I have a braided line from the fuel pump to the carb on a small block Chevy. When the car sits idling the fuel gets hot and the car runs rough. The line is kinda long could that cause the fuel to heat up? The shorter the line the better?
i would be more likely to think that the fuel line on the frame is too close to the exaust other than the one on top of the engine. is this a possibility?
electric or mechanical pump? i'm going to assume mechanical. does your fuel line run anywhere near your heater hoses? i was getting vapor lock when i ran a mechanical pump, because the fuel line rested on one of the heater hoses. have you measured the temperature at the fuel pump? does your car run hot? a hot engine will heat the fuel pump, and then heat the fuel. do any of your fuel lines come close to any exhaust tubing or your manifolds? this will also heat your fuel. what size fuel line? too small of a fuel line with a high performance fuel pump will cause heat, because you're trying to squeeze a lot of fuel through a small opening. pressure causes heat. your fuel pump and lines should be matched to your power level and fuel requirements. mechanical pumps are designed to only provide a certain amount of fuel, and that is based on rpm. but, it's not an exact science, either. if you have a hipo mechanical pump on an otherwise stock motor, it's trying to provide more fuel at idle than you need, which can lead to all sorts of trouble, vapor lock being one of them. electric pumps, especially those for modified applications, usually produce more pressure than you need, and need to be controlled with a regulator. when your pump wants to put out 14psi, and you limit the pressure to 6psi, you build excess pressure in the line, and most pumps will have an internal relief to relieve the pressure. again, pressure causes heat. the best solution for this is to install a return line, typically in 1/4" diameter.
Look for a heat source (header/exhaust manifold, mufflers) near your fuel line. At idle (with the car sitting still) there is no way of moving the heat away so a fuel line located near by gets hot. When the fuel gets hot it will start to vaporize and causes the rough running. BTW be sure that you fuel gradually goes uphill toward the carb. If not, you will trap the gas bubbles in the line and and not return to a liquid until it cools down.
Are we talking about a stock mechanical pump on the engine or an electric one mounted somewhere else? What makes you think that the length of the fuel line would affect how the car runs when warm? I don't agree with your premise. I'd be more inclined to suspect an ignition problem that acts up when it gets hot and not the length of the fuel hose. JMO
Well, I can tell you this If it ain't running optimum, it ain't Fixed ! Exactly how long of a period idleing does it take to get the fuel hot ? Is the carb hot? The hose hot ? Or just the fuel hot? How long in inches is "kinda long" and what route does it take?
I would be curious to know what you mean by "hot" and why you attribute rough idle to the fuel temp ...... engines run best (But wear faster) at about 180 degrees .... whats the engine temp?
Flowing fuel in a line don't get hot - it is the same temp as what it left the tank at. You can have a problem when it sits, if you have an edelbrock-carter type of carb where the fuel bowl is internal then fuel will get hot and boil in the bowl and it pushes the fuel into the engine. When this occurs the engine will be hard to start, you crankcrankcrank until fuel is brought up to the carb from the pump and then it'll lite off. If this seems what you are going thru then get a sandwich heat spacer from mr gasket (like #98a i believe) and that should do you pretty good.
Bingo! Had a 302 that would boil the fuel after I killed the engine. I put in a phenolic spacer and it was like night and day. No more hard starts!
ever run an electric pump? it sure can. many high volume pumps run fairly warm as is. running a high volume pump, choked down with a regulator, and no return line, causes the pump to work harder than it has to, which creates even more heat in the fuel. fuel coolers weren't invented for nothing. while fuel won't warm without an outside influence, it certainly can and will. on mechanical setups, this typically can't happen. it needs to be heated by the engine or exhaust.
What I got from his statement was that as long as the fuel is flowing, it won't get hot. At idle, the engine doesn't use as much fuel so the needle valve is closing, stopping fuel flow. A regulator that returns excess fuel to the tank will keep the fuel flowing like modern fuel injection systems do. As has been said, the fuel sits in the carburetor longer at idle so it can absorb heat from the engine. Also, the fan isn't turning as fast at idle so it isn't moving as much air to cool the fuel line and carburetor.
