'76 350: Carburetor Barely Warm but Boiling!

Old June 1st, 2019, 06:12 PM
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'76 350: Carburetor Barely Warm but Boiling!

I have worked in fuel systems for 30 years, but this is the first time I have ever seen anything like this:

This morning (which was cool so the A/C was not in use) we had driven my wife's '76 Salon with 350 4-bbl about 15 miles at moderate speeds when it began stalling at idle, requiring me to re-start it with foot-to-the-floor. When we could safely pull off the road, I opened the hood, removed the air cleaner . . . and was greeted by a QuadraJet boiling to beat the band but which was barely warm to the touch! I had never seen anything like it.

We use only 93 octane ethanol-free gas (from a local chain called Quality Plus Gashouse) in that car and we have never had a problem with it in any vehicle. My only surmise is that either I pushed to wrong button on the gas pump or somebody put gasohol into in the wrong underground tank at the station. Still, even though gasohol boils at a lower temp than "pure" gas, this was ridiculous --- I could very comfortably "hug" the carburetor with both hands!

We proceeded to the nearest Quality Plus and filled the less-than-1/4-full tank with 15 gallons of 93 octane E-free . . . and before too long all was back to normal.

What the heck is going on? Has anyone had an experience like this? Or am I the only "lucky" one?

Jeff Dreibus
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Old June 1st, 2019, 08:35 PM
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I dunno what to say. Here in Phoenix all we have is 10% ethanol fuel and even when the summer temps are 120F there isnít any issue with fuel boiling.
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Old June 2nd, 2019, 03:53 AM
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Thanks, Fun71; this came out of left field for me, too.

Jeff
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Old June 2nd, 2019, 06:23 PM
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Was it boiling or did the needle and seat get stuck open with some dirt and just flood it ???
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Old June 2nd, 2019, 06:39 PM
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Just wanted to add that a 1976 350 probably should be running on 87 Octane fuel. 93 might actually make the situation worse. You will most likely get better mileage with the correct octane gas too. If you have the owners manual see what it calls for. Any higher octane than what is called for is just a waste of money.

This is also true for any other engine.
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Old June 2nd, 2019, 06:57 PM
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I just found a copy of a 75 owners manual and it calls for 87 Octane. 76 is most likely the same.
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Old June 2nd, 2019, 07:05 PM
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Block off the heat riser path under the carb (intake removal, weld plugs into intake) install a heat dissipating gasket under the carb. Disable the stove IE remove the thermal flap in the exhaust manifold weld up the holes and adjust the choke accordingly and never look back.
Always make sure the tune is correct, no vac leaks and choke is adj properly before any extreme modification measures are taken.

Last edited by droldsmorland; June 2nd, 2019 at 07:08 PM.
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Old June 3rd, 2019, 03:23 AM
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Thanks for all of the responses!

BillK, I am actually a carb tech so no, the needle and seat are definitely not leaking. The only reason to run lower-than-specified octane in this era is because the higher the octane rating, the higher the ethanol content to "slow the burn". This issue "goes away" when ethanol-free premium is used as I do . . . or unless some bonehead pushes the wrong button on the pump (might have been me) or dumps the wrong fuel into the "E-Free" tank.

Good suggestions, drolds; I do indeed plan to install a phenolic spacer soon.

Jeff
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Old June 3rd, 2019, 05:17 AM
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You can throw a bottle of Marvel Mystery oil in your tank, run it down a bit and change gas stations.
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Old June 3rd, 2019, 05:28 AM
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Eric,

I'm not sure what Marvel Mystery Oil is supposed to do for the gasoline.

Also, very few gas stations in western NC carry ethanol-free gasoline; if the wrong gas got into the E-free tank at Quality Plus, I'm confident that it was simply a one-time mistake.

Jeff
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Old June 3rd, 2019, 07:18 AM
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Originally Posted by 76CutlassSalon View Post
The only reason to run lower-than-specified octane
I did not suggest using lower than specified octane. I said that running higher than specified octane is a waste of money and can actually hurt performance and mileage.

Originally Posted by 76CutlassSalon View Post
the higher the octane rating, the higher the ethanol content to "slow the burn".
I cant say 100% for sure but I do not believe that is true. I am fairly certain that the ethanol percentage is the same and other additives are used to adjust octane. That is assuming all of your fuel is E-10 (10%) like it is here. If your higher octane fuel does have more ethanol (E-15 ?) it would be labeled as such on the pump. I have someone I can ask to verify this.

If your owners manual calls for 87 Octane, have you tried it ?
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Old June 3rd, 2019, 11:22 AM
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BillK,

This car pings on 87 octane.

If a tank in this area was filled with E15 then it was a mistake on the hauler's part; we are not supposed to be running E15 here.

