QuestionHi Howard,
Next question on this bucket of bolts. My '80 MGB seems to be running on 2 or 3 cylinders. It requires a lot of choke to start.
This car has the twin HIF4 carbs from a '74 and uses a non-vacuum distributor from a '76 Mini. Last week it was running good.
What I have checked so far:
Carb fittings to make sure it is not drawing excess air.
Points - they were running too small a gap. Readjusted.
Dist cap - cleaned all contacts.
Checked spark plugs - all good.
Do you have ideas on what else I should check?
AnswerHi Ron,
In the dealerships we learned early to not bother checking items when an engine fails in any manor.
You need to test functions first. All engines only need three items to run and run correctly. Compression, Fire and Fuel with conditions on each. They must be tested in a specific order too. Compression FIRST and fire SECOND and fuel ALWAYS LAST.
It does not mater that the car was running correctly five minutes before, you MUST be sure the combustion chambers and the function of the engine is good BEFORE proceeding. The compression test must be conducted with the throttle OPEN. You should see about 125 PSI to 165 PSI on each cylinder with little difference between cylinders on an 80 MGB.
Only then can you proceed to ignition. Most people don't have a scope that can test the whole primary and secondary of the ignition system in seconds but there are other simple ways to do some tests. FIRST set the ignition points gap (which you already did) and then set the ignition timing to specs. Then remove all four plugs and put each plug wire on each plug and lay each plug on a metal part of the engine. Now have someone spin the starter. You want to see a thick blue spark (not in sun light). "Thick" is about the thickness of a pencil lead. Blue in color is a strong spark. A thin wire like yellow or orange spark is a weak spark that may or my not run the engine. If you have that confirm the firing order of the wires in the cap. 1,3,4,2 in your case.
Only then can you look at fuel as a cause. Again there is a simple method to do some of the testing. Remove the air filters of the two carbs (you made a good choice of replacing the one carburetor with two SUs) Set the throttle at a speed that it runs badly and don't move it. Then take a spray can of starting fluid and spray a little into each carb to note results. Did it improve by spraying into either or both carbs by spraying?
If so test intake manifold vacuum with a vacuum gauge. (vacuum depends on throttle setting and load) You are just looking for good steady vacuum. Then test fuel pump pressure (1.5 lb to 3 lb.) Now with the engine running and running poorly put your hand with fingers spread apart over one of the carbs so as to still let a lot of air in. Then slowly start to close up your fingers so as to very slowly restrict the air flow to one carb. Note the response. If an any time in the process the engine speeds up or smooths out you have a problem either in the fuel level, jet or a intake air leak on that carb. Do both carbs that way.
That is the procedure I used in several dealerships and several shops I have owned for many years and it NEVER failed to find the problem no matter where the problem was.
Howard
