QuestionQUESTION: Hello Howard, I'm attempting to fault find a starting issue with my 79 B and I
thought I would balance and tune the carbs. I followed the SU instructions
and now the car backfires and rides lumpy at operating temperature - a
complete mess. How can I 'reset' these carbs to begin again?
Thanks as always!
Lance
ANSWER: Hi Lance,
While in MG dealerships in the 60's and 70's I found it was 100% necessary to be sure everything else was correct before I attempted to tune the SUs. Even if the car owner said he just checked everything. First a compression test (if any cylinders were much off from the others even if the readings were all up to par set the valve clearance)
Then set ignition timing to specs. (very important)
Then I could start on the carbs. First confirm float levels are correct and the pistons are free and the needles are not damaged nor out of place. Then loosen the linkage and back off the throttle stop screws and set both at zero meaning the stop screw at the throttle but not opening any at all. Adjust the linkage so it opens both throttles at exactly the same time but maintains a little free play. Adjust both jets to zero (flush with the Venturi) Adjust both exactly the same number of turns so the jets look about 1/8 inch below the Venturi. (this is just a starting point)
Open both throttles with the stop screws exactly the same number of turns (about 1 and 1/2 turns, this is a starting point also)
Start the engine and warm it up to operating temperature. Set the idle below 1000 RPM. Lift the adjusting pin on the FRONT carb and listen to what the engine does. If it raises in RPM and stays fast the REAR carb is too rich. If the engine drops off and dies the REAR carb is either too lean or you didn't sync the two butterflies correctly. Never adjust more than a half turn on the jet adjustment before checking and rev the engine between adjustments to clear the engine and not foul a plug if you are on the rich side. If while adjusting the jets the idle changes NEVER adjust only one carb to correct, ALWAYS turn both stop screws the exact amount to correct an idle while adjusting the jets.
When the mixture is correct, the engine will drop off in RPM and run like an old two cylinder tractor.
Then lift the pin on the BACK carb and adjust the FRONT carb the same way. When finished recheck both again.
Now, if you have one of those Unison gauges, you can put a final stop screw adjustment. Some mechanics use the Unison gauge early when setting the throttles.
Last set the fast idle stop screw for choke idle.
Howard
---------- FOLLOW-UP ----------
QUESTION: Thanks Howard, I discovered some more variables: two different throttle
screws one of which is stripped for a few threads in the middle (affects the
hunt for idle) I've replace with two of the same type of screw. I noticed from
the timing gun that cylinder one was firing odd ie. missing - the others are
all fine. I pulled the plugs and discovered a real assortment of plug
diagnoses (number denotes cylinder): 1) very rich burn dark and sooty 2)
same 3) gap almost non-existent carbon deposits 4) 'kind of' normal ...
'replaced them all for round two, fuel filter too.
Carbs are balanced but I think still a bit lean (but close) I sprayed carb cleaner
around the throttle shaft to see if I have a vacuum leak - no change thus
good(?) But still it wants to die after reaching temp and reasonable idle
(1450 rpm- note that a previous owner put in a Kent mild road cam, valves
and lifters - at 800-900 the car sounds VERY chunky and unhappy) so I
unplugged the vacuum advance from the manifold and the idle sounds much
better! What could be happening? Is there a problem with the vac advance?
There are many variables here I appreciate your patience and advice :)
Lance
ANSWER: Flat black soot on either the front two or rear two plugs is an indication you did not adjust that carb correctly. If they are glossy black they are just fouled either from oil in the combustion chamber or an mechanical or electrical problem. Trying to diagnose by symptoms is a dead end road. All diagnosis should be done by testing.
Wet and dry compression test first. Then set the timing to specs and then adjust carbs as I indicated before.
Howard
---------- FOLLOW-UP ----------
QUESTION: Hi Howard, I should have been more clear: the plugs that were pulled were
from before the attempt to balance the carbs, lower the idle etc. I knew the
car was burning rich so I wanted to adjust the mixture as well. My question is
do I need to have the vacuum advance connected? Will this harm the engine
(the timing is good)?
Thnaks!
Lane
AnswerIf you still have the car set up as it was originally the vacuum is not operative until the car is in 4th gear so you don't need to disconnect the vacuum when setting timing.
However you need to test to see if there is any vacuum on the hose at idle to see that someone before you had connected everything up wrong. If you do have vacuum on the hose, disconnect it when setting timing and then check to see why you have vacuum at idle anyway since a vacuum advance unit is suppose to be on ported vacuum not manifold vacuum. Only vacuum retard units use straight manifold vacuum.
Howard