Ahh, gotcha. That makes perfect sense. It can still suffer from radiant heat, obviously. My carb gets fairly warm, so I'm wondering how hot it would have to get to affect the fuel.
Then how does Vapor Lock occur? My understanding of running vapor lock is where high engine heat (or some other source) on the pressured side of the pump (between the pump and the carburetor) boils the fuel in the lines, also starving the engine of enough fuel to run. If the fuel boils, isn't it hotter?
I had the same problem on a sbc with a stock 4 barrel intake. The fuel got so hot it would continue pumping into the engine when you shut it off. I had to set the idle up 1500-1700 to keep it running. An old mechanic installed a heat shield under the carb and I never had the problem again.
The Carter/Edelbrock style carbs will peculate fuel because of the transfer of engine heat and their small fuel bowls. You can go a long way to stop it by using a thick insulating gasket between carb and manifold. Of course you would need to by using this style carb to be relevant
do they oxygenate the fuel in maryland in the winter? i know they mess with the fuel up here that effects the way things run when the weather changes. not saying it is what is wrong just food for thought.
I am running a edelbrock carb with a mec. performance pump. When I am in a drive though and the motor gets a little hot 210 degress it starts running a little rough if I rev the motor it will recover. I think it's the fuel line becauce it does nort do this at any other time. Reving the motor makes it recover. The line is about 7 inches to long I have it looped down by the fuel pump. it is near the block but not near the exhaust pipes or headers. I wonder if the pump is getting hot I will check that with my infared gun. I also might install a spacer at the carb base.
like they say insulate the edelbrock it will boil also run a fuel pressure regulator or they flood out i learned after south of atlanta and insulated the carb even a 2 in spacer and some gasgets help keep the heat off the carb sure surprised me too and they will not stat after shuf off and run crappy at low speeds, try it aned report back
That sounds like its loading up, A good throttle blip and it clears up is usually is a sign that too much fuel is gathering and the rush of air clears it out. Could also mean its running lean and the extra squirt from the pump helps temporarily. Without seeing it, hearing it or a more detailed explanation its like throwing darts blindfolded. Could be caused by too much fuel pressure, incorrect adjustment, needs more tuning, or dirty carb. They like about 4 psi at the inlet. Eldebrock carbs are great, very simple, but the must be clean. Think of them as having an OCD about being clean only you need to do it.
The carb is running abit rich. I have changed the metering rods and it has a good take off from the line and it has good start up. I just ordered a spacer 9 plastic) from summit. I will see if this helps. I think the problem is that plus some minor tuning. If I am getting to much fuel at idle how can i minimize it? float adjustment?
You need to know what the fuel pressure is. Put a gauge on it and see. If its over 4 psi you'll need a regulator.
I may be the only one that will worry with my engine running at 205. I never thought that that was normal for a 55 Chevy. I know it won't boil over with a pressurized system but I prefer my engine to operate between 180 and 200 tops on a 98 degree day stuck in bumper to bumper traffic. JMO
Tommy, this is on my 1940 Chevy street rod. It runs at about 185 since I put a good ground strap on it. My gauges were not reading correct before. i can be in traffic bumper to bumper however in a drive though it has this problem. Once you get back on the open road it's ok. I drove to york last year and was in some contruction traffic. No problem. I think the spacer will do the trick, we will see.
The fuel in the bowls of that carbs is expanding/boiling at idle because not enough fuel is going thru it to cool it down. I bet it is hard to start when shut off too. The bolws of the edelbrock/carter are integral to the carb and absorb the heat, the fuel expands and pushed up thru the vents/booster and into the engine. If you look into the countersink wells for the rear throttle/gate you will see dried fuel in the bottoms, the only way fuel can get there is from being pushed up thru the vent. You need to isolate the carb with a wooden spacer or something along those lines.
The carb is now running alot cleaner, I have changed the rods to a smaller size. The car starts well even when hot however I did order a heat dissapating plate not a billit spacer. This phalic spacer will disapate heat. I did notice the metering rods when removed were very dirty with dried gas.
While changing the and installing the spacer I took the top off the carb. Bowl was filthy and the floats where out of adjustment I guess that's what was making it run rough. Clean everything and installed the spacer car ran great. case closed.
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