[Schurkey]

Originally Posted by Pierre1967

Hi,

I have a vapor lock issue with my Firebird 67 400. I have just a simple braided line from the pump to the carb with AN fittings (see attached pictures).

Is it best to add a return line just at the output of the mechanical pump runing under front engine or to split the fuel line just before the q-jet inlet ? (or change the pump to add one with a separated return output line ?)

Any other options are welcome ! Thanks to post pictures of your setup !

Pierre

Originally Posted by Pierre1967

I have a fuel filter near the thank and in the carb inlet. Hose is not so old.

When the engine temperature climbs near 195-200 deg (never above), it tends to stall when I brake hard at low speed. It's hard to restart too.

I have all the good stuff (Cliff carb, KRE heads, 30 deg. timing, alum. radiator, electric fan, 160 thermostat, phenolic spacer, new alum. water pump, water wetter, 40/60 mix, etc..)

(I'm thinking of going with an electric pump setup to solve the problem ... but would really like to stay with mechanical)

Originally Posted by HoovDaddy

I'm not dismissing everybodys suggestions, but stalling when braking, I would check the float level? Hard to restart, as said could be the fuel boiled out from heat, but also the bowl might be empty due to a low float level.

Half the problem in this thread is that folks are using the term "vapor lock" incorrectly.

"Vapor lock" is NOT "fuel boiling in the carburetor".
"Vapor lock" is NOT "fuel evaporating out of the carburetor".
"Vapor lock" is NOT "fuel boiling in the pressurized plumbing between pump and carb".

"Vapor lock" is the inability of an engine-driven (or, potentially, an inline electric) fuel pump to pump vaporized fuel. The fuel may boil (vaporize) IN the fuel pump, or it may boil in the suction plumbing between tank and pump inlet. At any rate, the diaphragm-and-check-valves of the typical engine-driven fuel pump are inefficient at pumping vapor rather than liquid fuel. Therefore, little-or-no fuel pressure, little-or-no fuel volume, the engine stalls or has reduced power because the fuel pump is "sucking air".

The fuel boils in the pump or the inlet plumbing due to engine- or engine-compartment heat; exhaust heat radiating from manifolds or headers, or the pipes under the vehicle, and it's compounded by the lower boiling point of a liquid at less-than-atmospheric pressure ('cause it's on the suction side of the fuel pump.) A lower-temp thermostat is of marginal usefulness since that doesn't change the temperature of the exhaust much. It can change the temp of the engine block, and the air coming off the radiator, though. So in some cases, it can help. Others, maybe most--no. He has a 160 thermostat, but the engine runs at ~200 degrees. Finding out why the thermostat has LOST CONTROL of engine temp would be high on my list of priorities--but fixing it may/may not help the stalling-when-braking problem.

A restriction in the inlet-side plumbing makes the pressure-drop worse, which makes the boiling point lower. This is why fuel filters belong on the pressure side, not the suction side. If a filter is used on the suction side, it's typically a fairly "coarse" "primary" filter, with a fine, "secondary" (smaller micron rating) filter on the pressure side.

^^^And all of that is probably moot, since the actual problem the guy is having is NOT typical "vapor lock" symptoms. I'm with HoovDaddy. Until the carb is proven good for float level, with proper float-bowl baffles installed correctly, float bowl vents in-place and not plugged, and the whole carb proven to not be overheated, vapor lock is not something I'd be considering. Assure that the brake booster isn't bleeding excessive air into the manifold when the pedal is pushed, too.

Adding a vapor return system (1/4" tubing) is most unlikely to cause a problem, I've done that to most of my carbureted vehicles. So even though this is unlikely to be a vapor-lock problem, a vapor return is still something I'd recommend on general principles. Re-plumbing for a return-style regulator and full-size return plumbing (because at idle, almost all of the fuel being pumped is getting returned to the tank) would be far more complex, more expensive, and--maybe--not any more effective than a plain vapor return.

A friend of mine had his daily-driver hot-rod burn due to rubber hose on the pressure side of the fuel pump--which rotted from the inside, under the steel braid. He had to throw his transmission into "neutral" then push his flaming car out of the driveway and into the street or it'd have taken the house with it. So I'll say it again: Get rid of that steel-braided junk from fuel pump to carb. Install a proper, double-wall seamless steel tube that's not going to rupture when you least expect it. If you HAVE to use one or more short sections of quality hose on the pressure side of the pump, inspect it regularly, DON'T use cheap-junk unshielded worm-gear clamps, and replace it at the first sign of trouble. I use the steel-braid-over-Teflon (PTFE) liner hose rather than steel-braid-over-rubber, or plain rubber.