Carburetor drip anomaly

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asiab3
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Carburetor drip anomaly

Post by asiab3 » Tue Mar 01, 2016 5:45 pm

I used to disbelieve in ghosts.

This week I helped a friend install some speakers in his dual carb '73 bus. When he parked at my place, I smelled fuel after a few minutes. Opening the hatch, I saw a steady but small drip from the right carb throttle shaft front bushing. Speakers were put on hold while I ripped apart the carb, cleaning out the passages, ensured the gaskets were correct and sealing, blew through the needle valve with Gum-Out, and checked bowl fuel level. Nothing stood out as unusual, so I reassembled, and found the drip-drip-drip after a few seconds of shutoff. It's occasionally present at idle, and it is pronounced right after shutdown, probably due to the lack of airflow swallowing up the fuel.

https://www.youtube.com/watch?v=pDlHlD-3c7U


Startup yielding a couple seconds of dry running, then I saw a slight drip. As a reached for my screwdriver, my friend shut it down and I cussed as the screwdriver rolled down my long and steeeeep hill. Wait a tic, what is the incline limit for a bus with factory dual carbs parked facing up the hill? With confidence, I told him "Ah yes, see the problem is not you, it's me." I ASS-UMED that my checks were good and his bus would be fine when not parked at such a slant that we needed wheel chocks...

Speaker install went fine. Until I got a phone call after he got home.
"She smells like gas and dribbles fuel on flat ground doofus."
"Ok, be right over."

This time, with my tail between my legs I swapped needle valves left to right to see if the problem follow. The problem completely ceased. I was hoping the drip would follow the needle valve and we could call it a day and a six dollar bill. Or maybe the drip would stay and I would find a bad accelerator pump check-valve or something. But no. The problem is gone, and that actually bothers me. Why is the issue gone??

So the bus is on probation, until he drives it for a week and checks for drips at Every Shutdown. We're two days into dry shutdowns now, but I want to know why my diagnostic step seemed to fix the problem.

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

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SlowLane
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Re: Carburetor drip anomaly

Post by SlowLane » Wed Mar 02, 2016 8:33 am

Amount of fuel in the tank have any bearing on the drips? Evap system intact and functional?
'81 Canadian Westfalia (2.0L, manual), now Californiated

"Wisdom comes from experience. Experience is often a result of lack of wisdom."
- Terry Pratchett

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asiab3
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Re: Carburetor drip anomaly

Post by asiab3 » Wed Mar 02, 2016 1:18 pm

SlowLane wrote:Amount of fuel in the tank have any bearing on the drips? Evap system intact and functional?
It was about 3/4 tank the first time I noticed, but it continued to drip well on its way to 1/4 tank. The tank is vented to atmosphere, so no excess pressure build up. I'll see if my spare charcoal canister will fit and if I have enough spare hose to make it happen.
1969 bus, "Buddy."
100k miles with me.
279k miles on Earth.

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Amskeptic
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Re: Carburetor drip anomaly

Post by Amskeptic » Fri Mar 04, 2016 8:56 am

asiab3 wrote:
SlowLane wrote:Amount of fuel in the tank have any bearing on the drips? Evap system intact and functional?
It was about 3/4 tank the first time I noticed, but it continued to drip well on its way to 1/4 tank. The tank is vented to atmosphere, so no excess pressure build up. I'll see if my spare charcoal canister will fit and if I have enough spare hose to make it happen.
Electric fuel pump retrofitted buses do seem to suffer this classic PDSIT dual carb behavior. The stock fuel pumps with the cutoff valve in the top of the pump do not.

I got rid of the drip by dropping the fuel level by means of a thicker needle valve. He will still suffer fuel smells and rich starting on hot days because such is life.
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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whc03grady
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Re: Carburetor drip anomaly

Post by whc03grady » Sat Mar 12, 2016 9:45 am

Ludwig had a somewhat janky electric fuel pump and a Weber regressive. Never had a drip. When Colin and I retrofitted a pair of Solexes, the dripping started. (Twice it filled the crankcase.) Colin (correct me if I'm wrong) surmised the pump itself might be at fault somehow? I'd gone through several needle valves to no good effect. We replaced the janky pump with a less-janky one. Not a drip since. <knocks on wood>
Ludwig--1974 Westfalia, 2.0L (GD035193), Solex 34PDSIT-2/3 carburetors.
Gertie--1971 Squareback, 1600cc with Bosch D-Jetronic fuel injection from a '72 (E brain).
Read about their adventures:
http://www.ludwigandgertie.blogspot.com

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asiab3
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Re: Carburetor drip anomaly

Post by asiab3 » Sat Mar 12, 2016 7:22 pm

whc03grady wrote:Ludwig had a somewhat janky electric fuel pump and a Weber regressive. Never had a drip. When Colin and I retrofitted a pair of Solexes, the dripping started. (Twice it filled the crankcase.) Colin (correct me if I'm wrong) surmised the pump itself might be at fault somehow? I'd gone through several needle valves to no good effect. We replaced the janky pump with a less-janky one. Not a drip since. <knocks on wood>

Iiiiinnnnterrestiinnngggg. I'll have to vett the mechanical fuel pump on this '73. I know it's factory-style mechanical, but I don't remember if it is original or an aftermarket replacement. Hasn't dripped a drop in two weeks of shutdowns. I'm following it to the mountains next week in my '76, I'll see how it does up steep grades at elevation compared to the 2.0 FI beast. :geek:

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

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