QuestionQUESTION: I have a 1976 MG Midget which is running rich at idle. I can see black smoke from tail pipe, sometimes in puffs sometimes lightly but there. The idle is somewhat rough and inconsistent sort of loping. The car seems to run well at accelerating and cruising but mileage not so great, in mid twenties. I have fitted with spitfire OD and mileage should be better. I have done the following:
Checked spark and plugs. They seem to be fine.
Checked vacuum tubing and advance along w/timing set at 3 degrees atc.
Replaced adjustable needle jet. At best run setting needle is off the adjuster.
Removed choke assembly cleaned and reassembled. My auto choke has been replaced w manual.
Checked for choke in full off position.
Replaced and adjusted to the best of my ability, under the circumstances, the accelerator valve.
The carb is essentially clean inside and out.
The engine is overhauled 15K miles ago perhaps and does not burn oil.
What may I be missing?
Thank you!
ANSWER: Hi David,
You need to remove the vent hose off of the carburetor float chamber, check the carburetor diaphragm for cracks, check the float level, remove the air cleaner for a test. Check the fuel pump pressure 1.5 to 3 PSI max.
You need to clear up one of your statements. "Replaced adjustable needle jet. At best run setting needle is off the adjuster."
I have no clue what you are talking about. The main jet is fixed and not adjustable. The needle in the piston is adjustable and lowered leans the mixture and raised makes it richer. Adjusted by a 3mm Allen wrench down through the top.
For it to run rich, it must have a supply of fuel.
Also what are you calling a "Accelerator Valve"
Howard
---------- FOLLOW-UP ----------
QUESTION: Hello Howard,
Thank you for getting back with an answer so quickly. Just a quick clarification. It is the piston needle I have replaced. At it's most extended position that is to turn it counter clockwise it will come off the threads but is retained by the built in spring. I wish I could extend it further. This is of course probably not the cause. To clarify the "accelerator" valve is the bypass valve or "decelerator bypass valve" which lowers the manifold vacuum when the throttle is released. I induced this error in my text when using the spell checker.
I need to check the fuel pressure. I am using an electronic fuel pump. It does constantly tick. There is no actual regulator on the output end of it. I will try the carburetor vent hose. The carburetor diaphragm is in good shape and the piston is oriented correctly in it's slide. I had checked the float level and it was within spec. I will also remove the air cleaner and test. Thank you again for the advise I will attempt to let you know how these measures workout.
AnswerDavid, run the adjusting screw back in all the way and find a drift or punch about the size of the ID of the tube and put the piston down on a flat surface that has a hole in it for the needle. Now tap the retainer clip down on the adjusting screw until it seats. Now adjust the needle until it has the shoulder of the needle about flush with the bottom of the piston. (leanest position)
The decel valve can not make any mixture change.
Fuel pressure is important as a high pressure will over power a float needle and flood the carburetor. 1.5 to 3 PSI is Max.
Howard
