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521 fuel delivery issue?


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so, truck i've noticed these past couple days when you accelorate it is more and more hesitant and sluggish. i thought it was choke, fuel filter is recent, but why i think its fuel is now you i have to hold gas to floor and it barely idles! *chug-chug-chug* u put in gear it stalls..


looked at bowl glass, its empty...




thanks in advance



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"looked at bowl glass, its empty..."


No carb change is going to fix that problem.


Step one, take the fender mounted fuel filter off the truck, and see if it has a lot of crud in it, and I will almost guarantee you it will. In an emergency, while it is still full of gas, shake it, and while holding it upside down, with the inlet side lowest, blow air into the out port, and it will push a lot of the crud out of the fuel filter.


I have had the fuel pickup in the gas tank get clogged. One way to temporarily fix that is to blow air into the fuel line going to the gas tank, that you just took the fuel filter off of, and that will dislodge crud stuck to the fuel pickup. This is a temporary fix, the crud will collect around the fuel pickup again, until you clean the tank.


now, hook everything back up, and see what happens.


Here is how to test the fuel pump. There are two tests.

First test:

Put a "T" in the fuel line between the fuel pump and carburettor. the fuel pressure should be between 3.41 to 4.27 pounds per square inch.

Second test:

Disconnect the fuel line fron the carburettor, and put it in a container to catch the gas. At a 1000 RPM, you should get at least 1000 CC of fuel a minute.

Your engine may not run that long without the fuel line hooked up. 500 cc for 30 seconds, 250 cc for 15 seconds. A 1000 cc is a little over a quart. 500 cc a little over a pint, 250 cc a little over a cup.


Original stock fuel pumps are pretty reliable. If it does not pass the flow test, on the truck, I would disconnect the pump from the filter, and get a short hose into a gas can, and try it again. Isolate the stock fuel lines on the truck out of the possible cause.


Your truck is 40 years old. It has had 40 years to accumulate crud in the gas tank. Most gasoline now also has ethanol in it. You may need to remove the gas tank, and clean it out.

The ethanol in the tank will dissolve stuff that built up in the tank before gas had ethanol. it will also dissolve any water that was in the tank, and on a 40 year old truck, you did have some water in the tank. All this crud will get moved to the fuel filter.


Once you get the fuel tank cleaned out, ethanol in 10% should not cause problems.

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My 720 with an L motor did that too. I replaced the carb and it went away. Every time i gave it to much gas or floored it going up a hill after a minute it would starve itself for fuel.


Float was set too low.....course changing the whole carb works too. :P


Radio, the easiest thing to do is change the filter. It's the most obvious cause and even though you say it's new ish they can bung up in 20 miles of driving if the tank is dirty. It's a cheap easy way to eliminate the filter as a cause and way easier than a carb change.


If that doesn't do it, disconnect the pump outlet and run it into a pail. Crank motor there should be a strong gush of fuel


Yes there is fuel at pump: The problem is in the carb. The inlet screen is plugged or needle valve stuck. Float may have stuck in the up position. Look in front of carb, float up or down?


Float Up: Tap carb lightly with hammer or remove sight glass and fix stuck float.

Float Down: Remove float and inlet valve and screen and clean or replace.


No fuel at pump: Since the filter was changed, you did change it right?, there must be a blockage somewhere back to or in the tank or the tank wasn't filled this week.Look at gas gauge with the key on...


No gas reading: rap the tank with your knuckle, does it sound empty or does it sound like it has fuel in it?

Yes sounds empty: Put gas in tank .

No sounds full: Finish fixing fuel delivery problem then replace gauge or sending unit


Yes gas reading: rap the tank with your knuckle, does it sound empty or does it sound like it has fuel in it?

Yes sounds empty: Put gas in tank, replace gauge or sending unit.

No sounds full: Continue fixing fuel delivery problem.

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I have been hearing and seeing(myself) a lot of fuel issues lately, I have two tanks in my 521 work truck, I can't even use the spare tank anymore, if the gas sets in it for even a month it goes bad. The fuel being sold nowdays is crap, if you don't burn it right away it separates into two layers, a darker murky golden layer on the bottom, and a lighter cleaner layer on top. Needless to say, the lower layer will not run my vehicle. I beleave the only way I will ever be able to use the second tank is to seal it with a modern tank sealer so the gas(if you must call it that) can not touch metal again, as it is eating the insides of my tank out. I guess this is one way to get rid of all the older vehicles. wayno

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