I have researched the issues with the fuel pumps and know there was a problem a few years ago. I bought a good used pump here and I am getting 9.5 psi at idle? My Holley HP handled it ok but the 850 mighty demon not so much...... How much pressure do I need o feed a 550HP motor with -6 lines? I am thinking 6 or 7 max? I will try increasing th fuel pump,gasket thickness as I have read his will move th arm out and decrease lever travel to reduce pressure. I would rather not use a regulator as I have the fuel lines all done now. Does this sound like a good idea?

Demon recommends 6-7.5 psi. 9.5 is a little too high but I would be surprised if it is causing your issue. I have always run 7 psi on Demons with good results.

You don't need a fancy pump to support 550HP. Even a stock merc pump will work. I'd swap out the pump though before shimming it, you do NOT want to mess up the pump pushrod or cam lobe. But yes, 9.5psi is too much pressure. It will flood the engine with gas. A self regulated pump is what you want. 6-7psi is about where you want it.

MT, like the OP stated I have also had a Holley function just fine on around 9.5 psi. But only long enough for summit to ship a new regulator to me. I cant say that I have ever seen the effects of more than about 8 psi on a Demon. Are the Demons more sensitive to fuel pressure or where the Holleys just lucky.

I have the engine on a stand now and have about 2 hours run time so far.... I used the holley first as the Demon uses the 9/16-24 fuel lines that I had to order .Got the lines on and the Demon does not like the high pressure. Its ok for a while but then starts to flood out at idle. If I run it higher around 2,000 or so it is fine but obviously I need to have this fixed before it goes in the boat. I have an LM-1 installed and trying to get the idle dialed in but it floods out and goes rich ..... I do have another pump to use so maybe I will clean it up and see how it works...... Another question is what A/F ratio is good for idle? around 13.0 - 13.5 or so? That is what I am shooting for....

Ya try the other pump. Generally, you can go leaner than 13.5 at idle, but you may find the engine will want to stall when shifted. Every boat is differnt. A boat with 18" diameter 6 blades and a big cam, might want to be rich at idle to not stall when shifted into gear .

Thanks... I definitely want good dock manners and no stalling as single engine docking can get dicey with wind...... My cam is not too radical and it will idle at 750 without choking itself but still has a healthy rumble.... I had idle afr's at 13.2 bouncing a bit until the Demon started to flood and went rich..... then I just ran the engine around 2-3,000 for a while varying speed until my gas was gone...

Be sure to check the dipstick for gas smell. Don't want gas in the oil, esp on a new build

It's not really a thing of how much pressure you need but volume. I've messed with fuel pressures from 5-15 lbs with carbs and never really seen a difference. I've noticed they started leaking fuel around 11 lbs. no noticeable difference in the 6,7 or 8 lb range. After messing around with fuel pressures I've pretty much settled on 6lbs.

Exactly. You need enough fuel psi to fill the bowls when the float drops, and not too much to push the needle off the seat when the float is at its correct height. More psi will get you nothing as far as performance in this application. Volume is the key.

Holleys current marine pumps, are available in 110GPH, and 130GPH. The 110GPH will not require a regulator, and the 130GPH will. The pumps visually look the same, both having 3/8NPT in/out. I wish they never stopped making the ones I use. Which are the older six valve pumps. These pumps are much better, have 1/2NPT in/out, preset at 6.5psi, and do not require a regulator. Mercury used these pumps on the 800 and 900sc engines. You can still get them, but need to modify them if you want to "marinize" them with the vent tube.

Heres the current holley pumps
http://www.summitracing.com/search/p...%20fuel%20pump

I attached a photo of the pumps I run that holley no longer makes.

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