Do I have a fuel pressure problem?

[62 Dart Convertible]

Hi Guys

I'm trying to solve a general lack of power at high engine speed. Engine pulls like a freight train down low and in midrange, then starts to cough and splutter as revs climb spoiling all the fun..
It has been like this since I bought it a year or 2 ago.

Engine is a stock dual quad 413 ('62 300H engine) with mechanical fuel pump. I replaced the stock dual point dist with a new electronic system, new leads and plugs, so I am pretty certain bad ignition isn't my problem.

I hooked a fuel pressure gauge to the fuel line just prior to the front carb and went for a drive. At low engine speed fuel pressure is 6-7 psi. Foot to the floor with engine coughing and farting, fuel pressure was 2-3psi.

Is this low pressure likely to cause fuel starvation?

Thanks in advance..

 

[MikeyT]

Wow, that is low! Tell us more about the fuel system. What size line are you running on it? 5/16, 3/8...? Also what type of fuel pump are you runnning on it?

Mike

 

[RustyRatRod]

Line size is irrelevent. 5/16 is plenty big. Pressure comes from the pump. It needs more pump.

 

[62 Dart Convertible]

Wow, that is low! Tell us more about the fuel system. What size line are you running on it? 5/16, 3/8...? Also what type of fuel pump are you runnning on it?

Mike

Hi Mike

All is stock. New tank fitted when car restored around 15 years ago. Pipe from tank is 3/8" to stock looking mech fuel pump (which is a non-serviceable pressed metal affair - manuals show stock pump is a serviceable cast aluminium, thing...), then 5/16" line through stock filter and to 5/16" TEE to carbs.

If that pressure is a definite problem, then I will first check for any blockage on the suction side, then move on to remove and investigate pump and pump pushrod.....

 

[MikeyT]

Line is relevant, especially if their is a leak or something that gets worse under WOT. However, I think3/8 should be big enough. My guess is that pump is bad, but I like to look at all things first. May I suggest a carter strip street pump.

mike

 

[punisher33]

Dito MickeyT.. Line size is very important. Just replace the pump with a HP mechanical pump that can handle your horsepower. Your line size is good for your engine. If that does not work then you may want to check the valvetrain. I have seen many times where the springs were weak and let the valve float. GOOD LUCK>

 

[Hemi Rebel]

How is/arethe filter/filters? i would expect a certain amount of pressure drop with dual quads going full tilt. Fuel Volume is what we really want to see. If there are no restrictions before where you have the pressure guage, i would think bad pump. Not sure what the rating is supposed to be for that engine, but if you have nothing better to do you could place the pump outlet line in a jug and crank the engine for like a minute and see how much comes out. Then measure that amount. Multiply that by 60 and that would be how much the pump flows in an hour. Pumps are rated at gallons per hour. Like i said though, i dont know the specs or at what rpm the pump needs to run to test it, but it may give you a ballpark figure. Or just put a pump and filter on it and see what happens. Course could be a too much fuel issue also. Could try running it hard to the point it breaks up then shut it down and pull a plug to see what you have going on. That may help determine which direction to go in.

 

[RustyRatRod]

For what he's doin, 5/16 or 3/8 makes no difference. Been there done it.

 

[bigmanjbmopar]

I went from stock 5/16" mech pump on the 383 and moved it to the 440 and it worked fine but made some knocking noise so I tired electric pump and wow unless you are going to the strip every other weekend no need for that. Switched that out to an Edelbrock mech pump, up sized the 5/16" to 3/8" and pressure stays at 6-7 all the way through. So unless you have clogged filters get a new pump. just my 2 cents. and for the record there was no pressure difference between the 5/16 and 3/8 lines just seems to make more sense that a larger tube will have less restriction but only as much as the pump can pull in so I think in some cases the smaller may be better to keep the pressure up if you know what I mean. I have 800 cfm card on 493 ci so for me getting a lot of gas to the pump is important for me.

 

[MikeyT]

I see your point but I'll also argue that if mopar didn't need to upgrade to a 3/8 inch fuel line they sure as heck wouldn't of done it, and while I agree a 5/16 inch set up works well in a 383 car. It isn't the case with a mildly hot 440. My lil 440 with a 750 Db pumper doesn't compare to a 493 with an 800, for a fuel need, but it does to the point I upgraded the fuel system to 1/2", but I go and flog this thing on the weekends too. Now I'll admit.. I might be overly cautious, but I've seen enough engine at the track china syndrome due to poor fuel systems then I would ever want to be a part of experiencing.
Mike

 

[WP29440SE]

we run built 440 6 packs , 440 dual quads all damn day long through a 5/16 lines and have never had a fuel starvation issue. 99% of it is in the fuel pump not the difference between a 3/8 or 5/16 line..................

