68 300 convertible - Overheating? Gas tank vent lines?

[TMB2]

New to the forum and glad I found you guys – hope someonecan help with this.
We have a 1968 300 convertible. Thirty-six thousand miles, 440 cu. in. (withthe Holley) all original survivor car – it was my uncle’s.
Been having a couple problems and I think they’re related.
The car starts and runs fine but once it warms up on a hotto very hot day, the idle picks up a little, it has a slight stumble on initialacceleration (when pulling away from a light) and a slight surge under steadythrottle. All of these indicate to methat it’s running hot.
We only have an “idiot” light on the dash and have nevergone to the trouble of installing a temporary temp gauge. The hot light does not come on but I don’thave much faith in it.
The other problem is when the car has a full or more than ½full tank of gas and we start to experience the above, gasoline will piss outof the two vent tubes big time. A littleunnerving to say the least. When I’mbehind and under the rear of the car checking on the vent tubes, I can hear thegasoline gurgling in the tank. When Irelease the non-vented gas cap, I get a pretty significant whoosh and thepressure pushes the cap off the filler neck. It’s pressure not suction.
Anybody else ever had this problem with a 440 C body? Any ideas???!!!

 

[Fury440]

First problem to address, the gas tank is vented and should never have any pressure build up. A pressurized gas tank is going to force fuel past the pump and most likely be the cause of your carb problem.

Remove each tank vent line and blow them out with compressed air, my bet is some bug/spider has set up residence and plugged the vents.

 

[TMB2]

The vent lines is on a long list of things we've tried/checked. Both are free and clear.

 

[1970cat]

a vented cap should help with your pressure problem. sounds like you are running a little lean when warm. keep in mind the "new" gas has a lower boiling point.

 

[TMB2]

Thanks. I tried a vented cap but due to the damn near horizontal filler neck, I ended up with another gas leak! We've also tried Cam2 high octane leaded (runs cooler!) but that didn't help.

 

[blackmamba]

220 degrees is about normal for a 440. Get a temp gauge on it so you can monitor the temp! Check your Thermostat sounds like it might not be opening and closing all the way. Look at your Carb, see that little arm with the holes in it remove that small JESUS CLIP! and move it down to the farthest hole to give you a bigger shot of gas. Remove your Idle Mixture Screws and shoot some Carb Cleaner in each Screw Hole. Put them back in but DON'T bottom them out! then back off 2 1/2 turns, this will get it started, then adjust from there.

 

[bluefury361]

the surging at steady speed suggests a vacuum problem/leak, which will cause a lean condition and cause a high temp issue.
While the Holley was a good carb in it's day they do not age well. A rebuild may not even improve it. A 750 CFM Edlebrock with an electric choke will really wake the 440 up. Put the Holley in the box and on the shelf.

 

[polara66hardtop]

x2. I had to do the same to my 440 GTX. And no ethanol!

 

[TMB2]

Thanks to all -

Have heard good things about the Edlebrock and may end up going that way.

In the meantime, we still have gasoline pissing out the vent tubes when a full or nearly full tank on a hot day. Any ideas on that? Anybody else ever have that happen? Every time I see a C Body or 440 at a car show or cruise-in, I ask the guy if he's had that happen and he looks at me like my name is Caitlyn.

One thing I am going to try is taking off the small canister fuel filter I put on a couple years ago. Before that, the only fuel filter on the car was the small, porous inline filter at the line going into the Holley. That didn't seem enough to me so I added the canister. Is it possible that two fuel filters are affecting the flow from the fuel pump? Could the fuel pump then be kicking pressure back to the gas tank? I doubt it but just asking the question.

 

[440Sled]

My vent tubes do the same thing! It really ticks me off and it is on warmer days. I hear the bubbling fuel and release the cap to vent it out so I don't waste fuel or possibly blow up the car and myself. I don't understand how fuel is being pushed out of these lines either. I smell gas while I'm driving too, but I chalked that up as running an exposed K&N filter and not the stock enclosed air cleaner set up. I also get the surges at a medium to high speeds. I have a Holley 670 Street Avenger, but I've messed with the jets because I live at a high altitude. I might switch them back or go up a size or two.

 

[TMB2]

FINALLY! Someone else is having the vent tube problem! Thanks 440Sled -

We're going to try high-octane, leaded CAM2 for a couple tanks. Not sure if that'll help but will let you guys know either way.