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AIR POWER
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157 Posts
Discussion Starter · #1 ·
My engine stumbles at wot like its starving for gas. I checked the plugs after a wot run and they are brown. I richened the jets and still the same color. I checked all the fuel lines for blockage and they are good. I have a carter electric fuel pump and it reads 5 psi at idle and at wot it fluctuates between 3 1/2 to 4 psi and the guage doesnt stay constant on psi. I am wondering if i dont have enough psi to keep the fuel flowing? Also shouldn't the fuel psi guage stay steady and not bounce around.
The Carb is a 800 holley and I have a liquid filled psi. Thanks
 

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E-7 Sheepdog (ret)
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6,834 Posts
3.5 to 4 psi is pretty minimal, but, should do the job. Borderline IMO. Could well be the problem.

My mechanical covers 4.5psi at idle and 9PSI wound flat out.
No problems here. ;)
 

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SLOW MEMBER
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470 Posts
I would put a better fuel pump on, better safe than sorry. The stumbling at wot could be a carb problem, I dont think it has to do with the lower fuel pressure.

This thing is naturally aspirated?
 

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AIR POWER
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157 Posts
Discussion Starter · #6 ·
I do think that the lower fuel presure might be the problem but the question is what should your fuel presure be at wot? and is the fuel pressure supposed to bounce up and down on the guage? I put a new liquid filled guage on thinking that was the erratic reading problem.
 

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SLOW MEMBER
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470 Posts
I installed "good" needle and seats in the carbs on my boat. I set the pressure at 8.75-9 psi. I am running a big mechanical fuel pump and a regulator. I sometimes will see my fuel pressure gauge (dash mounted) bounce a little bit, but it is only when the boat hits a wave hard. My fuel pressure does not drop at all when I mash the gas, if it did, I would vamp up the fuel system some more. Your holley carb should be able to hold back at least 6-7 psi with the stock needles and seats in it. Hope this helps, Ryan.
 

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AIR POWER
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157 Posts
Discussion Starter · #9 ·
i am thinking that 3 1/2 to 4 psi isn't enough to keep my carb feed well enough with fuel @ wot. IMO the engine needs more fuel, I was just getting opinions before I bought a new fuel pump. I also think that this carter electric pump is not a very good pump.
 

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E-7 Sheepdog (ret)
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6,834 Posts
does your pressure stay at 9psi. flat out with out without the presure bouncing all over on the guage?
My pressure is very steady, and only exceeds 7-8psi after I run across 5-grand, very linear pressure change as RPM climbs. I run 5400RPM off the gas making 9psi.
I run 5900 RPM tapping the same fuel suply for nitrous.
Don't know what the fuel pressure is @ 5900 on the fun-gas, as the gauge is under the engine cover, not in the dash, and to read it wife has to drive. She has never lit up the nitrous system for a test run.
My fuel pressure safety switch doesn't cut in, so I am unconcerned.
 

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E-7 Sheepdog (ret)
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I'm no fan of electric fuel pumps.
 

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I'm No Expert
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3,142 Posts
Have you checked your float levels? I had a stumble, well really what felt like a bog that would start after about 4-5 seconds on a WOT pass. Turned out my secondary side float level was set way too low. reset the float level on the water and i never had the problem again. I'm running a mallory 110 pump, regulated at 6 PSI.
 

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senior member
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1,814 Posts
it,s always good to go much bigger instead of good enough on the fuel pumps. i went from a holley blue to black then to a mallory 250 gph. on the liquid filled fuel pressure guage i was told they suck & were inaccurate & i thought the guy at the parts store was a tard & they wouldn't sell the things if they didn't work & then i had mine act up & the reading was set at 7 then 9 then 3 then 0 its was all over the place so i put a new reg on .. same thing so i got a new guage... liquid filled & it did the same thing so i got a non liquid filled & no problems.. i guess the guy was right ... the brand was a marshall .hope this helps i was also told 6.5fuel pressure is the best setting. for the fuel pressure over that it puts bubbles in the bowls & then it's airrated...less fuel.a guy told me that's what holley said.
 

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Sit N' Spin
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1,423 Posts
Typically carburetors wanna see about 7psi of pressure. 3.5-4 does seem low...but the fact that you have pressure tells me that you're not starving for fuel. If the engine were demanding more fuel than the pump could supply, you would have no pressure.

Pressure only exists behind the needle and seat assembly. Once inside the bowl, there is no "fuel pressure". Since the system has enough fuel volume flowing through it to build some sort of pressure, this tells me that the bowls are keeping full and the needle seat assembly is closing at some point, otherwise you would have no fuel pressure (if you were to unhook the fuel line from the carb and turn the pump on, there'd be no pressure as the fuel would simply be free flowing. This again disproves that this is a fuel related issue.

Also, the brown color on the plugs after a WOT run points to the fact that you are definitely not starving for fuel.

I would look elsewhere myself. Ignition maybe? Just my $0.02.
 
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