The info regarding ethanol's use as an antiknock compound was provided by an American Petroleum Institutes engineer.

Jeff
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Old June 3rd, 2019, 01:10 PM
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is your fuel line near the exhaust? pre heating it first maybe
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Old June 3rd, 2019, 04:49 PM
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Originally Posted by 76CutlassSalon View Post
BillK,

This car pings on 87 octane.

If a tank in this area was filled with E15 then it was a mistake on the hauler's part; we are not supposed to be running E15 here.

The info regarding ethanol's use as an antiknock compound was provided by an American Petroleum Institutes engineer.

Jeff
I had no issues running 87 with 10 percent ethanol and a lot of timing on my 76 350. Is the EGR stuck partially open? Only time I carb boiling issues was with 230+ engine temps.
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Old June 3rd, 2019, 04:57 PM
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Check for vac leaks and confirm all 3 timing events are working right...base, vacuum and centrifugal.

I had to perform the aforementioned heat stove & crossover delete. Once I did this my hot start & boilover issues were nixed. Car runs much better when hot and the paint stays on my intake. Its a tad cold-blooded especially with the tweaking required on the divorced choke...which you won't have with the electric on the 76.
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Old June 4th, 2019, 04:55 AM
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Originally Posted by olds 307 and 403 View Post
Is the EGR stuck partially open?
EGR has a high-temp gasket beneath it with no holes and a steel ball in the vacuum hose.
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Old June 4th, 2019, 04:56 AM
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Stan,

The fuel line is in the factory-routed location, hence nowhere near the exhaust --- but that's a good suggestion.
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Old June 4th, 2019, 05:01 AM
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Originally Posted by droldsmorland View Post
Check for vac leaks and confirm all 3 timing events are working right...base, vacuum and centrifugal.

I had to perform the aforementioned heat stove & crossover delete. Once I did this my hot start & boilover issues were nixed. Car runs much better when hot and the paint stays on my intake. Its a tad cold-blooded especially with the tweaking required on the divorced choke...which you won't have with the electric on the 76.
Actually, I converted my choke to electric when the heat pick-up tube in the crossover burned out, installing a flat plate where the pick-up assembly once was. I have not, however, blocked-off the crossover passage itself yet; I should probably do that. Thanks.
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Old June 4th, 2019, 05:50 AM
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I don't run 87 here because all our regular by law has 10% ethanol which means a drop in mpg. Where 91 premium is a non ethanol. Where is your timing set? Just strange you are getting pinging with a 7.9 to 1 350 on 87.
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Old June 4th, 2019, 09:31 AM
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Here's a thought. What if that station still had winter blend fuel instead of summer blend? The winter blend would be much more volatile in the warmer summer temps.
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Old June 4th, 2019, 11:54 AM
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Kenneth,

Thanks for the suggestion, but that is not the case. I gas all of my vehicles at Quality Plus and have done so many times since winter's end, including the Oldsmobile. I would estimate that I filled-up the Olds with that specific station's gas no more than a month ago.

Jeff
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Old June 4th, 2019, 03:28 PM
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Jeff, you didn't accidentally put in E85? I think that is 85% ethanol and only 15% gas. That might boil at a really low temp.. I dunno.... A friend accidentally filled up his brand new(1st fill up) Diesel Chevy truck with E85. DOH!!
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Old June 5th, 2019, 05:23 AM
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Greg,

Not unless the tanker driver put it into the underground tank by mistake! Quality Plus Gashouses in western NC don't even sell E85.

Besides, I think that I would have had more problems than just percolating and stalling at idle if I had put that in the tank . . . and that said problems would have occurred long before I had run it down to less than 1/4 tank! But it is an interesting suggestion . . .

Jeff
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Old June 5th, 2019, 05:32 AM
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Jeff,
Just another thought. Not sure if your 76 has a vented gas cap or not ? If the tank vent system got stopped up for some reason the hot day may have caused the pressure to go way up in the tank and overwhelm the needle and seat ? Maybe when you pulled the cap to fill it up it did something and cleared itself up ? Does it have a canister purge valve ? I remember when our Riviera was almost new there was a recall for the purge valves sticking and causing similar issues. I remember because I would not let the dealer touch it. Talked them into giving me the new purge valve so I could do it myself

Thinking out loud,
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Old June 5th, 2019, 05:53 AM
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That's a good suggestion, Bill; thanks.

The first thing I did when I realized that the carb was flooding was to run around to the back and remove the gas cap. No great expulsion of built-up pressure from expanding gasses came forth, so I knew that wasn't it.

I have worked in carbureted fuel systems for 30 years and I am always suggesting to my customers "Check your fuel tank vent" . . . whether the problem is fuel starvation or fuel tank pressure build-up.

Jeff
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