Rebuilt Motor Suggested Break-in procedure
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Jesse Ybarra has published a well thought and thorough article on mechanical verses electric fuel pumps and their effects on a freshly rebuilt motor.  Sit down with your favorite beverage and get ready to know everything you'll need to protect that high dollar engine that you just rebuilt and are ready to fire up.  Thanks so much for the information Jesse!

In an FC a mechanical fuel pump at cranking speeds has very minimal priming suction or vacuum, it works well if it is started regularly, in cool temperatures, but gas evaporates with any warm days, even after it starts, at idle it has low volume, low pressure until it fully starts up and all the lines, filter, and carburetor bowl are all filled. This is because of how the function is of a mechanical diaphragm rubber over spring actuated fuel pump, as found in the older fuel pumps of all FC's were never designed to operate on today's unleaded research additive octane mentholated fuel. A new or rebuilt fuel pump is the best you think you are doing, however after testing 6 new mechanical fuel pumps on a test motor only 2 each from different suppliers, were consistent to 4 lbs after warm up, one from Walcks, Napa, 4 wheeler supply, the reasons are, from when were they assembled, how long were they on the shelf, was the diaphragm made of rubber or neoprene, is the spring pressure / diaphragm able to pump liquid fuel only with or without vapor, but be confused with new vapor methanol fuel, these tests were on F 134 fuel pumps with low octane unleaded fuel, after. With a mixture of high octane unleaded mixed at 18-1 with 2 cycle oil and lead substitute added, I got 6 psi from the worse pump from, that was from Walcks.   

Unlike the electronic fuel pumps, in new vehicles, when an FC is started you pull the choke, it cuts back the air mixture, the carburetor accelerator pump, in the primary circuit makes a spray, out of what is still in the bowl, because it is an open vented system, the lines are only full when the engine is running, (the check valve in the electronic pumps hold pressure) not when it starts, the choke is pushed in, as of yet they do not have enough pressure to the carburetor needle and seat, so you let the clutch out and the engine dies, so that sometimes the engine needs to be restarted with more choke until the engine warms up. In later motors automatic chokes did the same think only automatically holding the choke in.
Newer vehicles even at first idle they can be given full throttle because the fuel management system allows for immediate high volume.

Hopefully when they started the engine the first time, they filled the fuel vent tube with gas to fill the fuel bowl and poured some down the carburetor, at idle or slow rpm a motor like yours should run this, way for 15 seconds.

Caution! You have a new freshly honed and bored block, the rings are new and not broke in. those rings and the valve guides are not broke in they told you that the motor is tight, it only takes a few minutes to cause a lot of premature damage. It is the fuel that starts the engine, and the fuel is supposed to prevent the cylinder walls from being dry, with today's fuel that has no lead in it, cylinder walls are able to only get marginal lubrication until they start then the piston oil rings keep the oil on and off the cylinders, if it does not start and run and be able to stay running for a break-in, don't keep cranking on it, the rings will get damaged. this is why, I always use an electric fuel pump, and always uses a premix fuel, to prevent dry starting the cylinder walls, next bad issue is break-in, the cam needs oil pressure, to have the oil on the lobes, or they go flat, after a new engine is started, it needs to be run at inconsistent speeds with light load in order to seat the rings and break in the cam lobes, the crank and rods sit in oil all the time, yet this is what people worry most about.

Bad fuel pump? Unless you have a fuel pressure gauge you are hoping that it makes enough pressure, as needed, when it is needed.

A. The correct carb fuel / air mixture is corrected at an experienced guess and by looking for it to run smooth, by adjusting the carburetor fuel air mixture adjusting screws, this is to correct the burning of the fuel in the engine, there is no way of actually knowing through the different speeds and load needs, without a sniffer computer, so after a vehicle is run for a few months, assuming the timing is correct and the engine has been operating at normal temperature ranges, the color of the spark plug tip tells you if you have the proper fuel mixture. If there is metal beads on the plugs, it is to late.
Because of the small inadequate output of the FC motors, most the power / torque needs happens at the low rpm, this means the fuel needs to be there, with your foot down, the motor fuel mixture sounds the same after 600 rpm, even if it is putting out half the pressure, because the fuel pump relies on rpm, however it may be only putting out only 1.5 pounds and running lean, at this point the engine temp goes up, people think they need a new radiator, fan or water pump because the engine is burning up metal.

