Where is my gas going?

[xsdg]

My '85 720 (Cali emissions, 6-wire carb) is hard to start every morning, and I've finally figured out that it's because the fuel bowl is empty every morning.

 

Where's the gas going?

 

Especially given that this happens pretty much regardless of temperature, I'm pretty sure it's not evaporating, which means that it's leaking somewhere.  When I've noticed this, I've tried to look under the bowl and haven't seen any obvious signs of leaks.  When I get a chance, I'll get in there with a mirror and a flashlight to give it a closer inspection.  But for now, I'm wondering if there's anything I should be considering other than the large gasket around the float bowl front cover.

 

Also, I should note that I don't know for certain that the bowl is empty.  I know that the fuel level is way below the sightglass, and that the float itself is angled down at a pretty steep angle (so the fuel level is definitely really low in there).

 

Once I get everything primed in the morning, the truck runs and starts great for the rest of the day.  Hot, cold, rainy, whatever.  But every morning, it's hard to start until the fuel pump runs for a bit.

[Ranman72]

make sure the throttle is returning and not hanging open allowing fuel to drain 

also check fuel hoses from pump to the carb if they are bad at all it can allow a siphon effect in the fuel bowl 

I also believe there is a check valve/ball or something like that in the fuel pump to keep the fuel from gravity feeding backwards 

[Charlie69]

Weber!

[datsunaholic]

  The most common external leaks are the bowl cover and the jet plugs, but you'd see those leak.

 

Generally the fuel leaks into the manifold so you don't SEE the leak, as it's internal.  Then it can be a whole lot of things, from leaking through the jets to power valves or even cracks in the casting or a loose power valve.  Since my 720 has an electric fuel pump and stock carb it'll start up even after sitting for YEARS (started it last month after sitting for a year and a half, before that was 4 years) but my mechanical-pump equipped 620s will take 30-45 seconds of cranking to fill the bowls just sitting overnight (a problem I've had with all my Webers, as those DO evaporate, and badly).

 

But you have an 85 720, so the pump should be electric too.  Have you tried just turning the ignition on, letting the pump run for 30 seconds or so, and then cranking?  Fixing the disappearing fuel would be optimal, though.

[datzenmike]

 

But you have an 85 720, so the pump should be electric too.  Have you tried just turning the ignition on, letting the pump run for 30 seconds or so, and then cranking?  Fixing the disappearing fuel would be optimal, though.

 

I have an '84 FSM and the fuel pump relay only allows the pump to be on when running or when starting so it won't be on just by turning the key on. It's job is to turn the pump off in case of an accident but is a stupid set up and should work like an interior lamp timer. 

 

I think my 710 is doing this also.

[datsunaholic]

Hm, my '83 pump runs all the time.  Must be a difference in year.  '83 didn't have a feedback carb either, though.

[Charlie69]

The 720 Pumps run all the time to recirculate cool gas to the carb after starting.

[datzenmike]

Hm, my '83 pump runs all the time.  Must be a difference in year.  '83 didn't have a feedback carb either, though.

 

This is what I figure. It even has an input from the accessory position. The pump doesn't work with the engine off so why have this input? It's the weirdest thing on the 720 and shrouded in mystery. I bet every year is different or changed. Another thing the 'relay' does not look like any regular relay but it does look like the room lamp timer and the pulse delay timer. I bet it was originally a timed on and then shuts off. Certain inputs would keep it on when running and shut it off in an accident.

[Rhino13]

I had a similar issue when my fuel pump was going out. It turned out to be a mixture of a shitty fuel pump and a little diaphragm on the carb. The arm the to diaphragm broke and for one reason or another it would drain the bowl and I'd have to give it a good 20-30 seconds before it'd get gas. I solved it by putting a Weber on and getting a facet fuel pump that Charlie recommended. Now the truck will start immediately every time with no problems since the install of both about 6 months ago.

[Charlie69]

Mike the fuel pump relay is also a timer in that it will shut the pump off if you loose oil pressure and If your are in accident to help prevent a fire.

[datzenmike]

It appears to be a timer just like the room lamp and the pulse delay relay but the '84 FSM makes no mention of a timer or how to check that it shuts off. It just shows a diagram where the pump can only run during start and once running. All other combinations of oil pressure, alternator charge, key on off the pump is off. I agree it is a weird part.

[khargis]

Hello all. I have a question regarding the hard start that was mentioned in this post. I just bought a 1985 720 4x4 and in the morning, she is pretty hard to start, it takes a bit. Granted it is about 25 -30 degrees, but I was wondering if there is a proper way to get her cracking'? I have never had a carbureted vehicle before, so I figured I'd ask some of y'all who are in the know. 

Thanks,

Kris

[datzenmike]

If you remove the top of the air filter and look while the engine is cold you should see the choke plate is mostly open. Have someone depress the gas pedal and it will, or should close with a snap.

 

 

Step on the gas pedal twice. This will set the choke to on, which forces a rich running condition. Crank engine over until it starts. Engine should start a have a fast idle which speeds up the warming. The choke is electrically heated and within 8-10 min it should be fully open or off and the idle reduced to normal.

 

 

 

 

 

 

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[Buzzbomb]

I  have this issue with my truck if it sits for more than 3 days. Both my Datsun and Chevy have this issue, and they have it with Holley, Hitachi, and Rochester carbs... I honestly think it has to do with the quality of today's gas; it just is not that resistant to evaporation, IMHO.  I think this also depends somewhat on the blend in your area. If you're running real gas, it might not be such a problem. If you're running 10% ethanol (or more?) year 'round it can be an issue. Just IMHO...

 

Since this happens, my starting sequence is to crank for a few seconds to fill the bowl without touching the pedal. Then hit the gas, foot off gas, try to start. Rinse and repeat until it starts.