All carburetor problems ARE NOT ignition « on: August 26, 2019, 11:11:55 AM » You know sometimes it’s real easy to overthink things. My motor (511 inch, roller cam, CNC heads, 10.2:1 compression) continued to run fantastic as long as it was moving at speed but once i slowed down it would cut out and stay “dead” for a while before I could start it again. It would cut out abruptly, so it MUST be ignition, right? Well I have gone through no fewer than 5 ignition systems ending up with an MSD. a-6. Same results. Finally had eyeballs on my AFR meter when it cut out again and guess what? Not ignition, boiling fuel and vapor lock. Finally realized that running the 1/2 inch fuel line up the inside of the fender well about 5 inches from the headers and no thermal insulation under the carb might not be the best way to go, especially with 96+ degree days. Not that I tend to overkill things but when I get back home (I’m cooling off in the Blue Ridge Mountains for a while) I’m going to re route the fuel line to a bypass regulator away from the aheaders and run a return line back to the tank. If that doesn’t do it, look for a 73 coupe de Ville in the for sale section. Greg Surfas
As an installing dealer for the AirSensors brand of aftermarket electronic fuel injection I fitted these units to any number of vehicles and their engines. There was one customer, actually a senior couple, who was ready to pony up the cost of a F'I conversion to fix what sounded like a simple case of vapor lock, just like yours. The vehicle was a motor home with the engine setback into a doghouse which acted like "an oven designed to cook fuel." This older couple ventured from Utah to portions of Canada every summer and they had tired of becoming stranded, most noticeably on the hills and mountains. Of course that was just when the engine was under the greatest load -- with less atmospheric pressure keeping the gasoline in a liquid state. I knew there was a simpler, cheaper solution than fuel injection. I asked if they would trust my suspicions and allow me to purchase and install a thermal barrier between the carb and intake manifold. They agreed and a couple of hours later I had made friends for life! The trips to Canada were event free from that time forward - a point I was reminded of each and every Christmas by a card from that lovely couple. May they rest in peace. bro. d
Bro D. It was the suggestion that I install a carb insulator, and my actually doing it that brought my thinking around to the point where I actually started looking at what I had. 5 inches from the passenger side header is NOT where a 1/2 inch fuel line should be run. As usual don't ever stop at 100% so I'm going to re-run the supply line and insulate it, replace the pressure drop regulator with a bypass regulator and run a 1/2 inch return line back to the tank. I've jerked around with this for too long and still need to get 500 miles on the motor and rear end before I go back to the track. Greg Surfas
Your observation sounds like the ticket. Let me know what you use for insulation on that fuel line as I'm going to need some here as well. Best, bro. d
Bro. D This is the stuff I used, both for the fuel lines and for my external oil bypass line. It comes in all sizes. https://www.summitracing.com/parts/dei-010473 Greg Surfas
The larger 1/2" line is actually making the heat soak worse. The larger diameter = lower velocity of fuel through the hose at idle, and = more time in the engine bay to pick up heat. A fuel return line is the ticket, keeps the fuel moving faster through the engine compartment by constant circulation. If you are running a mechanical fuel pump the Cad '77 and '78 425 fuel pumps have a return line on them to prevent vapor lock. Insulating the fuel line or adding a header heat shield will also help.
First of all I have been "nose blind" to even the thought of vapor lock as the cause of my engine crapping out, so I chased my tail through several ignition systems. Once I stopped running on the treadmill and stopped to think rationally the vapor lock was a no brainer. The 1/2 inch fuel line seems to be necessary as I "out ran" a 3/8 inch line with the same Holley "Blue" pump. A long time ago this motor out ran even the beefed up Pontiac HP pump so the mechanical pump was taken out of the circuit. Plan (this week) is to replace the pressure regulator with a bypass regulator and run a 1/2 inch return line back to the tank. Both lines will be insulated and run as far away from the heat sources as possible. More to come. Greg Surfas
Greg, Good logic on the larger fuel line in order to not starve the engine for gas. I do have a question: where is your Holley "Blue" pump located? As you know it is best to have an electric pump in the rear as close to the tank as possible. That way the entire column of fuel in the line is under pressure and better able to overcome the inertia of acceleration - and hold off vapor lock. Caddylackn, Your point about the fuel moving more slowly due to the larger diameter line is worth noting. That reality will be especially critical in my Caddy powered Studebaker due to the 5/8 inch (dash #10) size of the line. The good news is that the Holley pressure regulator does have a return port which will feed either a 1/2 inch or 5/8 inch return line to the tank. Best, bro. d
Greg, Nice setup! First thing that came to mind was the accumulated cost of all the AN fittings and parts. They don't come cheap, do they. Russell filter, ball-valve safety wired, everything done nice and tidy. Hope mine will be as nice when put on the car. Right now there is a sack full of fittings, a Holley Dominator 12-1800-2 pump, the appropriate pressure regulator, pre and post filters but so far it all looks like this: $$$$$$$. Thanks for sharing that picture as it helps to see what things look like when done right. Best, bro. d
Good thread. I’m about to change my ROBBMC regulator over to bypass mode and send a -6an line back to the tank.........just to keep fuel moving.
bro. d Thanks Over the years I have done many, many projects on the cheap with the "that's good enough" theme playing. In retirement this is one of my long term projects and I am enjoying doing it right to the best of my ability. FWIW I have been told "there ain't no pockets in that shroud". Every time I have drained the tank or cleaned the rear filter or pulled the pump down to check it I am glad It went together this way. Greg Surfas