B. Because the mechanical fuel pump is seldom ever changed, unless it is leaking to the outside and shows a drip, it is assumed that it is fine, because the actuation happens by mechanical movement, most of the time, if there is a leak, rarely is it to the outside, most of the time they leak inside, this means, fuel leaks into the crankcase, as the fuel leaks into the engine, it is still running fine and running lean, what happens next is devastating, after the engine is shut down, so is the pressure, then the leaking fuel pump, drains gas into a crankcase, the truck sits for a while, and you go to start it up, because fuel has some water, gasohol, methanol, ethanol, the oil floats on top of the fuel in the crankcase, at the bottom of the crankcase is the oil pump pickup tube, as the engine is restarted the next time, fuel diluted oil is then pumped up into the engine, the contaminated oil goes into the upper end when lubrication is needed the most. After it comes out of the engine, oil goes into the oil filter, so now you have filtered oil again, after a little while, that clean 10-40, has lost most of it's quality, and yet the filter traps the dirt but allows the oil and fuel to mix better, as the engine runs, so does the temperature, burning the fumes into blow-by, then destroying the valve seals. Because the new fuels have quick vapor evaporation, there is almost no smell on the dipstick, because these engines have open air breathers.
C. A fuel pump normally works with fuel on both sides, however, nobody ever pre-primes a fuel pump they rely on the slow moving diaphragm, sucking all the air out until a siphon starts, then the gas comes up while cranking a dry engine. Here is the problem some fuel pumps create the siphon sooner than others some can't get past the air. Example to why some motors will vapor lock and quit running because the gas is boiling.
D. Fuel pressure of a mechanical pump as it is measured in working psi, actually it is measured after the float is full and the needle and seat stop the flow, so a measurement of a fuel pump with a gauge at the end of a hose gives the static pressure, unfortunately, as the motor is in actual operation, under a load the needle and seat are wide open, at high rpm Max fuel pressure is made by the fuel pump because mechanically it is being stroked as much as it can put out, at lower rpm when going through the gears from standing still, worse under a big load, now you have vert low rpm, most of the torque is required, so the throttle is fully open, the fuel pump is not moving very fast, but only at the rpm of the motor. The needle and seat are open because the float is down regardless of if enough pressure is there yet the carburetor is at full throttle.
E. A mechanical fuel pump in theory works fine, in an FC-150, when at idle, the fuel pump is at full pressure, but after the throttle opens the pressure drops, at the time you need the most pressure, until the rpm matches the consumption, if a fuel pump is faulty the engine will appear to be running fine, but it is burning lean during that period the throttle is open, once the rpm catches up it doesn't need the full pressure any more, so it will appear to be working fine..
F. A lot of engines would only last under 60,000 miles years ago, as a result of needing rings and valve jobs, that was because they were burning oil, these engines sometimes were overhauled or rebuilt, but would never last as long as the original. Typically because the carburetor and the fuel pumps didn't leak so they put them back on. Toadies vehicles can and should last 400,000 if serviced properly, no mechanical fuel pumps on them and the spark plugs last for 100,000 miles.
The older vehicles carb throttle shafts get a lot of wear, letting extra additional air to change the fuel air mixture, we already went over the weak fuel pump diaphragm output at low rpm issues, now imagine your FC fully loaded with say firewood, you are in low gear going up a long winding hill, but it is steep, you have all your throttle, and yet your rpm is only 400, but the fuel pump is barely keeping the bowl full, worse, the temp is going up, this goes on for a while soon, you have blow by out the breather and oil starts dripping, because when you adjusted the fuel air mixture, it really didn't do much even at idle you are running lean, because those idle screws are useless because of the air coming in from the worn throttle shafts.  

A better example of how a mechanical fuel pump works is a wiper motor, that fuel pump sucks the gas under siphon / vacuum from the tank then pushes the gas / pressure up to the carburetor, it is the same on a double action fuel pump, these have the vacuum pump on top, these are on the FC-170 and a few FC-150s, or look at manifold vacuuming on a rainy day the vacuum wiper works fine at idle, depending on where the throttle is the vacuum wiper will stop working because of a lack of vacuum, a fuel pump needs to be good enough to able to suck and push at least equally to what it is requiring under a constant pressure.

