QuestionQUESTION: My 1975 Midget (with a Weber carburetor and manifold)runs great much of the time. However, when it is re-started and driven after it has been stopped and the engine has been off for a few minutes, it sputters and dies. It runs better after the engine temperature goes down a little. Is it possible that the temperature of the carburetor is causing the problem? The car has a new fuel tank and fuel filter and clean fuel lines.
ANSWER: Hi Leland,
That is a possible cause along with dozens of other causes. You need to run a few tests to try to catch what is going wrong when hot.
First, the 75 Midget has a Triumph 1500 Spitfire engine and it uses a mechanical fuel pump. Yes, the heat can cause a problem with the carburetor or even the fuel lines, so you need to know if it has the proper fuel pressure at the time of the "Sputtering". This is not hard to do but you need a fuel pressure gauge and a long piece of fuel hose and a "T" connection with hose clamps. All of this is available at most any auto parts stores and not expensive.
Put the "T" in the fuel hose close to the carburetor and run the long hose out from under the hood and place the pressure gauge under a wiper arm so you can watch fuel pressure at the time of the "Sputtering". Be sure to put clamps on each joint of the hoses.
You want to see from 1.5 to 3 PSI at all times. If you have that at the time of the "Sputtering" then you can eliminate fuel to the carburetor as a possible.
Next is the possible heat in the carburetor is boiling the fuel in the carb and upsetting the fuel mixture. This is a little more difficult to set up but look at the large hose that runs from the front to the heater box and remove it and get a short piece of hose about the same size from either an auto parts store of a salvage yard and put it in place of the hose that supplied air to the heater box but aim it at the carburetor to cool it down and drive the car. If that is the problem you will note a change right away and you will need to either put a heat shield at the carburetor or cut another hole in next to the radiator and direct air at the carburetor.
Some car manufactures and some owners of cars that heat to the carburetor is a problem, have to take steps to address this. Some manufactures put louvers in the hood to promote air movement and one car (Fiat) even put a electric fan to blow air on the carburetor to correct it. Many car manufactures put insulation around their fuel lines under the hood. Heat shields and insulation are often used to address this.
Heat is trapped up under the hood on most cars and only a few keep this in mind and suffer from it. I am presently building a project car and I am aware of the problem so I have had to take several steps as I have stuffed a V-8 in an MGB with 4 Weber carbs on it. I have directed fresh air from over the radiator directly on the carbs and opened vents in each fender to exhaust engine air.
Good luck, Howard
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QUESTION: Howard,
Thanks for your reply. Earlier I failed to mention that the problem has occurred using both an electric and mechanical fuel pump.
Recently, the car (after being run for about 15 minutes) sputtered and died and would not restart.
I removed the fuel line at the carburetor and there was no fuel being pumped at all -- not a bit. (There had been proper fuel pressure when checked when the engine was cool.) The fuel tank, all the fuel lines, and fuel filter are new and the fuel pumps (electric and mechanical)seem to be working.
After the engine cools down some, the engine runs great.
Would excessive heat to the carburetor keep fuel from being pumped into the carburetor?
ANSWER: The carburetor has no control to stop a pump (mechanical or electric) from pumping. If you did the test correctly.
For a mechanical pump pull the line off of the carburetor and put it into a jar or can and have someone spin the engine. For an electrical pump just turn the key on with the hose in a can.
This test MUST be done when the car is in it's failed mode. (NOT starting) It is a 100% waste of time trying to test when you know the engine will run.
A Real Time test is what I told you to do the first time. I learned a long time ago that trying to diagnose a problem by symptoms is a waste of time. TESTING is the ONLY method to fix it and the tests must be done when it is in it's failed mode.
When it is hot and failed pull the line off the carb and turn the key on (Electric pump) If there is no fuel, do you hear the pump running? If not test the electric circuit to the pump. If the pump is running loosen the gas cap, is fuel now coming out the hose? If not, leave the pump running and open the line between the pump and the long line to the front of the car. Is there fuel pressure there? If not is there a air leak on the intake side between the pump and the tank? Remove the line coming from the tank and lower it down and see if fuel runs out from the tank freely? If not blow a small amount of pressure into the filler neck of the tank and see if fuel is flowing. If not blow compressed air into the line (with the filler cap off) to see if the line is open to the fuel.
Somewhere there you have to find the problem.
Howard
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QUESTION: Thanks for your help. I actually mis-stated my last question. I meant to ask if the heat from THE ENGINE (not the carburetor) would cause the fuel not to flow. I was wondering if the problem might be a "vapor lock" in the fuel line itself.
I know from the test I ran (and I always did them when the car was in the fail mode) that there is no fuel getting to the carburetor. (I only tested the fuel pressure when the engine was running correctly to see the difference.)
I have not had an opportunity to run the car lately enough for it to fail, but when I do, I will try your other recommendations concerning "loosening the gas cap," etc.
Thank you very much for you help.
AnswerVapor lock is caused by heat from the engine but it is usually when the line going TO a fuel pump gets too hot. The fuel's boiling point is lowered by the lower pressure caused by the fuel pump trying to draw fuel from the tank. Once the fuel starts to boil and turn into a vapor which expands and forces the fuel in the line to return toward the tank and the pump tries to pump the vapor towards the carburetor (which it has trouble doing) and as long as the fuel line just before the pump stays hot (or the pump itself) there is no fuel pumped to the carburetor. Thus Vapor Lock.
This is partly why MG use to only have electric pumps in the rear near the tank, because even when you heat up a fuel line under pressure (between the pump and the carburetor) it is more difficult for the fuel to boil since the boiling point has been raised due to the pressure.
When you put a fuel pump on an engine and it is trying to draw fuel from a long pipe and a tank way low in the rear you lower the boiling point of that fuel in the line and make it more susceptible to vapor lock.
When you heat up a carburetor and metal line from a engine driven pump and heat up the pump you can still boil the fuel that is under pressure but it takes a lot more heat to do it. Engine mounted mechanical pumps are mounted on either thick gaskets or on fiber blocks to help stop engine heat from transferring to the pump. Some engines put insulation on the fuel line from the pump to the carburetor to stop engine compartment heat from the fuel. Most engines that are not "Cross Flow" (carburetor on one side and exhaust on the other) have to put a heat shield between the carburetor and the exhaust manifold. And most carburetors are mounted on fiber blocks to help stop engine heat from transferring to the carburetor. One car had a small fan mounted on the side of the engine blowing air on to the carburetor to try to keep it cool.
Your car has a Triumph Spitfire engine which came with a engine driven mechanical pump and does have more "heat to fuel" problems then the original Austin engine used in the Midget prior to 1975.
If it were mine and you have trouble pinpointing the problem but suspect heated fuel, I would look at what can be done to insulate the hard lines before and after the pump, check that insulating blocks are in place between the pump and engine and between the carburetor and some kind of heat shield is in between the carburetor and exhaust manifold and look at the possibility of opening up some of the fresh air from beside the radiator to point cool air to the fuel system.
I have had to address that problem ahead of time on a project car I am building with a Olds V-8 stuffed into a MGB-GT.
I have watched a lot of things tried by car manufactures over the years and some worked and some didn't work to address heat to fuel problems. The series III XJ-6 Jaguar solved the problem by pumping most of the fuel sent forward back to the tank but first running it through the AC system to be cooled first.
let me know.
Howard
