Hello I have a 1967 Pontiac Firebird with a 400 motor. For the life of me I cant figure this out. I have a Edelbrock 560 cfm carburetor with a Holley mighty might electric fuel pump 4-7 psi range. I have a Edelbrock fuel pressure regulator with a return back to the tank along with a vented gas cap.
OK here is the problem when I start the car the fuel pressure on both the regulator and the one on the carburetor fuel rail going to the carburetor is a 4PSI. But after driving the car after maybe 30 minutes BOTH gauges read down to 0 PSI including at idle. The car runs fine it not overheating no surging no stalling. I looked at the plugs very carbon fouled and i was using AC Delco irredieum plugs.
I initially though that the fuel pump was too big for this car it had a Mallory 4140 fuel pump so I bough a Holley mighty might fuel pump still having same issues as listed above. Then I bought a Edelbrock return type regulator due to the fact the regulator I had on there was a deadhead type regulator (no Return Port) still having the same problem. Then I rerouted the fuel lines along the frame rail away from exhaust with 3/8 inch rubber lines and also dropped the fuel tank and bought a 1969 fuel sender that has the supply and vent line attached to it. Be advised that my fuel tank has a sump welded on the bottom of it and goes from the bottom of the fuel tank to a fuel filter then to the fuel pump then to the regulator and the return fuel line goes back to the fuel tank via the original 69 fuel sending unit and i did cut the fuel supply screen mesh screen off so it does not have any restrictions when the fuel return back to the tank. the other line with is the vent line is capped. I tried to take the gas cap off and see if the pressure return to normal after a few minutes but no avail.
I keep thinking I am having a c aviation issue but the Holley fuel pump is small and the vented cap should be sufficient for venting. so my question is this normal for the pressure to drop to zero on both my gauges or should they remain a 4 psi all the time while the car is running. I also check with I open the throttle I am getting plenty of gas from both the primary and secondary jets. Any suggestions please.
the ground is to the bottom of the car near the fuel tank straps. as far as wiring new wire harness along with fuel pump relay next to the fuel pump. will put a meter to the 12 volt theory going to the relay just to make sure.
Do I read this correctly that the gauges are registering 0 psi, but the car remains running fine?
Regarding the wet plugs, have you adjusted the mixture screws on the front of the carb? There are some videos on the Edlebrock web site that will help you with that. IIRC, they both go in snug and then back out 1.5 turns as a starting point. Not to throw any more parts at the problem, but a 560 cfm for a 400 engine seems a little on the small size; did you mean to say a 650 cfm? Cal
yes the car runs fine both gauges are at 0 psi. no surging no idle problems im going to move the grounds directly to the negative terminal of the battery to see if this helps as far as a possible grounding problem per big chief i am getting 11.6 volts at the relay and fuel pump both are mounted near the gas tank. . as far as the plugs I did adjust the idle mixture screws i watched the Edelbrock videos.
Are both gauges fluid-filled mechanical gauges and located under the hood? If so, the dampening fluid is expanding due to the heat under the hood. The gauges are sealed, so the expanding fluid builds pressure which "pushes back" against the fuel pressure and the gauge reads lower - even 0 if the dampening fluid pressure is greater than the fuel pressure.
I took out the rubber stopper and drained the fluid from my gauge and it reads correctly at all temperatures now.
Agree with Pete. Most likely the fluid filled gauge reading wrong. Mine reads 6.5 cold and goes down to around 3-4 after warmed. But you should have 6-7 pounds on nitial startup. 4 doesn't sound like enough even cold. If that is truly all you're going to get from that pump, at the carb, it sounds like another pump is needed.
Typically the low pressure FP's, [4-7psi] don't require a pressure regulator. With pressure fluctuations at temperature, Holly recommends a by-pass regulator, [p/n: 12-803BP]. Is this the one you have? The regulator should be set to 7 psi, 4psi is too low. Also, were is the fuel filter mounted and what micron is it? To fine a filter on the input side of the pump may restrict flow. A 100 micron filter may be used on the input and then a 40 micron before the carb.
re: black/wet plugs; this may be caused by a leaky needle valve in the carb.
