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Slopey1939 09-11-2019 11:20 AM

Sudden Drop in Gas Mileage

I hope someone out there can give me a little advice on this issue. So, a 39 Fordor Sedan Deluxe, flat head V8 (motor might be later 40's) normally getting about 14+ miles to the gallon highway suddenly dropped to about 7. Symptoms are: idling a little bit rough, vacuum gauge shows 1 inch fluctuation from 20 to 19 and back, more when idle increased, more soot than normal out of exhaust pipe, some surging when being driven, plugs are black as heck but dry. My guess is a carburetor issue. Has been at least 15 years since a rebuild. Any chance of fuel pump leaking internally? No outward fuel leaks anywhere.
Thanks very much for any help you can provide.

Kurt in NJ 09-11-2019 11:46 AM

Re: Sudden Drop in Gas Mileage

Fuel pump leaking internally usually raises the engine oil level and makes it smell like gasoline

Perhaps the power valve in the carburetor--- that also would cause long cranking time because the fuel bowl would be empty

mfirth 09-11-2019 11:49 AM

Re: Sudden Drop in Gas Mileage

Power valve on the carb. or fuel pump diaphram are both possibilities. Check & smell your oil for gas. Power valve would make it run rich 4 sure.

Mart 09-11-2019 11:53 AM

Re: Sudden Drop in Gas Mileage

Could be one of the many reasons a carb runs rich. I'm only familiar with the 94 carbs. If you have a 94 you could check for:
Punctured float.
On seating needle valve.
Ruptured power valve.
incorrect or failed power valve gasket.
Vac leak into power valve chamber. (lower and middle card bodies warped).

I'd be looking at all those.


scicala 09-11-2019 01:43 PM

Re: Sudden Drop in Gas Mileage

Has all the symptoms of a power valve in the carb going bad.


Slopey1939 09-11-2019 07:25 PM

Re: Sudden Drop in Gas Mileage

Thank you all for your suggestions. Seems the power valve (general consensus) could be defective although it starts right up if it has been run in the last 4 or 5 days. Otherwise I use a quick blast of my electric fuel pump which would nullify that issue. Will also look into the fuel pump and gas effect on the oil.
Thanks again all!

keith oh 09-12-2019 01:36 PM

Re: Sudden Drop in Gas Mileage

Any obstruction of the air filter such as dirt, or the wrong filter will affect mileage. Even a worn choke butterfly bearing surface can cause air flow obstruction. Also make sure the choke butterfly is opening completely vertical. these simple problems are often overlooked.

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