MG Car Repair: Fuel, feeler gauge, air mixture

Question
QUESTION: Hi, its Jon with the 1976 Midget (1275 in it). I replaced the jets--they now
can be adjusted. I just synchronized the carbs air mixture and respectively set
idle at 1000 rpm. I brought the twin SU's down 2 full turns from topmost
setting. I adjusted up and down...the best idle sound achieved was near 3 full
turns down. I tried lifting the pistons each .85mm to listen to a increase or
decrease in idle. I got a slight decrease. However, when I proceeded to adjust
down I started to notice slight backfires developing. So, I went back up and
stopped near the previous setting. I re-tightened the throttle linkages to test
a snap or two of the throttle. However, I would get (at near 3000 rpm) a
sound and response like the engine was "breaking up" (not sure if too much
or two little fuel) the rev's would not really increase without this breaking up
and usually the accompany of a backfire. After some fiddling, I would start to
drop fuel pressure--(as per my gauge mounted on the fuel rail). I tested the
pump and it was running out of fuel. Got noisy and was not pumping fuel out.
(tested with hose off bottom of pump) I decided to replace the pump with the
original. Now I have good pressure, but under tests got same results with
motor not running right--except for idle.

Howard..., I am at a loss. Am I too rich--causing a backfire, or too lean--
running out of fuel and backfiring>? I double checked timing...I am exactually
at 13 degrees btdc running with vac advance disengaged. The valves are set
right also.

Suggestions?
JGP

ANSWER: Hi Jon,
Rich can not cause a backfire. When you say "backfire" do you mean a backfire out the exhaust or are you calling a spit back in the carbs a backfire?
When you lift the pin on the front carb, what does the engine do? and which carb are you adjusting when you lift the pin on the front carb?
Howard

---------- FOLLOW-UP ----------

QUESTION: Hi Howard. When I lift the pin .85mm--with a feeler gauge, RPM drops a bit.
This is the same with both carbs. I plan to read front carb--adjust the back.
Read back carb, adjust front.

I am getting "spits" from the carbs. I am also getting a poping sound from
the exhaust. The backfire I hear from the tailpipe is moderate in sound.

The two may be related.

ANSWER: The pin is on the side of the carb and you lift the pin ALL the way up. This lifts the piston inside much more than .85mm. When you lift the front pin, you adjust the rear carb. The reaction you get when you lift the pin tells you which way to adjust. If the engine speeds up and stays fast, the other carb is too rich. To lean it out you turn the adjusting nut UP. If the Engine shuts off you either didn't sync the butterfly shafts correctly OR the other carb is too lean and to richen the mixture turn the nut down. Never adjust more than 3 flats at a time and recheck and rev the engine between settings.
If the engine slows down and starts running like an old John Deer tractor the other carb is correct.  
Where did you get that feeler gauge thing from?
Howard

---------- FOLLOW-UP ----------

QUESTION: so, start with the front carb...push pin up--I adjust down 3 flats on the rear carb--; recheck  with front carb pin and follow process adjusting the jet on the rear carb until the engine slows (with pin up on front carb); even if this is perhaps 3 down, 3 down..so on. I then can move to lift the rear carb pin and adjust the front carb down untill the desired drop in idle. I assume I should keep adjusting throttle inbetween to get idle>?

What about the spits and the pops from the tailpipe?

Thanks. JGP

Answer
Jon,
Three flats, not three turns. six flats = one turn. Yes, if you had the carbs so far out of adjustment you may have to reset idle. When you get close to correct you will not have to reset idle. Don't get them out of sync with each other on the idle adjustment. If there is popping in the exhaust after you get the carbs set correctly then it is something else not carb adjustment.
Howard