Fuel Pump/Pressure ???

[JR_VIC]

I searched around and can not find a chart for recommended fuel pump PSI relative to horsepower and/or top RPM. Something such as approximate recommendations for example 300-350 horsepower or 5K RPM would be 5-6 PSI and 350-400 horsepower 5,500 RPM would be a range of 6-8 PSI. I am doing some trouble shooting and want to make sure my fuel pump is working properly. I installed a new intake last week as well as a new power valve in the carb. Took the boat out Saturday and the boat ran good up to 5050 RPM where it had an occasional cutting out or missing...almost like the rev limiter but not a consistent miss like a rev limiter feels. I checked the timing before I left and it was 12 degrees.

On Monday I am going to get a gauge and verify fuel pressure as well as pull the bowls and check for any debris. Total rebuild on the carb in April and new fuel pump in August. Last trip out in the boat before the intake swap the boat ran great with no problems and up to 5180 RPM with no cutting out/missing. If anyone knows where the fuel pressure should be that would help. Carb is a 4160 Holley 600 CFM. Any thoughts as to what else I could check would help. Timing was good but if I was running low on fuel pressure and the secondary bowls were running low I would think the engine would slowly loose RPM's and speed and not just a sudden miss or engine cut out.

Thanks!

[ffmedic74]

RE: Fuel Pump/Pressure ???

I would recommend calling PCM. How many hours are on the motor? If the motor has a bunch of hours, do you think the valve springs might be a bit weak and your floating the valves a bit????

[JR_VIC]

RE: Fuel Pump/Pressure ???

I was able to check the fuel pressure and it was @ 6 PSI so all is OK there. What is weird is that I am running a rev limiter with a 5600 RPM pill. With the Edelbrock it turned near 5200 RPM with no misses. Changed the intake and it is missing at 5050 RPM with the same 5600 RPM pill. Went to the lake and decided to install a 6000 RPM pill to see what happens and bingo...engine now turns 5400 RPM which again is strange so there must be an issue with the rev limiter even though it is only a few months old.

I then worked on the carb checked the screen and primaries for any debris but it was all clean. I then installed my rear secondary metering block conversion with size 70 jets upon Holley's recommendation. That must not be enough because it leaned popped at 5400 RPM so I did a plug chop and pulled all 8 plugs. Each plug was milk white in color so it appears lean on the top side. Also slightly raised the rear float but it did not appear to change anytihng. A little hesitation out of the hole so it appears lean all the way around. I'll keep tuning and get her dialed in.

Thanks for all of the feedback

[JR_VIC]

Re: RE: Fuel Pump/Pressure ???

I would recommend calling PCM. How many hours are on the motor? If the motor has a bunch of hours, do you think the valve springs might be a bit weak and your floating the valves a bit????

Only about 200 hours on these heads. Spoke with a friend and he does not think it is valve float.

[TRBenj]

RE: Re: RE: Fuel Pump/Pressure ???

I doubt very much that its valve float- the stock cam is pretty tame and 5400 RPM isnt extreme (although very impressive in this application). Sounds like the high RPM hesitation issues are gone as well- although its still running very lean.

Jeff, are those standard GT40 heads?

[JR_VIC]

RE: Re: RE: Fuel Pump/Pressure ???

I was told when I bought these heads they were milled 20-25 thousandths, 3 angle valve job, and some minor port/polish work. It was a fair price when I bought them and was told by the machine shop they were not modified to the point where they would be too aggressive for low end performance on a ski boat. It seems as though that these Hi-Tek headers have really opended up the engine and let it breathe. It would be really interesting what a cam and a set of AFR aluminum heads would do. The headers, intake, and DUI ignition have added about 1,000 RPM's.

[TRBenj]

RE: Re: RE: Fuel Pump/Pressure ???

That may make those GT40's perform similar to the GT40p's that I have. Stock vs. stock, the p's flow a little better and have smaller combustion chambers.

