10. Trouble shooting

Assembly manual

10. Trouble shooting
Richard Taylor wrote this manual.

This section is a practical guide to fault finding, although it is not fully comprehensive it does cover the majority of problems that we have encountered over the years. This section offers no warranties and makes no guarantees, if your engine still does not start or run properly seek additional help from your Lambretta workshop manual or a competent Lambretta dealer.

Symptom Possible problem Check or perform the following
Engine does not start or stops after
short time
No fuel getting through Check tap, check filter, check float
valve is screwed home, check float
is free. Smother carb with hand and
prime by kicking over
Engine does not start, is hard to start
or misfires
No Spark, intermittent or weak spark Look for short in loom, Check
ignition and replace parts
accordingly. Faults in new ignition
components are not uncommon, test
as you go.
Engine does not start
No Compression but crank turns
Compression problems, from loss of
rings, seals, petals or hole in piston
Check reed valve and petals are
closing properly. Check for holed
piston, damaged rings/piston.
Check timing and jetting if piston
Engine runs erratically has
intermittent misfires
Possible carb or ignition problems Check ignition & loom for correct
operation, check carb for air leaks,
loose jets & needle valves.
Engine starts and idles but misfires
when ridden
Carb, fuel & compression is OK but
timing mysteriously seems to move
Possible broken crank, loose
timing/pickup, loose flywheel or
flywheel boss, damaged woodruff
Back firing & ignition problems Reed petal broken and pushed
inside reed block
Re-time ignition, or replace ignition.
Replace broken petals and clean out
petal debris from crank case & barrel
Light heat seizure, caught before
wheel locks
Barrel over heating and tight ring
gap tolerances. Possible weak
mixture, air leaks or no oil. Engine
not “run in” long enough
Light hone of barrel to remove
marks, replace piston and rings if
damaged and re-gap
Medium heat seizure, wheel locks
up. Barrel scored, rings bent and
light damage to piston.
Barrel over heating and tight ring
gap tolerances. Possible weak
mixture, air leaks or no oil. Engine
not “run in” long enough
Hone/re-bore barrel to remove
marks, replace piston (possible next
over size dependant on damage)
and rings generously re-gap
Heavy heat seizure, sharp
screeching noise, wheel locks up,
possible bang & rattle followed by no
compression. Barrel heavily scored,
rings shattered and blown into
exhaust, piston is history & looks like
Ben Hurr’s helmet.
Barrel badly overheating and tight
ring gap tolerances. Possible weak
mixture, air leaks, catastrophic
failure of rod & bearings, barrel stud
loose or pulled out or no oil. Engine
not “run in” long enough and running
flat out.
Re-bore barrel to next oversize.
Replace piston & rings with next
oversize. Clean out debris from
exhaust, and crank case. Inspect for
damage to crank, crankcase and
cylinder head.
In winter engines runs sluggish and
does not achieve peak power, but
runs well in the wet.
Possible carburettor restriction,
related to weather conditions.
Check to see if your over-trousers
stuffed under the seat are blocking
the air scoop before getting the
spanners out.
    Spark plug colours Colour indication Action
    Spark plug insulator & electrode is
    very dark or black and oily
    Fuel mixture is rich and or oily. This
    is expected during “running in”.
    Keep spare plug, brush, clean and
    rotate use during “running in”. If not
    running in reduce jetting. Check
    your oil / fuel ratios. Possible drive
    side oil seal failure.
    Spark plug is white, light grey or very
    light brown and slightly shiny around
    the insulator
    Fuel mixture is lean, weak and or
    fuel starvation.
    Warning, Prolonged running like this
    will cause damage.
    Check for air leaks, correct oil
    mixture and or increase jetting.
    Possible mag side oil seal failure
    Spark plug insulator is a dark dusty
    coffee brown colour (coffee with a bit
    of milk)
    Mixture is correct (some say a lighter
    brown colour is better but best to
    stay on the richer mixture side we
    All is well in the world, pat yourself
    on the back and go down the pub
    (do not drink and drive / ride)

    Section index

    1. Pre-fitting kit preparation
    2. Installation preparation
    3. GT186 Stock Touring Installation steps
    4. Air box installation amendments
    5. Carburettors, jetting & sparkplugs
    6. Running in
    7. Care and Maintenance
    8. Tuning & performance
    9. Disclaimer
    10. Trouble shooting
    11. Cylinder head specifications
    12. Suggested specifications