carb fuel missing

[Andyman]

I have a 77 GMC dump truck 454ci that does not get used much. The problem is that its extremely hard to get started after a week or two, and it costs a lot of money in fuel to start it up every couple days. Yesterday I removed the carb, and as suspected there was no fuel in the carb. There was also no sludge, so it does not seem like its all due to evaporation. I suspect that the carb is leaking fuel, so with the top off I filled it and waited, but could not determine any fuel leaking out of the bottom of the carb, so if it is leaking, its not obvious, and any leak is evaporating faster than it can appear. Also, I noticed that when I put fuel into the carb, fine bubbles were observed coming up from the bottom, indicating that the passages were completely dry. In this case it has been a couple months since it was last started.

My plan is to reinstall the electrical pump that was used to transfer fuel between tanks (I'm only using the auxiliary tank now, as the primary tank can not be fueled without raising the bed). I have a few questions:

1. Is it normal for fuel to evaporate from a Q-jet? If not, what could be the cause? I rebuilt the carb and it did not seem warped or anything, but it does already have newer gaskets. I can't be leaking backwards because the float needle is at the top of the fuel level.

2. If I install the pump on the flexible line near the outlet of the tank, will it pump fuel through the mechanical pump and into the carb? By pre-filling the carb it should start right up.

3. If the mechanical pump interferes with the electrical pump, should I disconnect or remove it? Is it OK just to use the electrical pump instead of both?

Thanks advance for any input.

[bake74]

     When you do get it started, does it smoke at all ?  This could be a symptom of fuel leaking into engine(the float bowl holds very little fuel when just sitting), which could be why your carb. is dry and why it is hard to start after sitting.
     To answer your question, no, fuel just doesn't evaporate when sitting in a carb.

[jaredts]

The carburetor fuel bowl is vented (it has to be to work).  Fuel will evaporate.  Any carbureted vehicle will have to be cranked a little extra if it sets for several days.  If you have a charcoal canister to catch those vapors, some of them can be burned to help start it, but this is a percentage of what evaporates.  A few seconds of an electric fuel pump running will fill the bowl back up without having to crank the engine if you want to go electric.  I usually just make sure I have a good battery and live with the cranking.  If your starter or battery is marginal you can pour a very small amount of gas down the carb. to minimize the issue.

[bd]

Fuel does evaporate, but very slowly.  Even though you couldn't see it, fuel probably leaks internally past the well plugs.  It's a problem that has plagued Q-jets since their inception. 

[Andyman]

thanks so far, but doesn't answer my question if fuel will pass through the mech pump before cranking the engine. I've tried priming with gas directly into the intake, but it takes a few tries before the carb fills, and runs really rough. I suspect one reason why it runs so rough when trying to start it this way is because it takes a couple minutes to fill all the passages in the carb. After only a few days it starts right up without any rough running, and there is no black smoke.

I was initially concerned with fuel leaking into the oil and diluting it, but my guess is that it prob. evaporates before it can get that far. Yeah the battery is not that great (usually drains it before it starts), but even a big new battery would have a hard time cranking a big old engine that much, and it is hard on the battery too. Its too much work to start it this way- with having to recharge the battery and prime it 2 or 3 times, and then it runs like crap for a couple minutes, which can't be good.

Just looking at the carb it seems obvious that evaporation (and possibly leaking) is going to be a problem, esp. on something this old. I've considered buying a newer carb, but not sure what would be a good choice. Certainly not a racing (Holly) carb. Prob. not  worth it unless I get a lot more work for this thing.

[enaberif]

If the mechanical fuel pump has a bad diaphram the fuel will dump back into the oil.

I am having that issue myself right now and when it does start it smells super rich and dumps a ton of blue smoke.

[Andyman]

If the mechanical fuel pump has a bad diaphragm, does it still work enough to run?
When I disconnected the fuel line from the carb, it did not have any fuel in the filter, indicating that the fuel is draining back into the tank, or perhaps into the engine? I'm not getting any black smoke.
I'm not sure if there is an internal check valve in the mechanical pump itself, because if there isn't then fuel in the line could slowly drain back into the tank once the float goes down enough to break the needle valve seal, making it even harder to get enough fuel into the carb to start the engine. If fuel does travel that easily backward through the pump, then it should go forward too, right?

[enaberif]

If the mechanical fuel pump has a bad diaphragm, does it still work enough to run?
When I disconnected the fuel line from the carb, it did not have any fuel in the filter, indicating that the fuel is draining back into the tank, or perhaps into the engine? I'm not getting any black smoke.
I'm not sure if there is an internal check valve in the mechanical pump itself, because if there isn't then fuel in the line could slowly drain back into the tank once the float goes down enough to break the needle valve seal, making it even harder to get enough fuel into the carb to start the engine. If fuel does travel that easily backward through the pump, then it should go forward too, right?

My truck will start and run fine. Then I drive around a bit and each it will start ok. Then all of a sudden it won't start. I have to essentially put my foot on the floor to start it then it goes and blue smokes dumps out and smells rich.

[bd]

To repair Q-jet internal leakage, gently peen, reseat and burnish the four primary and secondary fuel well plugs then shroud them in a thin coat of Devcon Plastic Steel.  To control fuel drain-back install a new paper fuel filter element that contains the integral check valve.

Carburetor inlet pressure should be 5 to 7 PSI.  The mechanical fuel pump houses two check valves - one inlet, one outlet.  If the electric fuel pump builds ~7 PSI pressure, it should push fuel through the mechanical pump.  But, if you are going to use an electric pump, run it directly to the carburetor through a pressure regulator adjusted between 5 and 7 PSI, and bypass the mechanical pump altogether.  At the same time, install a supplemental inline fuel filter so that it's easy to access for future service.

To check for fuel in the oil, let one drop of oil from the dipstick land on the "clean" pad of your finger.  If the oil quickly tracks along the friction ridges (fingerprint), the oil is saturated with fuel - if it holds together in a tight puddle, it's fuel free.

[Andyman]

thanks for the info. I did not see any cheap way to put a 5-7 psi regulator on, and my pre-test shows that the pump can easily pump fuel through the mechanical pump and into the carb.  So I just touched the pump wire to a terminal on the fuse box for about 30 seconds, then let it set for a couple minutes so the internal passages could fill too, then left the new electrical pump disconnected.  It started right up without the usual misfiring (from sitting for more than a couple weeks). Success! 
Ps I'm still not sure exactly where the fuel is going or why. To my surprise, when I disconnected the line coming out of the fuel tank- it had no fuel in it- when I was expecting to get soaked. Seems that it's draining back into the tank somehow? All I know is that I found a way to get it started without cranking it over and killing the battery every time.

[Andyman]

Should mention that it does have a new filter in the carb with the internal spring and check valve. It does seem like that check valve is not working properly. For whatever reason the fuel is draining out of the lines, so previously it was taking a long time to get the mech pump primed and supplying fuel to the carb.