QuestionI have a 79 mgb that quit running on highway. Since than i have replaced fuel pump, new ignition, and plugs and rebuilt carb. It trys to start but after a few trys it fouls the plugs. Than dry out the plugs and try again but same result. Does anybody have an idea whats wrong??
AnswerHi Preston,
You first need to stop purchasing parts and start testing. All piston engines only require 3 items to run, Compression, Fire and Fuel. (With conditions on each)
Compression --- An engine needs sufficient compression to operate. on most automotive engines that is about 125 PSI and higher. An inexpensive tool is needed to test the compression and that is a compression gauge. (Tested with the throttle open)
Fire ---- Spark should be strong and at the correct time. Shops have a lot of high tech equipment to test the spark but a preliminary test can be done with a minimum number of tools that are not expensive. (timing light) Timing must be close to factory specs and not altered unless major engine modifications were made. The power of the spark can roughly be noted by the color of the spark at the plug. A good strong spark is thick and blue in color. A weak spark is hair thin and yellow or orange. (must be checked in the shade)
Fuel ---- A combustible fuel must be available to the combustion chamber in a roughly correct air/fuel ratio and be atomized or vaporized. Too much fuel and it can wet the spark plug shorting it out. Fuel not atomized will not produce enough vapor. Gas does not burn easily, only the vapors of gas burns easily.
In your case you see wet plugs. So you either have no compression, no spark or you are dumping too much raw fuel into the combustion chambers.
If you had brought me your car with what you told me here is what I have to do with NO exceptions.
FIRST, I run a compression test on all four cylinders. I want to see from 125 to 165 PSI on all four cylinders with little difference between cylinders. Throttle open and at least 5 or 6 revolutions with the starter (all plugs out and the wires put back on the plugs and the plugs laid on a metal part of the engine) Two reasons for this, one is to look at the color of the spark and Two is because if you spin the engine with plug wires loose, a spark can jump down the coil tower to the primary of the ignition and destroy an electronic ignition system.
If this is ok up to now I check the timing and clean or replace the plugs and set their gap. (If I noted a weak spark I would have put the car on a scope to see why.) (there are a few rough tests that can be used if a scope is not available but they are time consuming.)
IF all is good up to this point and only if all is good will I look at fuel as a problem. There is a simple test here too. Disable the fuel pump by removing the wire off of the fuel pump and loosen the mounting screws on the auto choke that attach it to the carburetor. (don't remove them) (this prevents excess fuel from going into the engine while testing.
Now take a can of starting fluid and remove the air filter and have someone try to start the engine while you spray starting fluid into the carburetor. If it starts and only runs for a second or two try to keep it running with the spray. If you can keep it running with spray then you need to first test the fuel pump pressure (1.5 to 3 PSI max) If that is ok check the vent hose from the float chamber that goes to the charcoal canister to see that no droplets of fuel are present by shaking it or blowing through it. For a test leave the hose off of the float chamber when you try to run on gas again.
A problem with the auto choke is common. One of the associated symptoms is black smoke out the tail pipe. And flat black plugs.
Somewhere in all of that is your problem so don't skip any of it.
Howard
