QuestionMy 73 MGB is dying at idle speed. It only stays running when choke is pulled or when accelerator is pressed. Very recently I replaced the plugs, rotor and dist cap (Pertronix Elec Ign) as well as oil and filter, air filters, vaccuum line. car ram great for about 20 miles afterward, thought all was well. Then next day quick degrade to what I have. I adjusted the timing. What else should I be looking at? Not good with carbs.
AnswerHi David,
Each time an MG came to me in the dealership with a running problem I had to not assume anything so the first and most important thing is to check the combustion chamber by running a compression test. Be sure you hold the throttle open and turn it over 4 or 5 revolutions. It must be 135 to 180 PSI and little difference between cylinders.
When that is done I adjusted timing to factory specs. You have this complete so after the compression test you can test the carbs.
It is no use doing any carb tests unless the compression test is done because the compression and the timing effect the test and if either is off the test will give you false readings.
Since your engine will not idle you can't run the tests so you will have to set the carbs up from scratch.
Back off both throttle stop screws and the fast idle screws so that both throttle plates are closed all the way. Then loosen the linkage between the throttle shafts and reset them so that you have to pull the throttle cable a slight bit before they start to open the throttles and that they both open exactly at the same time. Then turn each stop screw in until it contacts either a thin piece of paper or I use a piece of cellophane. Use the piece of paper as a feeler gauge so you just feel a drag as you pull the paper out but it must NOT open the throttle. Now you have both throttles still closed and the linkage locked together so there is some free play and if you pull the cable it will open both throttles at the same time and both stop screws are EXACTLY the same but not touching the throttles. Now you can turn both stop screws in the exact same number of flats. (the heads of the screws have six flats) (six flats = one full turn) Turn each in 9 flats (for a starting point)
Now remove the top pots and do NOT get them mixed as they are a machined set and must NEVER be mixed. Note that each needle is in position and both have there shoulder flush with the bottom of the piston. Now adjust both jets until the jet is flush with the surface of the bridge (all the way up) Then adjust one down until it looks to be about 3/16" below the bridge but count the number of flats as you adjust it down. Then adjust the other down the exact same number of flats down. (starting point) Put the pistons springs and pots back on.
Now start the engine and warm it up. If it does not idle turn each stop screw down the exact same number of flats to make it idle. DO NOT TRY TO GET IT TO IDLE SMOOTH BY SETTING ONE STOP SCREW MORE THEN THE OTHER!!! Just set the idle speed to 800 to 900 RPM.
Now, lift the front carb spring loaded pin just under the lip of the pot and note the response. If the engine speeds up and stays fast the REAR carb is too rich so turn the mixture adjusting nut up three flats and rev the engine between each test. Lift the front pin again and note the change. If the engine had slowed down and died you either were not careful enough setting the throttle shafts and stop screws or the REAR carb is too lean. To correct a lean condition adjust the REAR carb down three flats and check again. If while adjusting the idle changes reset the stop screws but be sure to adjust BOTH stop screws the exact number of flats to get it back correct. When the jet is set correctly you will lift the pin and the engine will drop off and sound like an old tractor and idle very slowly. When you have the REAR carb set correctly then lift the pin on the REAR carb and adjust the FRONT carb the same way.
A cylinder that is low on compression or if the timing is off will make the carb response wrong and you will never get the carbs adjusted correctly.
Howard
