Tuning your carbs
Once you have completed the pre-tune check list and corrected all problems you are ready to tackle the intricacies of tuning and synchronizing the multiple carburetor setup on you MGB or TR4. How difficult is this? Not difficult at all. Even a first-time novice should be able to precisely tune carburetors in 15 minutes or less — only if you have completed the pre-tune check list and corrected all problems.
If you are adjusting an existing carburetor or set
- If the engine is running reasonably well you may skip these first steps.
If you are having problems or if the car has sat for a long time first set
the carbs to a base line. With a felt-tipped marker, mark a line where the
dashpots (piston chamber) meets the carb body. This will be used to reinstall
the dashpots. Remove the dashpots and pistons. With a soft cloth and some
carburetor cleaner clean the deposits from the inside of the piston chamber
and the both the small and large diameters of the piston. Clean the steel
rod in the center of the piston and the bore inside the piston chamber. DO
NOT USE ANY ABRASIVE such as sand paper – never sand blast or glass bead!
Liberally coat the steel center rod with WD40 or similar light oil. The
piston and side wall of the chamber should be clean and dry. Be careful to
avoid bending or damaging the needle – avoid removing it, they bend easily
and often stick in the piston.
- Raise the jet until the top of the jet is even with the bridge. Back
out the idle adjusting screw until it is clear of its stop, turn it back
until it just touches the stop and then 1.5 turns. Replace the piston,
spring and dashpot being sure to put the same piston back into the dashpot
that it came out of – do one at a time to be safe.
- Raise the piston using the lift pin on the underside of the dashpot
mounting flange, when released, the piston should drop cleanly to the bridge
with a distinctive “clank.” If the piston does not drop cleanly you may
need to center the jet. If the piston drops
cleanly lower the jet adjusting nut 2 complete turns. You now have a baseline
adjustment and are ready to tune your carburetors.
After you have a baseline or if installing new carburetors
- Bring the engine to normal operating temperature
- Loosen the screws on the interconnecting linkage – each carburetor
should operate independently of the other
- disconnect the choke cable or any automatic enrichment device – loosen
the cable stop nut or accelerator linkage
- Using a tachometer, adjust the idle speed to factory specifications.
Make your adjustment incrementally, turn the adjusting screw 1/4 turn at
a time, adjusting each one equally.
- Raise the jet adjusting nut, on one carburetor at a time, until the
engine RPM just begins to drop. Depending on where your base line settings
were you may first notice a rise in RPM before it begins to fall. Lower
the jet adjusting nut until the RPM begins to rise and then 1/6 turn (one
- Readjust idle to manufacturers specifications.
- Test fuel mixtures – using the lift pin on the underside of the dashpot
mounting flange, press up until you feel contact with the piston and then
lift slightly (specs are 1/32 of an inch – good luck). If the engine RPM
increases when the pin is lifted, and remains elevated, your mixture is rich
– raise the mixture adjusting nut. If the engine RPM drops when the pin
is lifted your mixture is too lean, lower the mixture adjusting nut. If
the engine RPM initially raises as the pin is lifted, and then settles back
to the original RPM or very slightly above, your mixture is right.
- Readjust idle speed if necessary. To synchronize the carburetors use
a length of fuel hose. Hold one end of the hose just in front of the carburetor
inlet – not so that it obstructs air flow – and listen to the ‘hiss’ of the
air as it enters the carb. You will easily notice any difference in airflow
by the intensity of the ‘hiss’. Alternatively reduce the idle speed of the
‘fast’ carb and increase the idle speed of the ‘slow’ carb until both produce
the same ‘hiss’. Evenly adjust both idle speed screws to reach the desired
engine idle speed. Recheck fuel mixture, and carburetor synchronization.
- Adjust the pegs on the ends of the interconnecting linkage until there
is approximately 1/16 inch clearance (free play) before the peg contacts
the slot on the carburetor tab. Tighten all screws on the interconnecting
linkage. Tighten the cable stop, accelerator linkage and choke cables.
- Pull the choke until the jets are just at the point of moving but have
not moved. Screw both fast idle screws in until they just touch the fast
idle cam. Alternately turn each fast idle screw until the idle speed is
increased to 1,000 RPM with the engine warm. Push the choke back in – put
on your air filters – close the hood – You are done!