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OT: Fuel Pump Pressure Question

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Dave in GA

12-14-2006 07:39:42

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I have a 1966 IH truck with the 392-V8. I don't drive it much but start it up once a month or so. When I started it the last few times, I ended up with the Holley 4-barrel carb leaking fuel all over the engine while the engine was running fine. The first time, it sprung a leak where the fuel line attaches to the carb. It had never leaked there before, but now was gushing out. I had to put a new washer on the fitting, which solved the problem. Then last week when I started it, and while the engine was running fine, gas began gushing out where the front reservoir-to-rear reservoir fuel tube comes out of the front reservoir. There's little o-ring in there that seals it. So I took the carb apart and found that the o-ring had blown. As soon as the rebuild kit arrives, I plan to replace all of the carb o-rings and gaskets. But I've been wondering if my 5-year-old fuel pump is building up too much pressure that's blowing the gaskets and o-rings? Is this possible? The last thing I need is an engine fire.

By the way, while the engine runs, the carb doesn't flood the engine. The needle/seat valves seem to be holding the pressure just fine.

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Earl-IL

12-14-2006 14:17:47

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Re: OT: Fuel Pump Pressure Question in reply to Dave in GA, 12-14-2006 07:39:42

When I had the store we sold a pressure regulator that was adjustable . I think most engines, it was set for 2 - 2.5 lbs. It worked good.It was installed between the fuel line & carb. Earl

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glennster

12-14-2006 09:24:55

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Re: OT: Fuel Pump Pressure Question in reply to Dave in GA, 12-14-2006 07:39:42

hollley carbs can handle 4-6 psi fuel pressure, ovet that, the pressure will open the floats and flood the carburetor. pick up a fuel pressure regulator and set it at 5psi. the other problem with the older holleys is the o-rings and rubber is not resistant to the alcohol in the fuel. its eats up the seals, the new kits are a little more resistant. one other thing to look at is the power valves in the metering block plates. they look like a screw in bottle cap sized deal, with a rubber diaghram and spring inside. a back fire can damage the power valve pretty easy, and you have to replace the power valve.

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big jt

12-14-2006 08:45:33

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Re: OT: Fuel Pump Pressure Question in reply to Dave in GA, 12-14-2006 07:39:42

Dave

Just like the replies below said the fuel pump isn't likly to increase pressure. Sounds like your kit should solve the problem.

Only thing that has been missed is that the bowls are vented to the air horn. That meaning that unless that vent is plugged you cant build any pressure in the bowl.

Clean and Kit the carb and all your problems should go away, supermodels will flock around you and life will be good.

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jdemaris

12-14-2006 08:39:05

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Re: OT: Fuel Pump Pressure Question in reply to Dave in GA, 12-14-2006 07:39:42

I had an awful lot of equipment over the years with Holley side-bowl carbs, e.g. IH 345s, IH 392s, small block Chevy's etc. Seems I was always changing the cork metering plate and bowl gaskets - and that was with old mix, good old-fashioned gasoline. I did have a few situations with what seemed to be excess fuel pressure - from electric or mechanical pumps. Usually happened more after a hot shut-down - not while running. But - all it did was force a little fuel through the inlet needle & seats, thus flooding the engine a bit. A lot of rigs with Holleys got inline adustable regulators installed. I suspect - all you need is two sets of new gaskets and o-rings and a resetting of the float levels. I assume the materials used today are better? One set you install, and the other set gets stored in the glove-compartment.

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NEsota

12-14-2006 08:19:14

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Re: OT: Fuel Pump Pressure Question in reply to Dave in GA, 12-14-2006 07:39:42

Could it be that the carb would do fine with 1966 gas but does not like the stuff that is now marketed? Seems to me that people have blamed alcohol for attacking non metal components in fuel systems for the kind of results you have had.

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Allan In NE

12-14-2006 07:56:43

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Re: OT: Fuel Pump Pressure Question in reply to Dave in GA, 12-14-2006 07:39:42

Don't think so, Dave.

That pump is only capable of pumping somewhere around 13 lbs max, so doubt if that is the problem.

I'd rather think that the old Holley is just tired from all the under hood heat it has endured over the years.

I use lubra-plate when I put those little o-rings on. Helps to keep from gougin' 'em when ya roll the tanks into place.

Allan

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Bob

12-14-2006 07:51:06

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Re: OT: Fuel Pump Pressure Question in reply to Dave in GA, 12-14-2006 07:39:42

The Holley carb gaskets dry out and the "O" rings turn to gummy mush after a while. Happens all the time.

ith a typical automotive mechanical fuel pump, the fuel pressure is determined by the strength of the diaphragm return spring. NO WAY will the pressure output increase with age.

(The cam pushes on the diaphragm, sucking in gas, then, as the pump "comes of the cam", it is the spring tension against the diaphragm that actually determines fuel pressure.)

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RustyFarmall

12-14-2006 07:49:43

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Re: OT: Fuel Pump Pressure Question in reply to Dave in GA, 12-14-2006 07:39:42

I am assuming that you are running the original style mechanical pump? I think those are supposed to have a pressure relief valve built in, and maybe that relief valve is stuck? I think I would replace the pump again, be less expensive than under the hood fire.

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Bob

12-14-2006 07:54:24

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Re: OT: Fuel Pump Pressure Question in reply to RustyFarmall, 12-14-2006 07:49:43

Rusty,

There's NO pressure relief valve in a typical mech fuel pump.

As pressure builds, fuel pressure overcomes the strength of the diaphragm return spring, so the diaphragm doesn't return and suck in more fuel, limiting pressure.

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