Reader Question: Feed ’78 Mark V Ethanol?

Here’s the latest reader question, along with my reply!

Gary asks: Should I only use ethanol-free gas in this car? Tuned up, carb seems ok but mileage (5.4 MPG) is really bad. Let me know. Thanks a lot. Listen to you on Real News.

My reply: I’m not surprised by the mileage drop; your car’s engine (and carburetor) were set up to run on 100 percent gasoline. Using E10 will result in a noticeable mileage drop because the 10 percent ethanol in the fuel contains less energy than 100 percent gasoline.

If you use unadulterated gasoline, you ought to see a noticeable uptick in mileage. You might even get up to 11 or 12 MPG! (The 5.4 MPG you mentioned is appalling even for a mid-70s American boat with – I assume – the 460 V8.)

I also strongly recommend having your car’s carburetor rebuilt with ethanol-compatible parts (float, gaskets, accelerator pump) because the ethanol is corrosive to the original-spec parts and will literally dissolve them over time, causing problems as the disintegrated pieces make their way through the fuel system.

I’d also preemptively replace the fuel pump (assuming it’s original or was made before the ’90s) for the same reason.

And I would re-jet the carb a little richer to adjust for the inherently lean ethanol-laced fuels.

If you can find and use unadulterated gas, no worries – scratch all the above.

Finally, check for exhaust restrictions, especially the catalytic converter. The early units were horrendously inefficient and if yours is clogged, it would help explain the 5.4 MPG. If it were me, I’d get rid of the cat altogether or at least replace it with a modern high-flow one.

Great car, by the way!

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