Lean mixture at high revs.

[FatBoy]

Hi,
I have just had my Cobra on a rolling road in order to address some minor carb related running issues. The engine was built by one of the respected FE builders and has been installed without any subsequent modifications.
Although there is nothing major wrong with the engine, it has never run as well as I feel it should do.
The maximim power at the wheels was 348bhp, but the curve started to get very lumpy above 5,000rpm and the air/fuel mixture became lean. The engine was quoted as having significantly more by the builder and the spec would suggest it should have according to Desktop Dyno.
I have the Holley Red Top fuel pump as fitted by Kirkham, running in series with a Carter mechanical pump as fitted by the engine builder.
The lean mixture at higher revs would suggest that not enough fuel is getting to the engine when the demand is high and while I was out in the car yasterday, I noticed that under hard acceleration, the fuel pressure would drop significantly, which seems to confirm it.
Many Kirkham customers run much bigger and hotter engines than mine, so I'm assuming there isn't a fundamental issue with the capacity of the Holley pump and the fuel pressure under normal running conditions stays at around 7psi.
Can anyone shed any light for me?
I'm sure additional information will be required in order to help diagnose teh problem, so please ask.
Thanks,
Paul

[Hotfingrs]

What size fuel line are you running, where is your regulator mounted. There are different factors that could cause this.

[Mdman352]

I have the Holley Red Top fuel pump as fitted by Kirkham, running in series with a Carter mechanical pump as fitted by the engine builder.
I had a similiar problem. What is the purpose of running a electric fuel pump behind a mechanical one??? Self defeating. Lose the mechanical one.

[Z-linkCobra]

I was thinking the same thing...loose the mechanical pump. Is it carburated? If it is then make sure your power valve is working. If you blocked it off you might want to put it back. Make sure your secondarys are opening..again if your carburated. Another guy asked what size your fuel lines are...make sure your filter isnt installed backwards...seen that before. Depending on what type of filter you have..some dont flow worth a damn when installed the right way and wont flow at all if in backwards.

Just some thoughts.

Gene

[jhv48]

I would disconnect, or bypass, first one and then the other fuel pump and see if the problem goes away.

If it doesn't, then I would look at the fuel filter. Then loosen the gas cap and try it.

Something is restricting the flow of fuel. Could be collapsing fuel lines, plugged or no fuel tank venting, etc. But my bet is that one of the fuel pumps are the cause.

I, too, question the use of two fuel pumps.

[FatBoy]

The fuel line from the tank through to the mechanical pump is S/S braided line as fitted by Kirkham. The line from the mechanical pump to the carb is a hard line of 3/8" O.D., which is the correct size for the Holley fuel manifold.
The carb is a Holley 750 DP.
I did check beforehand on the forum and many people seemed to be running electric and mechanical pumps in tendem.
Here is a picture of the filter, fitted immediately before the Holley pump.



Thank you.
Paul

[FWB]

your original post. the 7 psi seems high. when i go to 7 with my dual holleys it over powers my needle and seat. i run a mallory electric 140 GPH.
the fact that you have the mechanical pump restricting the flow, and it will, is keeping the pressure at bay so the 7 psi probably is more like 6.5 at the carb.
in the last post you mentioned a 3/8 OD line.......too small for a motor like yours at high revs. switch the line to 1/2 OD, loose the mechanical pump, and drop fuel pressure to 6.5 or lower. you should then have no issues feeding your carb.
at the high revs you need to overcome inertia also and the 3/8 OD just can't provide the volume necessary


p.s. put the filter after the pump.....

[FatBoy]

It is interesting that you say 3/8" isn't big enough. It is the same size as the manifold tubing supplied by Holley for this carb. Wouldn't that suggest that it is big enough, or am I being naive?
The fuel pressure sensor is in the carb manifold, so it should be showing the pressure at the carb rather than further back in the system.

