The MGA With An Attitude
TRASH in the Fuel System -- FU-100

At 09:58 AM 9/18/2006 -0600, Bill Mitchell wrote:
"My front carb overflow pipe is flowing. I've opened up to check the float was floating; all was good so I closed and problem went away. Then started again."

Not a float problem.

"Changed the Jet and Needle and problem stops for a while and comes back."

How much fuel pressure do you have? Original fuel pump outs out 2.0 to 4.5 psi at the pump. Pressure measured at the carb feed hose is typically 1.5 to 4.0 psi. If you have an aftermarket fuel pump it may put out too much pressure. Anything over 5 psi is a big problem and can blow fuel past the float valve. Vibration at certain resonant frequencies will aggravate the situation. If you have too much pressure you can replace the pump or install a fuel pressure regulator.

"Now I give the lever a slight bend so the bowl need not float so high and the cars runs till the bowl goes 'dry'...needle seems to have gotten stuck in close position. "

Do NOT bend the float arm as a "fix" for the overflow of sticking problem. The float needs to be adjusted to give the correct fuel level, and then leave it alone. Overflow or sticking valve is a separate problem not related to float level.

"I free it up and the overflow pipe flows again."

Most problems with the float valve sticking either open or closed are a result of dirt (usually rust particles) in the fuel system. Cleaning or replacing the float valve can make the problem go away temporarily, until it may ingest some more trash to make it stick again.

Carburetor float covers should have a small screen inside the inlet fitting. Check and clean the screens, no tears or holes allowed in the screens. If screens are damaged or missing, install new ones. Ditto for the screen at the inlet fitting of the fuel pump.

Theory is that any trash particles small enough to pass through the screens should also pass freely through the float valves and fuel jets and should cause no problem. This is not always true, especially when you have a lot of rust in the fuel tank. Gross Jets in particular have smaller internal clearances between the body and metering balls, and are a bit more prone to sticking caused by small rust particles.

The proper cure for chronic trash in the fuel system is to drain and remove the fuel tank and take it to a radiator shop to have it boiled out for cleaning. If it is badly rusted inside, cross your fingers and hope it does not develop pinhole leaks in the process. If you drive the car a lot and run a lot of fuel through it, the fuel system should stay clean, filters or screens should never clog up, and float valves should never stick. After boiling out the fuel tank you may optionally coat the inside with fuel tank slushing compound. This can help to prevent future rusting problems, especially for a car that is not driven much or is left to sit for long periods of time with less than a full tank of fuel. The rust problem occurs from moisture condensing out of humid air above the fuel level with ambient temperature changes. Such condensation will be greatly reduced in dry environments, or in a garage with controlled temperature.

Gasoline can absorb substantial moisture (1% to 2% by volume) and keep it in suspension where it will cause no harm. Modern fuel containing 10% alcohol can absorb up to 5% water by volume and keep it in suspension. Fuel as delivered to a gas station may already contain some moisture content, so you cannot necessarily add 2 quarts of water to 10 gallons of fuel and expect it not to settle out. If you think you may have a bit of water settled in the bottom of the fuel tank, adding a small can of gas line dryer (fuel antifreeze) may allow the fuel to absorb the water.
Drain plug with intake filter screen (arrow)

"I do not have a filter on the line."

Original issue has screens in the inlet fittings of the fuel pump and carburetors.

"Could my tank be so dirty that the 'junk' is fowling the 'needle and jet'?"

Could be.

"When putting on a filter, is there a type you like."

A filter by any other name is still a filter. Nearly all of them will have a paper filter element. A precious few may have a porous bronze filter element. Some may have a steel shell, some clear plastic. Some serious fuel filters (usually expensive) may be disassembled for cleaning and re-use. It may be cheaper to have the tank boiled out rather than to buy a disassembly type fuel filter. If you have a bad trash problem you should fix the tank rather than spending time fiddling with in line filters.

"Should I worry about the filter changing the pressure in the line?"

No. Pressure drop across a filter is negligible, until the filter may become clogged with debris, in which case you need to clean or change the filter.

"I would like to put it in the engine compartment in front of carbs."

For appearance reasons it is nice to put a filter near the fuel pump where it is generally out of sight. For convenience it is common to install an in-line filter in the hose between the body pipe connection and the carburetor.

"Should I try to bend the lever a bit more?"

No. Make a point to re-adjust the float arm per instructions in the shop manual to have the correct fuel level in the float chamber. Find other means to fix the sticking valve problem.

"If it was the gas, I think it would be effecting the back carb too, wouldn't it?"

Hard to say. It might run a long time without sticking, and when it sticks it could be either carb at random and statistically unpredictable (except for 50% between carbs on average). If it seems to affect primarily one carb then check the inlet fitting and port for accumulated debris. Also check the bottom of the float chamber for accumulated trash. You could try switching the float valves between carbs. If it is a problem with one float valve then this switch may make the problem go to the other carburetor. The cure for a sticking valve is to clean or replace it, keep the fuel clean, and maintain correct fuel pressure.

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