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Sputtering once warmed up
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MoPor
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Joined: December 03, 2010
Posts: 292
Location: Canada
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PostPosted: Wed Apr 07, 2021 8:42 pm    Post subject: Re: Sputtering once warmed up Reply with quote

I had a similar issue once. There was a head to cylinder leak. The hot combustion gases caused the fuel to boil. You could hear a slight popping at idle. The machine shop somehow bored one side of the head deeper than the other.
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Matt

'58 356A coupe
'00 Boxster S
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jvaleski
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Joined: May 19, 2020
Posts: 6
Location: Boulder, CO
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PostPosted: Sat Feb 24, 2024 8:45 am    Post subject: Re: Sputtering once warmed up Reply with quote

this has been quite the saga. multiple mechanics, multiple theories, multiple "fixes." things are finally to a decent state and I thought I'd update here.

in the end, I believe the issue is some sort of vapor-lock/pressure amalgam. my configuration is a bit unique.

porsche 356b motor transplanted into a '65 vw bug. I live in CO, USA (1mi above sea-level). the original config had the original VW bug fuel tank, and fuel-line, feeding the 356 engine/pump.

the most recent mechanic replaced the tank (including the fuel inlet nozzle and associated breather valve), and said fuel-line. He was starting to think there was a pressure issue with feeding the pump/carbs, so he wanted to get new flow in the loop. He also did some foil heat disbursement on the lines in the engine bay (from pump to carbs mostly) to dissipate heat. I'm less convinced this was the issue (I'd tried various heat diffusion methods before on my own to no avail), but, he did some anyway which is fine.

the issue hasn't presented in many drives of many durations/temps. I'm pretty sure it was mostly a pump/pressure issue, exacerbated by potential temp problems, and that the new end-to-end fuel line setup changed things just enough to make the system happy.

I read somewhere that the Zenith carbs are super sensitive to pressure, and will stop taking fuel if they get even slightly above where they're supposed to be.

whether the issue was pre-pump (in the fuel lines/gas tank/bleeder-balance nipple on the neck of the fuel inlet on the tank), or post-pump (vapor lock'ish) remains a mystery, but, I think I've personally resolved it was pre-pump. I'd had the pump rebuilt several times, and replaced outright a few times (different mechanics wanting to use "their supplier/guy").

if this flares up again, I'll go down the path of fully replicating the tank/fuel-lines from the original Porsche 356 to feed the pump/motor as originally intended. my gut says its mostly in there... not vapor-lock.
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