Should a filter even be there?

[caleb22]

I have a 76 stock 350 with a 1406 carb. Been having trouble with fuel boil and what appears to be vapor lock. I bought a spacer for the carb that I'm going to put on and replace the fuel pump. But I noticed there is a fuel filter inline to the card...should that filter even be there? It's up out of the way probably because it was boiling but I noticed when I get what appears to be a vapor lock issue the fuel filter is empty.

I don't think it should be there...but not sure with ethanol and crap these days.

I'm almost to the point where I can only drive it about 15 minutes before it won't crank or go into gear without dying.

At some point I will drop the tank and replace the lines but not now.

 

[shady]

Having a filter in line between the pump and carb is normal. I've had a few there and never any issue... Maybe if it was laying on the exhaust.... ...

I'd try leaving your gas cap cracked once.

 

[K5wrench]

Having a filter in line between the pump and carb is normal. I've had a few there and never any issue... Maybe if it was laying on the exhaust.... ...

I'd try leaving your gas cap cracked once.

Great point.
When I got my 76 K5 the previous owner had removed the evap canister and plugged the lines.
One of those lines is supposed to be the fuel tank vent.
Make sure it's still there and open.

Replace fuel pump is a good idea too if you are getting vapor lock.
Of course check all the lines to make sure they aren't near exhaust, hot parts.

Something else I found caused a condition very much like vapor lock was when the top of the carburetor comes loose.
Those screws can back out, or maybe it is the hot and cold cycling.
Whatever it is, Bout every 6 months I go through and tighten all the screws.
On Quadrajets there are two screws hidden below the choke blade above the primary venturi.
Those come loose and you lose vaccum to the primary air bleeds and it plays heck with driveability.

 

[diesel4me]

If it had a universal gas filter added in between the carb and fuel pump (assuming it has a mechanical pump on the engine,not an in-tank electric one),make sure the original paper or brass filter that is in the carb under the fuel line nut is removed,because having both can restrict fuel flow enough to cause issues on heavy acceleration or long uphill grades..

Some of those in the carb gas filters have a "roll over valve",a check valve that supposedly wont let gas drain from the carb if the vehicke rolls over...

Those can be troublesome sometimes--I either tossed those filters away and used a universal in-line clear type one,or put in the older style filter without that rubber check valve..
The oem filters suck,once they plug up,the spring behind them allows the fuel to push it out of the way and let dirty gas by-pass it completely..not a very good design..

Also look and make sure no one added a filter between the gas tank and fuel pump--having one on the suction side is usually no problem,but might cause enough restriction to starve a carb on an engine that guzzles fuel,especially under the conditions listed above..

The sock filter in the gas tank on the sending unit is very fine mesh,and water cant pass thru it easily,nor can any silty rust--it may build up around it due to the suction of the fuel pump after several minutes and eventually starve the carb for fuel..

Water in the gas can usually be eliminated with some dry-gas additive,as for rust,the best fix is to pull the tank out and flush it good (or replace it)--I have gotten around this issue on a few vehicles by putting a strong magnet on the bottom of the gas tank (like one off an old junk speaker,or hard drive)--place it near a corner,away from the sock filter ,so the rust it'll attract will be kept away from the fuel pick up tube and sock in the tank..

I had one truck I bought that sat with an empty gas tank for years--it clogged up several universal fuel filters with rust in a short distance--after I had replaced them about 5 times,an older mechanic told me about the magnet trick--after I added one,I didn't have to change it again for over a year,and it didn't look like much rust was in it,I just did it for peace of mind..

The tank not being vented could cause this as noted also..also any old or original rubber fuel lines wont be ethanol compatible,and can come apart inside and cause a blockage intermittently--best to replace all the rubber fuel hoses with newer stuff rated for ethanol--I see this on many riding mowers that have the same issue--runs great for a few minutes,then "runs out of gas",even with a full tank..old hoses can crack and be a fire hazard once ethanol eats away at them long enough too..

 

[Joe In Montana]

All good thoughts so far - but the fact that gas boils at a pretty low temp and that PULLING/SUCKING the fuel from the tank LOWERS the pressure in the fuel line, just LOWERS THE BOILING POINT of the fuel that much lower.

Think this way ---> the tank should be under very little vacuum - if the ECS system is working correctly, and the fuel pump can certainly overcome a small vacuum value that would be present in the whole fuel system IF it is working correctly.

ASSUMING the ECS system is working or is totally removed - then you have to look other places for the 'vapor lock' you say you are getting.

I personally had the following troubles.............this might be long, but it's true I swear!..................

1. I have an electric fuel pump PUSHING the fuel to the carb. NO mechanical (by this I mean: the original fuel pump) version is on my engine. I was too lazy to put the pushrod in and just blocked off the hole with a nice chromed Cal-Custom plate.
2. My electric fuel pump was installed just ahead of the tank (bad idea) and this got it pretty hot when sitting on a hot day at a long traffic light.
3. Idling was the problem since the cooling fan on the water pump wasn't pushing enough air down through the hood, past the engine and over the transmission/transfer case and out the back from under the tank and rockers. This is also most likely area to capture the heat rising off the exhaust system and muffler. Another bad thing!
4. The fuel between the electric pump and the steel line coming from the tank would start to heat up and actually turn the gasoline into vapor - the 'boiling' that you see.
5. The fuel pump would then starve for liquid fuel - it IS after all a liquid fuel pump and by that I mean it likes liquid fuel.
6. The fuel pump would start to whine and the fuel pressure dropped and the carb bowls went dry and the venturi ceased er, 'venturi-ing' and the engine stalled. The fuel filter looked empty.
7. Cranking a 'vapor locked' engine was fruitless since there's NO gasoline in the carb or the fuel lines or the pump.​

Even with a fuel pump pushing the fuel to the carb, the heat got to the whole thing and made it fail. Cooling down for an hour or so cured it - but by then I would be zombie food.

PUSHING fuel is always better than SUCKING it as the heat has LESS effect on the fuel because the fuel by being pressurized would therefor act to raise the boiling point of the fuel - but it too will fail after it gets too hot or collects enough heat to overrule the pump anyway.

The cure for me was to isolate the pump and the suction line from the pump (from the tank) - by insulating it all.

I bought a 6 foot piece lots of that 1/2" foam insulation for water pipes, and put it over everything including the pump. OK - the pump took a slightly larger size - but you get the idea - right?

No more troubles!

As for your problem - test the fuel pump to make sure that it's working up to design limits first. A weak or dying pump can act the same way with a vehicle that never had this trouble before.

But we tend to modify what the factory does (don't we?) - sometimes for the worst,
and we have to re-engineer the whatever-we-screwed-around-with
-------------> to get it back into working order.