Thanks for the replies I have both liquid filled gauges on my car and both are located under the hood. The fuel filter is mounted just above the gas tank just before the fuel pump which is also located just above the gas tank. and the pressure at startup when cold is about 7-9 PSI then drops when the car is warm. never thought about liquid pressure buildup in the gauges. I called Edelbrock about that and they said it should not matter about liquid filled or dry type gauges and they Edelbrock stated that my type of carburetor with vacuum secondary should not have more than 5.5 PSI going to it at operating temperatures. But Firebob mine does what you stated in the forum listed above. I also get 13.5 volts now at the fuel pump relay and at the fuel pump. I also removed the SUMMIT Racing -8 100 micron fuel filter summit part number SUM-230118R it had some minor trash in it but did not put it back on the vehicle . and the fuel pressure is exactly what Firebob described it about 7-9 PSI at cold start and now its at 4 psi when at normal temperature. im going to try to reinstall the fuel filter Sunday and see if I get the same results and previously listed [7-9] PSI at startup the [3-4] PSI at operating temperatures. In closing why have liquid filled gauges if they are prone to this types of problems I guess live and learn next time I am getting dry gauges. whats the advantage of liquid filled gauges compared to dry gauges. .
Only thing I can see they would be for is maybe a little more shock resistant. I wouldn't sweat it. If your car is still running you must have fuel pressure. The more ugly issue is when the car does a nose dive at the top of the gear because of a lean condition. Then you have a fuel delivery issue and more work is in your future.
Thanks Firebob I did go out today it was sunny and 82 degrees pretty humid here in florida today. I have not installed the summit racing fuel filter but i did test drive my car today from stop and go conditions wide open throttle and cruising on the highway at wide open throttle no issues at the top of the gear. what I am going to do Sunday is drain the liquid from both gauges and install the fuel filter. and test drive again and see if I have any problems at top of gear situations. In closing for educational purpose whats type of liquid is in the fuel gauges is it mineral oil. thanks for you all help.
Truth be told I really only experience the flat line at about the 1000ft mark at the dragstrip. Felt like the car just ran out of gas. Didn't stall or anything but just kinda gave up. Other people have had the same type of issue in the top of lower gears but that could be caused from not having enough fuel in the tank and the pickup becoming uncovered or just a small fuel bowl on the Qjet going empty. I did also run into lean conditions while on the dyno so I know it's real. I currently run two pumps. One electric and one mechanical. So far so good. I haven't really thoroughly tested it yet though.
Sounds like you have two separate problems, maybe related. If your plugs are getting fouled you have too much fuel not too little. Adjust the idle mixture as suggested, I would use a vacuum gauge for that. Also check the float level, it may be flooding enough to foul the plugs but not enough to cause running problems. Seems to me the regulator would be the problem with the low fuel pressure reading. Reducing the pressure by by-passing to the return line but having enough at the carb to run the engine.
Curious as to why you have an electric pump in lieu of the mechanical? The mechanical pump on mine feeds the 464 cubic inches through an 850 carb up to 7,000 rpm with no problem.
Al, I'm really surprised that a mechanical pump is able to keep up with that beast of yours. Must just be the right combination of parts. Bigger that stock fuel line from the tank? 1/2" pickup in the tank? I assume you're running a Holley carb. I was having issues keeping my 400 fed with a mech pump. Maybe has more to do with the qjet than I thought.
Typo, meaning I made a mistake, it's an 800 CFM not an 850. Edelbrock 800 EPS carb. Stock pick up and 3/8" lines, Holley fuel pump. Guys used to drag race Pontiacs and big block Chebbys, Fords and Mopars all day with mechanical pumps, should feed a 400. I don't know how anyone can stand the noise that comes from the electric ones, worse than Flowmaster drone. And, one electric hiccup and no fuel. I did have fuel starvation at top rpm a few years back, before I redid the engine. I ran a fuel line out the cowl and zap strapped a gauge to the windshield wiper, I was loosing pressure at about 6000 rpm. I replaced the fuel pump and presto, fixed. The gauge showes a constant 5-6 psi. The pump was only 45 years old, why don't they make things to last?
Robert, I don't think it's the type of carb that would make any difference, unless the input to the bowl is smaller on the Q-jet, I don't think so. If you're running say 12.5 to 1 air/fuel with an Edelbrock and 12.5 to 1 with a Q-jet, two engines with the same volume at the same rpm will use the same amount of gas. Do you still have the small filter in the inlet of the carb? Most guys I know who have Q-jets toss those.
What about the needle seats, are there some with a larger orifice than others? Or can one modify the seat to flow more gas if needed? I would think the pump at 6psi would be able to fill the bowl as fast as the engine is emptying it, with moderate street engines like ours.
I suppose I could calculate how much fuel I'm burning, at say 6000 rpm, using my air fuel ratio, swept volume, temp, air pressure and all that. Then see how it relates to the advertised output of the pump at a restriction resulting in a 5 psi head. Carb specs always tell you the rated CFM but not the fuel flow, how many pounds of fuel an hour at a selected ratio.
