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fuel pressures

 
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rogus
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Joined: 07 Aug 2005
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1976 Chevrolet Chevelle

PostPosted: Sat Sep 09, 2006 10:14 pm    Post subject: fuel pressures Reply with quote

With a stock Chevy Sb fuel pump you usually get around 9 PSI. Holley suggests about 7PSI for their carbs and Edelbrock suggests about 5PSI.

Can someone tell me what problems can be caused by to much fuel pressure to the carb?
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Big Dave
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Location: Tampa Florida
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PostPosted: Sat Sep 09, 2006 10:31 pm    Post subject: Reply with quote

It has been my experience the stock pump only puts out about 4.5 to 5 pounds of pressure.

The fuel pump compresses the spring inside the pump and sucks in fuel as it lifts the diaphragm on the down stroke (cam eccentric pushes rod to push pump lever arm). When the pump falls off the eccentric the flapper valve closes and the compressed spring pushes the diaphragm back down forcing fuel to the carb. The spring determines the fuel pressure and can be shimmed for more pressure. The diameter of the diaphragm determines the volume (since it travels the same distance on every stroke).

If the carb sees more fuel pressure than 7 psi the float (which floats on top of the fuel bowl's supply of gas) can not apply enough pressure to close the needle and seat in the carb. The carburetor then floods as the gasoline continues to enter the fuel bowl and exits out the top vent, and into the throttle bores.

If we are talking about an in tank, electric fuel pump for a TBI, it runs at 14 psi not 9 psi and will definitely blow the needle off it's seat.


Big Dave


Last edited by Big Dave on Sat Sep 09, 2006 10:38 pm; edited 1 time in total
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10sec.et
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Joined: 29 Mar 2006
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1969 Oldsmobile Cutlass

PostPosted: Sat Sep 09, 2006 10:37 pm    Post subject: Reply with quote

i cant believe im saying this but.... i dont think thats the answer he was looking for Dave. i think the answer hes looking for is... too much fuel pressure will force the float needle off of its seat and cause flooding. still good info Dave.
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af2 wrote:
It seems we can look at our magical Balls and come up with a fix?

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10sec.et
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Joined: 29 Mar 2006
Posts: 3473
Location: Houston,Texas
346658.74 points


1969 Oldsmobile Cutlass

PostPosted: Sat Sep 09, 2006 10:38 pm    Post subject: Reply with quote

HAHAHA!!! you edited as i was typing. i gotta move faster Laughing
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af2 wrote:
It seems we can look at our magical Balls and come up with a fix?

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rogus
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1976 Chevrolet Chevelle

PostPosted: Sun Sep 10, 2006 12:56 am    Post subject: Reply with quote

Thanks i do appreciate all the info.

I have guaged my stock fuel pump and it does produce 9 psi.

That having been said - how much damage have i probably done to my Holley carb and what might it take to fix it (besides a new fuel pump)?
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Big Dave
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PostPosted: Sun Sep 10, 2006 8:00 am    Post subject: Reply with quote

If it hasn't flooded by now no damage at all.

I would assume your float level is not set correctly if you have gotten by until now. You probably have the float level set too low in the bowl to get enough buoyancy to close the needle and seat. That results in there not being enough fuel in the bowl to support a top end charge; so the engine will run lean at WOT. But this is only an assumption.


Big Dave
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240Z8
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1972 Datsun 240Z

PostPosted: Sun Oct 01, 2006 8:45 pm    Post subject: Reply with quote

With a big electric pump you can hydrolock the engine in a matter of seconds. Embarassed Did that this weekend for the second time in my hot rodding career. Embarassed I'd worry more about the engine. Running engine with too high float level can cause over rich condition and wash cylinder lubrication. I'd get a regulator.
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af2
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Location: grassvalley, ca
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1933 Willys Coupe

PostPosted: Sun Oct 01, 2006 9:12 pm    Post subject: Reply with quote

240Z8 wrote:
With a big electric pump you can hydrolock the engine in a matter of seconds. Embarassed Did that this weekend for the second time in my hot rodding career. Embarassed I'd worry more about the engine. Running engine with too high float level can cause over rich condition and wash cylinder lubrication. I'd get a regulator.


6.5 lbs is the normal. Any thing higher, I DON'T know?
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10sec.et
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1969 Oldsmobile Cutlass

PostPosted: Mon Oct 02, 2006 11:05 pm    Post subject: Reply with quote

too much fuel in the cylinders will ultimately dilute the oil... not good for the bearings. iterestingly though, if you wash down the cylinders in a Jap motor, it will loose compression until it gets oil back. compression on those motors will drop well below 100 psi. just a bit of pointless trivia if anyone cares self-satisfied smirk
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