Mechanics vacuum gauge
 

Truck is a 78 351m 4 speed 4x4 Finally got a mechanics vacuum gauge. from Harbor Freight to see how well my truck runs. wanted to make sure I was useing it properly. I hooked the vacuum gauge to the vacuum tree behind the carb. I got a fairly steady 18 and was holding vacuum steady at 2500 rpm. thats want an engine in good condition should be doing right.



https://cimg1.ibsrv.net/gimg/www.for...51fbfc6f40.jpg

Looks good. Yepper, should be a rock steady needle somewhere between 18" to 20" at sea level. "Fairly steady" though isn't what we want, might mean a misadjusted carburetor, or might be something more serious if bad enough. What's it doing?

Study the vacuum gauge charts, there are a gazillion different things they will show.

Here are some fellow FTE members vacuum gauge reading indications. https://www.ford-trucks.com/forums/1...l#post16764138

It stays on 18 but the needle dose shake a bit first time I've used one so was not sure if that's ok now that I have a base line I think I'll try and rebuild the carb this weekend hopefully that will take care of the slight shake in the needle and raise my gas mileage some I'm sitting at about 5 miles to a gallon witch seems a bit low I was hopeing for something more along the lines of 10 although I've done nothing but city driveing with it for the most part so that might be part of the issue I'm suprised at how well this thing runs for how old it is

Thank you there is some good info in there

Make sure the correct Power Valve (economiser) is selected. Your vacuum gauge is a good check on this, run it into the cabin so you can keep an eye on the vacuum in different situations, particularly steady cruising on level ground. If the average cruise manifold vacuum falls below the PV opening point it will really waste some gas. You'll have to play with jetting to get top efficiency and economy.

Truck still has the stock 2 barrel I bought a rebuild kit for it off mikescarburators I'm gusseing it came with a stock power valve replacement I've read a 4 berrel will get better mpg if you stay out of the secondaries but I'm not ready to dump that much cash yet on the engine will probably wait untill I'm ready for a cam swap then swap to a 4 barrel

If you've done only mostly city driving than 5 isn't unbelievable. I only get about 7.5 city driving with a 460 engine (about 10 or so highway).