K5 fuel system temp - relink to '74 thread

[my kids took the truck]

Hey, during the discussion on fuel systems for the '74 K5 we have figured out there is a big difference that happens somewhere after '74; the GM engineers introduced the fuel return line.

So the number of people over on the '73+ side that can help me is not many. I think the thread should be here in the pre-73 world.

The short version: My fuel evaporates from my quadrajet in two days (I calc'ed the evap. rate and it is possible). My fuel is over 100 deg F in the fuel bowl and that causes the ethanol mix to evaporate - and some suspected back-flow down the fuel line does not help. The fuel bowl plugs seem to be leak free - no black smoke on startup, easy start after sitting short time. Should I insulate my fuel lines and put a wood spacer under the carb so at least the fuel in the bowl is closer to the 60 deg F in-tank temperature? Add a fuel return line? Add an electric pump to top-up the fuel bowl on startup? Live with it? Put my Holley back on? Find a source of q-jet design spec. lower voc '60s era fuel? Switch to fuel injection? Buy a prius?

Ideas --> http://coloradok5.com/forums/showthread.php?t=317338

 

[sponsoredbydad]

I'm gona say NO on the Prius

 

[my kids took the truck]

 

[blazer74]

The fuel return was used on earlier cars like the impala with the 400 and up engines etc.

The return I don't think will help heat soak after shut down. Im thinking the return is more for vapor lock issues IMO.

The wood spacer did not help much on mine. But I could swear I cold hear fuel boiling in the carb before I installed it.

I put a return line in my 74 but have not changed to a 4 pipe sending unit and 3 line fuel pump or filter to hook it up.
I really don't think it will help with fuel evap in the bowl, it's just an ethanol byproduct unless the bottom plugs are leaking making it worse.

I have the plug kit installed and still does it.

I'm good for about a day for turn the key start at best, after that requires a little help.
After a week or more I crank for
4 seconds then pump and it starts in less than 10 seconds.

 

[my kids took the truck]

our K5s live in the same world when it comes to dry fuel bowl

nice to hear you have worked on this problem and are ahead of me on it

too bad you have not found the solution


I ordered some insulation with pipe clamps for the two tri-y header pipes - I will add those and see what it does for fuel temp. I just realized I have not calculated the evaporation rate at 70 deg F. At 120 deg F in the first two hours 25% of the E10 fuel should evaporate, then in the next 24 hours another 25% will evaporate, leaving half of the original volume in the bowl. I have no idea what it should be at 70 deg F. It could be like you said, the wood spacer made no difference, probably becuase it still evaporates, just a little bit less. I looked online and I cannot find an easily used fuel evaporation calculator.

 

[blazer74]

I would say due to the small volume bowl with the Qjet the only solution would be an electric pump plumbed in as a priming pump to fill the bowl before cranking.

 

[my kids took the truck]

I used your advice earlier in the post and contacted Cliff. He suggested removing the clip from the fuel float and needle, to try a windowless seat if I don't have one already, and to check the accelerator pump cup, the cups in the NAPA kits can start leaking almost instantly when in contact with E10, Cliff supplied a replacement cup.

If all that fails, then I think it will be an electric priming pump. Most likely they will switch to fall blend, the weather will cool, and my problem will go away until next summer.

 

[dyeager535]

How about re-making the fuel line to carb, and coiling it (similar to the brake lines off the master, but more compact) so that there is a "reservoir" above the level of the fuel inlet? This "idea" could be short term tested with some questionable plastic/rubber components for the short term, to see if there is any merit to it.

I wonder if the fuel can siphon back to the tank after shutdown in some instances, over time?

 

[bigjbear]

Don't know if this will help, but I can at least give some input. My truck has a Q-jet and I run an electric pump. Basically, once I start it for the day it starts as soon as I turn the key. Once it sits overnight I need to turn the key to "run" to let the pump push fuel back into the carb. If it sits for a week or more I need to let it run for quite a while before the truck will start.