I'm not sure about that. I talked to a lot of guys that run Holley's that don't seem to have the same lean issue at the top of the gear. It may have something to do with the fuel inlet or the size of the bowl in relation to the float on a qjet not holding enough volume. Or it could be one of a hundred other things I suppose. It could be as simple as not having a return line( my car was originally a 350 car without the return). All I know is I've been chasing this problem for the last 4 motors. I think I got it licked now but I won't really know until I get it to the track or on the dyno for a varifyable test. No filter in the carb. Probably wouldn't hurt to replace the canister filter now that I think about it. Yes, there are different sizing of fuel inlets. They can be enlarged easy enough with the right size drill bit.
If it's the bowl size people would be running out of gas a lot. You'd think [or I would] the carb would have an inlet large enough to feed the bowl and maintain a proper level even when full throttle. I went over the Coca-cola highway at mostly 100+ mph. Some of the long hills are 7-8 miles long. In fourth gear the rpm was just over 5000, not 6-7000 as it would be at the track but high enough for long enough to drain the bowl if it were not getting filled fast enough. The trend these days seems to be large pick-ups, 3/8 to 5/8 inside diameter fuel lines and pumps made to feed a high pressure injection system. All that gas we have moving still has to go through the 0.110" to 0.135" orifice in the inlet seat. I'm no expert on Q-jets, but I have heard tell of a couple of items which could give you a lean feature at WOT. First is fuel stacking in the bowl. Long hard acceleration or cornering can push the fuel against bowl wall, that fuel can lift the float and cause the needle to stop flow out of the seat. Do you have a Pontiac Q-jet or one made for another brand? They all have suttle differences. I think there are 20 or 30 different shaped floats for Q-jets and they can also be modified by cutting them. The other is the secondary fuel metering, you've most likely changed the secondary rods and hanger a few times to get flow, but if secondary flow is still too low even with smaller diameter rods or a hanger that will hold the rod higher, the orifices can be drilled from 0.160" to 0.197". Of course it will be rather difficult to restore to the original diameter.
Your "stalling out" at the 1000 ft mark, are you sure it was fuel and not the point when your camshaft stopped increasing the horsepower? Mine flat lined at 535 all the way from just over 5500 to just about 6600 when it started to drop.
I did consider that when the motor gave up at the 1000ft, that it migh be all the cam had to give except that it was confirmed on a couple different dyno runs that it was going lean at about the same RPMs. The operator( not the same one each time) suggested I give a Holley a try. Still haven't done it but I might some day.
Hmm, the dyno guy use your fuel pumps or their own? If they went with theirs and bypassed yours you can eliminate fuel delivery. The dyno's air fuel ratio show a lean condition at a specific rpm then enrichen as the rpm got higher or once the lean condition started it stayed for the rest of the pull? The engine is stationary on the dyno so that would eliminate fuel stacking.. You have the same prob with two engines so not camshaft/ heads suddenly breathing better at a specific rpm. Same carb each time, why not test with a known carb? that way you could confirm or discount the carb or the fuel delivery as the problem. They guys at dyno shop I had mine on also suggested a Holley to me. Mine was running rich with the Edelbrock, we changed jets and needles five times and thought we'd reached the limit. After the last pull we swapped out for a Holley 750 they use at the dyno. I picked up both torque and horsepower with the Holley. Since I do most of my driving at normal street speeds and do not compete in racing, I just left the Edelbrock on. Maybe if I win the lottery I'll buy one of the Holleys but right now I can't see spending the money to get seven more HP at an rpm I seldom run at. I built mine for torque.
Yeah it has always been a chassis dyno so it was set up like it always is. I had it on maybe 3 different dynos over the years and every time the operator has come and said it was going lean on top and I might want to consider swapping over to a Holley carb. I'd have to go back and look but it starts to go lean around 5000-5200 and my limiter won't let it go past 5500 so it doesn't get any better. Actually the first time was too weak of valve springs so it never really got that far. I ve been through so many versions of 400s, I don't think I've ever dynoed the same motor twice but the carb has always been a Qjet. Those dyno operators probably always suggest Holleys just because they are just easier to tune. Like I said I hope it's taken care of with the fuel pump arrangement I have in place now. If I run it again and it's still going lean I will swap in a 750DP I bought just in case. Haven't done it yet because it would mean figuring out a new throttle linkage setup again.
