Gas in fuel bowl disapearing after 24-48 hours

[rwberinger4x4]

Hi all,

I'm sure this topic has been covered a few dozen times but I've tried just about everything I can think of.  I'm wondering if there is something I missed. 

I have two Quadrajets - one is an early 1965 Buick Wildcat Qjet mounted on a Chevrolet 283 V8.  It was rebuilt by me about 5 years ago with one of Cliff's kits.  At that time, I drilled, tapped, and epoxied the welch plugs. 

The second quadrajet I inherited from a different 283.  It was built by Sean Murphy Inductions (who I understand to be fairly reputable).  It's now mounted to a 350 SBC.  This one is a later 1978 "Mod Quad."  My understanding from Sean is that he ALSO epoxies the plugs, but doesn't see the leaking issue on the more modern varieties.   

On both engines, I have installed check valves in the fuel line just before the carb to prevent fuel from draining back down the line - in case there's a syphon issue or the check valve in the fuel pump doesn't work.

Both Quads are mounted on thick, 1/4" spacers, one of which is from Cliff's kit (the other seems decent quality but I can't remember where I got it). 

Still, on both engines, after a day or two of sitting, both fuel bowls are empty.  I doubt both carbs are leaking, and I know I'm not having gas drain back down the line.  My question is - where else could it be going? 

The SMI Carb/350 are in a Toyota Land Cruiser FJ40.  The under hood temps get pretty high. I put an IR thermometer to the Carb and it gets to be about 150 degrees.  Is this enough to boil off the gas after shut down?  I have NOT witnessed fuel dribbling into the venturies when the engine isn't running. 

I'm at a loss.  Does modern fuel just evaporate this fast? 

Any thoughts or insights would be appreciated.

[Cliff Ruggles]

The SMI unit will not be effectively sealed as they use epoxy that isn't worth two squirts of duck poop when it comes to sealing up the bottom plugs.  Best to pressure test the main casting on that one as described in my book and make repairs if/as needed.

All of those early units 65-66 will leak from just about every plug in them so good news that you sealed it up during the rebuild.  I avoid any of those carbs here if/when possible.  They also have the "goofy" N/S assemblies, originally used the plunger type but replaced with the later type and plugged bypass in all currently available rebuild kits.  They can be quite an adventure to get working correctly as there is only a weighted dampner for the secondary air flaps in lieu of using the pull-off and link up them.

If you get the problems with them effectively corrected they are OK, and probably adequate for the little 283 engine.

As far as the bowls being empty in a few days you can thank Obama for all that.  This new fuel evaporates very quickly compared to older formulations.  In the hot/humid months it's probably gone in 2-3 days after a hot shut-down.

NEVER use a filter with a check valve in it, very restrictive.  Remove the clip from the needle valve instead, then nothing can drain back and the clip is not needed unless maybe your vehicle sits for many months and could possible stick the needle on the seat from drying up, but we haven't seen that happen with this new fuel as it dries up with no stick residue behind. 

That's about the only good thing about it, as the older formulations would leave tons of "varnish" behind and glue the needle on the seat requiring the carb to have to come apart if you left it for long periods of time sitting without being used.

Coincidentally I did a test a couple of years ago with this new fuel, E-10 and 93 octane.  I did a hot shutdown on my engine (455 in the Ventura) and very quickly and carefully removed the airhorn to check the fuel level.  I sat the top back on the carb and checked it daily.

Remarkably on the hottest summer day the carb lost fuel very quickly, and was just about bone dry in 48 hours.

Did the same test in the Winter months and it wasn't down to the bottom of the bowl for 7 days.

Things that help this problem are having an electric pump for instant re-fills.  It also helps if your mechanical pump isn't allowing all the fuel to return the tank and the lines are empty coming up to it.  We see this with some vehicles and not with others.  I full return from the mechanical pump or after it (usually found on A/C equipped cars) will also allow the fuel lines to drain back to the tank.

So with a vented fuel system, bypass system and mechanical pump some folks will have issues with this new fuel and there not a lot can be done about it........Cliff

[rwberinger4x4]

Cliff,

Thank you for the very thorough response!  I suspect you are correct, in that this may just be the nature of the beast with modern fuels.  It seems like with a check valve in the line, no return line, and the plugs epoxied, it can really only be evaporating or boiling off.  Would a phenolic spacer help here if the thick baseplate gasket isn't?  I feel like if my hood gets to 140 degrees, simple radiant heat would get the carb about the same.

My last question - if it's evaporating - is it winding up in the intake and therefore in the oil?  My bigger concern is if the fuel is leaking or boiling into the intake I may end up washing my rings or diluting the gas.  Hard cranking it to start it ever couple days isn't a huge concern, the bigger concern is where that gas going.  If it's in my intake - there may be a problem.  If it's in the atmosphere, I'm less concerned.

In the meantime, have a happy 4th of July! 

[Cliff Ruggles]

We get the most complaints with this during the hottest months and seldom if ever in cooler weather so it's going to be temperature/weather related.

The folks who have the most troubles also report that it takes extending engine cranking times to get the carb filled and engine to fire.  This shouldn't be happening unless all the fuel in the supply lines are also draining back and the pump is taking a long time to re-fill everything.

Far as condensing in the intake, unlikely unless it's leaking in the bottom of the carb someplace and coming out under the throttle plates......Cliff

[rwberinger4x4]

We get the most complaints with this during the hottest months and seldom if ever in cooler weather so it's going to be temperature/weather related.

The folks who have the most troubles also report that it takes extending engine cranking times to get the carb filled and engine to fire.  This shouldn't be happening unless all the fuel in the supply lines are also draining back and the pump is taking a long time to re-fill everything.

Far as condensing in the intake, unlikely unless it's leaking in the bottom of the carb someplace and coming out under the throttle plates......Cliff

Thanks again, Cliff!

I just let the truck sit for 48 hours (two 96-degree days) and lo and behold it still had fuel this time...So I'm at a loss.  The truck doesn't get driven in winter (not much paint left, and too much road salt in MD) so I don't have a basis of comparison.  I've got a check valve (separate from the filter) installed in the line so I know the gas isn't draining backward.  That being said, the gas AFTER the Check valve definitely does - like you said, it takes 10 seconds or so of cranking to get the fuel back up to the bowl. 

I suspect the carb is boiling/vaporizing fuel after a trip long enough to get everything good and heat-soaked in the engine bay.  If this was the case - wouldn't the fuel be dribbling from the primary jets?  I still haven't witnessed this, which is why I'm so confused.

[Cliff Ruggles]

Unlikely it would be boiling fuel in the bowl.  Most likely just the added heat is just causing it to evaporate quicker......Cliff