Paul

[FWB]

personally i don't trust holley with anything, but that aside, the 3\8 is definitely too small, 4 and 6 cly street cars have 3\8 line, a small block has a 3\8 line. if you kept the rpms down to street levels of lets say 4500 you may never have an issue. but the 750, and i'm guessing here may be jetted in the 72 to 78 primary and 80 to 88 secondary needs fuel, if you feel the 3\8 will be fine then try it by all means. bypass the mechanical pump, i would bet the car will run better. watch for flooding with the 7 psi. if you still have a issue with performance then consider the 1\2 line. just my opinion, i've been messing with these motors since 1978, a 3\8 line came factory, and on the dragstrip, it felt like you were hitting a brick wall in high gear when you just plain ran out of fuel. we ran the holley blue pumps. and even in that environment 6 to 6.5 fuel pressure. switched to 1\2 inch line and it would pull the whole way.


if you retain the 3\8 up by the manifold that will probably be fine, its the volume from the rear to the front, the 3\8 probably travels rearward from the front anyway so inertia will help there.

[Z-linkCobra]

FWB... you need to buy some new needle and seats...lol. We run 8.5 to 9 psi on the race car all day long with no flooding problems.


Fatboy... FWB is right...3/8 o.d. is too small. You need to step it up to 1/2. Pressure is not the biggest factor its volume that you need. With a 750dp your going to run out of fuel with the small line and the restriction of the mechanical pump. Ever taken the line off of a mechanical pump and cranked the motor over...huge pulses in the fuel flow..not a steadt supply. Perfectly ok for cruising and pulling the CObra to the show...but not for going fast.

[FWB]

8.5-9 psi....i would bet not with regular stock neoprene needles. the solid needles will take that much.

i don't think the filter before the pump is a good idea either. the pump should be below any possible fuel level in the tank.

[FatBoy]

If I by-pass the mechanical fuel pump, can I just disconnect both pipes and join the ends together or do I also need to remove the pump and fit a balnking plate over the hole?

The base of the pump is level with the bottom of the chassis rails, so it is as low as it can go and is below the fuel level in the tank.

Many thanks for your input and the benefit of your experience.

Paul

[RICK LAKE]

FatBoy Paul Try the simplest way first. Take the mechanial pump out of the loop. road test the car and do the same test for fuel pressure. Have the pressure at 6 psi. If the car doesn't change and the pressure is still dropping more than 3 psi we need to look at the supply system and pickup in the tank.
I know that most cobras came with a 3/8" and 5/16" supply line for a carb car and mechanical fuel pumnp on the motor. The carb was only about 750-780cfm. Today 1,100- 1,300 cfm carbs are out there. Unless you are running a pump in the fuel tank, I see a couple of problems,
Under hard accel the fuel is being pushed to the back wall of the tank. If you are around or under 1/2" a tank of fuel the pickup may be sucking air/fuel combo. This doesn't work to well for a motor and lean condition happens. If under normal driving this is not happening, fill the tank to 3/4 full and try again the hard accel. If no problem happens the issue is the pickup in the tank. I have had to change my tank and rework the baffles. I can't run lower than half a tank of gas without having the same problem as you in high "G" turns or accelleration. If there is no change the next thing is to look at the fuel filter for possible clogging and partial flow. The newer gas out with 10% ethenol is eating any rubber hoses you have on the car. Very small pieces of the rubber are clogging up, filters jets, needles and seats of the carbs. This has happened to a couple of guys here this year.
If the electrical pump is not getting the job done, I doubt the mechanical will fair and better. The next thing would be to run a larger line from the pump in the back to the tank and repeat the drive. If car gets better you have your answer. If not start looking at the carb. Ps I run 1/2 line to my all my cars and 3/8" return line with a good fuel pressure regulator at 6.0-6.3 psi. This system never drops more than 1 psi at WOT. The other idea I have is the fuel is cooking in the lines. I would rap the supply line with a deflector tape and foam insulator. Last Question Does this problem happen only when the car is hot? and not when cold or warming up?? Rick L.

[Z-linkCobra]

In the end you need to remove the pump. The pump sitting there thry to pump air will not it any good at all. If it trys to lock up it might cause a problem with the esentric on the front of the cam. To do the test and see if thats the problem just bypass it for now.

It's better to push the fuel through the filter with the pump then try to suck it through. electric pumps are made to push not pull.

Rick lake has some good tips and things to look at as well. It only takes one little piece of crap to give you major headaches.

[Z-linkCobra]

Originally Posted by FWB

8.5-9 psi....i would bet not with regular stock neoprene needles. the solid needles will take that much.

i don't think the filter before the pump is a good idea either. the pump should be below any possible fuel level in the tank.

Of course....who runs stock anything on these cars....lol.