If you have understood how the mechanical fuel pump works, then the same is the example of an electric fuel pump to an electric wiper motor, these two different components are never effected by rpm or throttle position, both have consistent regulated output, for the fuel pump, as the key is turned on, then instantly fuel fills the carb, it stays full no matter where the throttle or rpm is, until the key is off, if you are running out of fuel it will run at a loud high speed, as it warns you that, it is starving, rather than your engine is melting.  


Electric universal fuel pump.
Any auto parts like an Autozone, O'Reilly's, Napa, Pep Boys, will carry a universal electric fuel pump with 5/16" inlet/outlet, you need a pump that has 4 lbs outlet pressure, but not more than 6 lbs.
Buy a new metal or plastic universal 5/16" fuel filter, not a glass reusable type.
Buy at least 6 fuel clamps, stainless American made screw type clamps.
Buy 3' of 5/16" fuel hose, preferably fuel injection hose, it lasts longer with the new fuels, later after the break-in replace all of the existing hose, buy more hose if you need more hose, after you read all the following procedure. 
To the new electric fuel pump attach the new fuel fittings, some of the fuel pumps come with a fuel filter already on them, take it off, use Teflon tape on the fittings.
Remove your old filter, if it is after the fuel tank, before the mechanical pump, to mount the new electric pump, near where your old filter is, drill 2 holes into the frame and mount it, make sure you observe the direction of the arrow on the new fuel pump, or if it says inlet /outlet, mount with one wire, the ground (black) wire to one of the frame mounting bolts, the other wire mounts to the 12 V side of the ballast resister, the ballast resister, is before the + side of the coil, add a short hose, then the new fuel filter, a new 5/16" hose, to the fuel line going directly to the carburetor.
New engine break-in.
I have a boat tank from an outboard motor, that I use only for starting motors, it is 5 gal, I put it in the bed, I use the hose that comes out of it, These tanks are usually on Craigslist or garage sales or barrow one, or use a regular tank, with more new hose, (but use care it may tip over) I put the tank close to the new pump, connect that hose to the inlet. Then start the motor as it warms up check your timing, run the motor through the break-in period, at least 10-15 minutes continuous, or more as we previously discussed, before shutting it off, only off this tank. Using this tank with 5 gal of premix, in the tank, prevents any chance of a clog or water from the truck tank. Expect a little smoke, 18-1 premix but none should be visible, after the choke is pushed in.
After the motor is broke in.
If your stock original tank has been boiled out, and is clean, then hook it up to the inlet of the new fuel pump. If you think the tank is clean and has recently been run within the last few months, then disconnect the hose after the electric fuel pump before the filter and place a different hose on the outlet side to drain into a glass sample bottle like a soda bottle. Now you have a drained a clean sample from it, let it sit over night, and look for any sediment, or rust that has separated. If you see any moist clouds, rust, dirt or yellow varnish, then remove the tank and have it boiled out at a radiator shop, they may tell you that it will destroy the bladder, but FC tanks have no bladder. After that put it all together, by pass the mechanical fuel pump. Hopefully you buy an electric wiper from Craig, if not and you are getting the vacuum from the fuel pump, get the Vacuum from the intake manifold.  
Fuel, during the rest of the break-in, the next 500 miles.
If you paid to have hardened valve seats, use only High Octane grade unleaded, if you did not have hardened valve seats installed then add a lead substitute, follow the mixture directions carefully.
These motors will run fine on unleaded low octane, if it has hardened valve seats, later after it is broke in. But it will continue to need lead additive, if it does not have hardened valve seats.   
Later if you want to run a new mechanical pump, hook it up inline, if not, eliminating the mechanical fuel pump, remove it, use the block off plate, that is commonly sold for a Chevy V8. 

A Great 12 volt fuel pump from Carquest Auto Parts

This 12 volt in line unit the can mount on the passenger side frame rail just past the fuel filter.
Part # E8016S
Rating:  2.5 to 4 PSI.