side has power valve plug and I believe 74's on the jets. So, this is my guess.. I need a pressure regulator..?? Help would greatly be appreciated.. Thanks..!!
William |
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troutwilly
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Joined: Sep/14/2007 Location: Carriere, MS Status: Offline Points: 992 |
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Posted: Sep/11/2021 at 9:59pm |
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What does your fuel pressure do as you move up through the rpms? The RobbMC pump is rated at 7 psi +/- 1. So if your showing 8-9 your not that far out of spec, provided your gauge is accurate. That said, it sounds like your carburetor is fine. The float bowls aren't overflowing. But I don't mess with much on carbs other than air/fuel mixture, choke, and idle speed. That brings me to ignition. Have you watched your timing mark as you bring the engine up from idle to 2,000 rpm? Is it smooth or jumpy? If jumpy I would check the distributor advance mechanisms, its bearings, and maybe even the distributor gear. If smooth I would move the timing light inductor from wire to wire and make sure each plug is firing consistently. If not check cap, rotor, wires and plugs. |
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Bill O.
70 AMX |
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BrotherBamc
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Posted: Sep/11/2021 at 10:22pm |
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Thanks for the reply...
Just had dizzy out the other day and gears look fine. I should add that dizzy is a unilite and there is a MSD 6AL ignition box. Just had all the plugs out and cleaned. Cap is good and so is rotor and wires. Timing is smooth also. It's also hard starting when cold. Runs kinda rough but once it warms up for 45 to 60 seconds or so it idles just fine. Fuel pressure gauge is kind of a piece of junk. It dances pretty good now and when you give it gas it kinda holds at about 9 psi. Still jumps around tho. My buddy is thinking it might be in the carb. Somewhere..  |
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BDCVG
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Joined: Dec/19/2007 Location: Endless Mtns. Status: Offline Points: 993 |
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Posted: Sep/12/2021 at 7:10am |
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Put a vacuum guage on it. Should be steady at 16" or higher. A leak can cause surge at cruise but the DP floods so much gas in when You floor it You can't feel it.
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1970 AMX 390 5 spd full Control Freaks front and rear suspension
2014 E63 AMG-S wagon 1965 Austin-Healey 3000 MK III |
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Ken_Parkman
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Joined: Jun/04/2009 Location: Ontario Status: Offline Points: 1814 |
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Posted: Sep/12/2021 at 7:58am |
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Often surging under light throttle cruise is too much timing; often a result of a performance mechanical advance curve and added conventional vacuum advance, also possibly full vacuum to the distributor instead of ported.
This is assuming you are using vacuum advance. If so, pull off the vacuum advance hose and go for a cruise to see if that helps. Dunno the Unilite but it should have an adjustable vacuum advance; if it does just knock some advance out if that is the problem. Or try ported instead of full vacuum.
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BrotherBamc
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Posted: Sep/12/2021 at 10:11am |
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Thanks Guys..
Got a few leads now and a few things to try out. One is the power valve might be too small. I have about 10-12 inches of vacuum at idle and should be running about 9.5 power valve or even a 10.5. I'll have to order those. I do have a vacuum gauge in the car, and I never thought to look at it when the hesitation starts.  I'll take some timing out and plug the vacuum advance, see what happens later on today. One thing that I wish I had was a fuel pressure gauge in the car or where I can see it. Keep you all posted on what happens. |
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tufcj
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Posted: Sep/12/2021 at 11:52am |
The mechanical advance in a unilite is below the module. There are 3 set screws holding the plate. My mechanical advance was really sticky after years of use. Check it, disassemble and lightly lube it, and see if your surge goes away. Mine had the same symptoms. I now re-lube it every couple of years as part of winter maintenance. Bob tufcj
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69 AMX
74 Javelin AMX 67 Rogue If you need a tool and don't buy it... you'll eventually pay for it... and not have it. Henry Ford |
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BrotherBamc
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Posted: Sep/12/2021 at 11:58am |
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Thanks Bob..
Will check this afternoon along with a few other things.. Ordered a couple power valves also. A 9.5 and a 10.5
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BassBoat
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Posted: Sep/12/2021 at 1:17pm |
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The symptoms you describe are the definitive ones from too much vacuum advance. If you have a vacuum advance, disconnect it at least temporarily and see if the surging doesn't go away. In my experience a 6.5 power valve works for almost everything.
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PHAT69AMX
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Posted: Sep/12/2021 at 1:32pm |
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And yes, get and run a fuel pressure regulator regardless, what's it's gonna hurt, it eliminates a possible or probable variable, it can only help or prevent, 5 psi is plenty. Imho at 2,000 rpm and below most likely it is running on the idle circuit & transfer slots and the Boosters and main circuit is not even delivering fuel so main jets, power valve, and main air bleeds are not even coming into play, other than stepping into it to accelerate beyond the 2,000 rpm mark... As fuel pressure increases, the fuel level in the fuel bowl rises as it is more difficult for the float to close the needle against the higher pressure. With a float set to work with a higher fuel pressure, then the float has less drop to allow opening the needle during high demand situations. There are basically no benefits to higher fuel pressure at these horsepower levels. one half pound of fuel per hour per horsepower, so would imagine we are at about 200 lbs per hour fuel requirement. Do the math on 2 inlet needles of x.xxx diameter at 5 psi to see how many lbs per fuel they deliver. Depending on age of the carb may want to get some size 008 O-Rings to replace on the inlet needles. Peace
Edited by PHAT69AMX - Sep/12/2021 at 1:47pm |
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BrotherBamc wrote: