Carbs tuning bases:
(First of all, I'm sorry
for my bad english. I have not been helped to make a good translation, it should
be corrected soon...)
A correct carb
tuning is very important as problems can occur to the engine (overheating, holed
pistons) if it is not the case. Keep in mind that the best results (performance)
are achieved when the tuning is on the lean side
however in that case, the
engine temperature is too high. Then, the ideal tuning (for durability) is a slightly
enrichment with regards to the best performance obtainable.
First, it is necessary
to set the carbs with the original values: main and idle jets, needles, atomizers,
throttle slide cutaways, float height, ect... It is also necessary that the carbs
are clean and in good state.
In most cases, it will be sufficient to change
the worn parts and to clean the carbs (ultrasonic way is the best) but, in extreme
cases, it will be necessary to replace the carbs. In other words, it is VAIN AND
TRY TO TUNE A WORN OR BLOCKED CARB!
Only when the carbs are
perfectly cleaned and set along original conditions, all seals and gaskets replaced,
without any air leak, then you have to analyze the symptoms:
- Symptoms of lean
main jets, needle level, float height, air leak:
plug has a satin white colour, even light yellow. Watching with a magnifier, the
electrode edges are slightly rounded, eroded.
- The engine starts but it stalls
when you suddenly accelerate.
- The engine splits and misfires.
- Slightly improvement when the choke is on.
2- Concerns the idle jet, throttle slide cutaway,
air screws, air leak:
- To start
a hot engine, you have first to action the pumps or the choke.
- Bad idle,
sometimes the idle speed remains too high during 3/4 seconds then shut down quickly.
- Splits or misfires, above all at low engine speed..
3- Concerns choke
jets, choke cables or plungers state:
- Starting the engine is easy when
hot but not when cold.
- Symptoms of rich mixture:
1- Concerns main jets, needle level:
- The plugs have a sooty black
or dark brown colour
- Easy starting when the engine is cold but not when
- Lack of power of the engine, it is weak, it vibes during acceleration,
black smokes from the exhausts.
- When the throttle is wide open, engine at
high rpm, it seems that it has almost more power if you slightly release the acceleration
- Slightly worsening if choke is on.
Tune-up the carbs: Method:
To do first: Carbs must be perfectly cleaned. Needles, jets, throttle slides must
be new or in perfect state. Carbs must be balanced.
There must be some plays
to the cables, gaskets and seals must be new or in perfect state. Choke plungers
must be free and the springs must be in good state.
The rubber caps of the
choke cables (on the carbs) and the seals of the choke system on the carbs must
be in very good state to avoid air leak at these points. Pumps levers are free
and in good state.
Pumps membrans are new or in good state. Basic tuning is
original (see above). Floats height is correct. No air leak.
methods can be applied: The dynamic tuning and the plug "reading".
- Dynamic tests:
1: Main jets: Start the engine, warm up some seconds then accelerate at WOT
through the gears:
-If the engine has a good response when cold but less when
hot, tuning is too rich.
- If the performance are better whe hot that when
cold, tuning is too lean.
Phase 2: Main jets, needles: In 5th gear,
keep the acceleration stabilized around 5500 rpm (at around 8/10 of the WOT) then
suddenly decelerate until 1/2 of the WOT:
- If the engine hesitates or misfires,
mixture is too rich.
- If the engine seems almost run better, mixture is too
- If the engine slows down immediately, without misfires, it's correct
or close to correct.
Phase 3: Idle jets, needle height: From idle,
- If the engine seems weak, feels sluggish, vibes, mixture
is too rich.
- If the engine hesitates, misfires, splits through the carbs,
even cuts then accelerates, mixture is too lean.
Phase 4: Main jets, needles:
Engine at constant speed on highway, action the choke and watch what occurs in
the firts seconds:
- If the engine hesitates, misfires, decelerates or even
cuts out, mixture is too rich.
- If the engine slightly decelerates without
too much misfires, tuning is correct or close to correct.
- If the engine
seems to run better, mixture is too lean.
Phase 5: Idle jets (or air leaks):
Suddenly accelerate through the gears on a highway then suddenly decelerate:
- If the engine slows down too slowly, with a deep and "weak" sound,
mixture is too rich.
- If the engine misfires, detonates, mixture is too lean.
- If the engine slows down quickly without detonations, mixture is good.
6: Idle jets: Engine at idle speed.
- If the engine idles some seconds
then slows down and cuts out, mixture is too lean.
- If the engine, after
having briefly and slightly accelerated, revs then has some difficulties to slows
down, then slows down and cuts out, mixture is too lean too.
- If the engine
idles but is weak when you biefly accelerate (too low acceleration), mixture is
- If the engine revs immediately, without hesitation, when you briefly
and suddenly accelerate, it's ok.
As a good result, the engine must run
well at any speed and any condition, without hesitation, misfires or detonations,
with a great power, and must get a great and stable idle. Once these conditions
just make the modifications below for durability:
- On triples,
fit larger main jets: + 5 points on lateral cylinders, + 10 points on central
- On twins, increase the main jets: + 5 points.
Plugs readind (main jet test):
several brand new plugs (at least 20), good thermic range. Reading a plug must
be done only with a new plug.
Fit a new plug before each test.
On private road od track, stabilize the speed to top speed during 10 seconds,
then shut the engine before you shut off the throttle.
Remove the plugs and
watch to the electrode colour and shape (a magnifier is required):
black colour, soot is clearly visible with the magnifier, electrode shape is intact:
- Black or dark brown colour, not or few soot, intact electrode
shape: Slightly too rich.
- Satin white colour, even light yellow, eroded
electrode shape (rounded off at the edges): Too lean, DANGER! Don't restart without
fitting larger main jets!
- Light grey colour, intact electrode shape (or
very small erosion visible with the magnifier): Slightly too lean, danger for
- Light brown colour on the electrode, dark brown on the plug
body, electrode sha
Bigger carbs? How to calculate the new main jets size?
the formula: New carb size/old carb size=X. Old carb main jet size multiply by
X squared, then substract 2%, you get the theorical size of the new main jets.
Then you can apply the advices above.
Replacing the 32mm carbs for 36mm ones on a triple engine:
130 old jet X 1.266=165 - 2%= Circa 162, new main jet
- Dirty carbs,
In this case, check the mixture
screw/pilot jet/progression galleries. in the abscense of a compressor a full
tin of carb cleaner used with the extension tube will usually do the job. