Original Mechanical Fuel Pumps

Carbs & F.I.

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asiab3
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Location: San Diego, CA
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Re: Original Mechanical Fuel Pumps

Post by asiab3 » Tue Apr 14, 2015 1:53 pm

Latest post of the previous page:

hambone wrote:How is that 2.8 PSI working out? I don't like it.
Mine is working lovely around 2.5-2.8psi. I have not noticed any overly-rich hot starts, nor any spike in CHTs in long third gear climbs. I have not, however, climbed at 4,000 RPM for more than three miles at a time.

This same engine on my old dual port was run at 5psi for a long time. Hot starts were maddeningly rich, but I do think that there is more to it than JUST the reduction of fuel pressure. The new engine runs with the surface temp of the fuel pump, case, and carb/manifold cooler, so there is less boil-over due to heat. There is also a new needle valve in the carb, so I think bets there are off too.

Robbie
1969 bus, "Buddy."
100k miles with me.
279k miles on Earth.

kreemoweet
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Re: Original Mechanical Fuel Pumps

Post by kreemoweet » Wed Apr 15, 2015 2:54 pm

asiab3 wrote: There is also a new needle valve in the carb.
I'm beginning to think those "needle valves" should be replaced at a regular interval of, say, every year or two. Not too long ago I disassembled 5 valves
I happened to have, and found: one, probably from some rebuild kit, had a ball bearing tip instead of a needle; one with non-Solex markings, but otherwise
of identical construction, in good shape; and three Solex valves with a very pronounced wear ring where the "needle" met the seat.

The "needle" is actually an approx. 60 deg. cone that seats on a fairly narrow chamfer at the valve orifice.

While those worn valves probably do an OK job of regulating fuel bowl level underway, they may well leak enough at hot shutdown or when subjected to
fuel tank gravity pressure to cause problems.

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asiab3
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Re: Original Mechanical Fuel Pumps

Post by asiab3 » Wed Apr 15, 2015 5:16 pm

I have speculated wildly that there is a variation in needle valve quality going around. It would be interesting to test somehow, though I don't know how we'd do it.
1969 bus, "Buddy."
100k miles with me.
279k miles on Earth.

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Amskeptic
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Re: Original Mechanical Fuel Pumps

Post by Amskeptic » Sat Apr 18, 2015 7:26 am

asiab3 wrote:I have speculated wildly that there is a variation in needle valve quality going around. It would be interesting to test somehow, though I don't know how we'd do it.

Consider this:

I have only filled my crankcase with gasoline when I installed new needle valves and went to bed
*without starting the engine*.

Fuel pressure, paradoxically, helps the valves close. Fuel rushing in will push the float up against the needle valve with some verve.
Lack of pressure can allow fuel to trickle in. You can park a car, fuel evaporates, float drops, fuel is allowed to trickle in due to heat soak or full gas tank, and the needle valve will never be pushed enough to close.

The only crankcase floods I run into out in the itinerary is due to contamination.

You can test a needle valve by blowing into the nipple and pressing lightly on the tip. I suggest exercising a new needle valve in this manner, but with more closing pressure to help "lap" it. Blow hard all the while, press/release/press/release/press/release, etc.
Colin
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 93,996 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,113 miles

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