With the GT40p's, intake, cam and DUI, I have gained 400-600 RPM. The 200 I gained from the DUI are suspect though, since I didnt get a chance to play with my timing before my ignition module died. It was really underperforming at 41 MPH @ 4200 RPM, so the DUI really brought me back to stock performance (42 MPH @ 4400 RPM). I gained 400 RPM from the other mods. My guess was that the exhaust manifolds are the major restriction at this point... and it sounds like you are confirming that. I cant wait to get my hands on some!

[DanielC]

RE: Re: RE: Fuel Pump/Pressure ???

Make sure you also check the fuel pump volume, too.

[JR_VIC]

RE: Re: RE: Fuel Pump/Pressure ???

What is a range of volume for the stock fuel pump?

[Mikeski]

RE: Re: RE: Fuel Pump/Pressure ???

JR,

What does it smell like at an idle? Fatten up the idle circuit until is starts to smell rich then work down. It's always safer to run it on the fat side rather than the lean side.

[TRBenj]

RE: Re: RE: Fuel Pump/Pressure ???

Richening the idle wont help his lean condition at WOT. He'll have to do some plug readings and change the jets accordingly- first by dialing in the primaries and then moving on to the secondaries. Jet sizes should be about 5 steps higher on the secondaries to account for lack of power valve.

[JR_VIC]

RE: Re: RE: Fuel Pump/Pressure ???

I spoke with Cam research because I will be buying a cam from them over the winter. Explained my engine mods etc. as they are all around Ford pros. He beleives (and I concur) that the engine is lean down low and up on top. He stated the Stealth intake has larger plenums and slows down the carb signal which is basically not pulling on the carb down low so it needs a little more fuel. He recommended fatten the idle circuit and leave the primaries alone and put in larger secondary jets.

Mikeski...I don't smell hardly any fuel at idle and it is not smoking either. After Thanksgiving I will mess with the jets and idle circuit.

[Mikeski]

RE: Re: RE: Fuel Pump/Pressure ???

sounds like a good plan.

Do you have an IR thermometer? If you have access to one it wouldn't be a bad idea to take some readings right at the exhaust manifold/block junction area. I don't know what it should be on a boat but it should be consistent to a good running boat. If it's hot it's lean. Obviously you need to read before the water jacket comes into play which may be difficult with your long tube headers.

[Fast351]

How big is the engine? 351? A 600 CFM carb is pretty small for higher RPM work. You might want to try a slightly bigger carbureter, see if you can borrow one from a buddy.

Stumbling/power loss at the high end is almost always fuel related unless you have a weak ignition system. Sounds like you have an MSD, and while not one of my favorites, they do work reliably and you're not really taxing it at 5500 RPM.

Start by fattening up the primaries and secondaries by 4 jet sizes. I realize it will run rich, but you will definitely be able to tell whether fuel is your problem. I recommend 4 jet sizes because 2 is usually not enough to tell especially in a boat where the engine is harder to feel anyway.

Another thing you can do that doesn't involve changing jets for trouble shooting purposes only is to raise the float level (this should only be done temporarily for testing purposes). If you have a fuel supply problem, this may point that out.

Also check simple things like plug gap, loose wires, bad distributor cap, etc.

Sorry it sounds like a bunch of rambling thoughts, I need some coffee :grin:

[TRBenj]

I think a 600 CFM carb should be plenty. According to the CFM calculators Ive found, it should be enough for that 351 up to 5900 RPM (assuming 100% volumetric efficiency). A 750 CFM probably wouldnt hurt- but I dont think the 600 CFM is the limiting factor yet.

It sounds like his high RPM stumble has been corrected by playing with the rev limiter. Can you explain the advantage of using the MSD in conjunction with the DUI, JR?

Let us know what you end up with for jet sizes. Remember to disable the secondaries and get the primaries dialed in first. Secondaries should be ~5 sizes bigger to account for the PV. Id start with 68/78 or 70/80 and go from there.