 

[Meep-Meep]

Check the length of your fuel pump push rod - I bet it's worn to the point you don't get full stroke out of the pump. It is also normal to get a pressure drop as you go for max power at WOT because the needle valve is wide open and the flow has increased. Pressure will be low at max flow.

I'm running a 5/16" line on a slightly warmed over 440 and it's fine. I even had a stock replacement pump that produced a whopping 2.5 PSI of fuel pressure and I didn't notice a damn bit of difference between it and a Holley 7.5 PSI unit (no track time; just driving around town including a quick blast at WOT). I drove the car with a fuel pressure gauge to verify.

Consider this: You have two AFB's and a total of 4 needle and seat assemblies with an orifice in each one not larger than .080". A single 5/16" line delivering fuel under pressure to a stock engine will be just fine.

 

[lewtot184]

you probably need about 4psi at WOT. do a volume test. might have a valve spring issue but you don't state at what rpm the engine craps out.

 

[RustyRatRod]

you probably need about 4psi at WOT. do a volume test. might have a valve spring issue but you don't state at what rpm the engine craps out.

4PSI is probably about perfect. I know the Thermoquads don't like much more than 5 before they start to push the needles off the seats. I had two 850 TQs on a tunnel ram on a very hot 340 many years ago and found as long as I maintained 4-5 PSI I never ran out of fuel. Oh yeah, and I had 5/16 line from the fuel sender all the way up and it was a screamin little bitch in heat.

 

[62 Dart Convertible]

Thanks for all the replies guys. Considering everthing is pretty stock, I never thought fuel line size could be the problem. Chrysler built how many cars with these size fuel lines without issue??!! I think I am looking at either a blockage (crap in the line, blocked filter...), faulty pump or worn/shortened pushrod. I'm hoping to get at it this weekend, but to make things harder, here in Oz it's not like I can pop down to Summit and pick up another pump and be done in a day.... I will update this thread once problem is resolved.

 

[Hemi Rebel]

4PSI is probably about perfect. I know the Thermoquads don't like much more then 5 before they start to push the needles off the seats. I had two 850 TQs on a tunnel ram on a very hot 340 many years ago and found as long as I maintained 4-5 PSI I never ran out of fuel. Oh yeah, and I had 5/16 line from the fuel sender all the way up and it was a screamin little bitch in heat.

You wouldn't happen to have any pictures of that would you? I have seen some things, but 2 thermoquads on a tunnel ram ain't one of 'em. Not in person or in any magazine. Had a conversation with a guy at work a couple of months ago and we were talking about it. Cause I have a tunnel ram and we joked about doing it.

 

[lewtot184]

there was a chevy pro-stock in the early '70's that used 2 thermo-quads on a tunnel ram. in fact i seen a picture of a chevy pro-stock engine with 3 on a tunnel ram.

 

[RustyRatRod]

You wouldn't happen to have any pictures of that would you? I have seen some things, but 2 thermoquads on a tunnel ram ain't one of 'em. Not in person or in any magazine. Had a conversation with a guy at work a couple of months ago and we were talking about it. Cause I have a tunnel ram and we joked about doing it.

I don't, but my sister probably does. When my first wife left me, she got what she thought were all of my pictures from back then and burned them in the driveway....along with a couple of really valuable guitars and various other things of worth to me. I'll see if I can come up with somethin.

 

[62 Dart Convertible]

According to the Carter AFB factory service data, the low limit for fuel pressure is 2.0 PSI.....

 

[62 Dart Convertible]

Dito MickeyT.. Line size is very important. Just replace the pump with a HP mechanical pump that can handle your horsepower. Your line size is good for your engine. If that does not work then you may want to check the valvetrain. I have seen many times where the springs were weak and let the valve float. GOOD LUCK>

I'm wondering about the same thing re valvesprings. I have checked the fuel system and can find no obvious fault. Engine has a hi lift MP mechanical cam and I have no idea what springs.

In a few weeks engine is coming out and I will know once and for all...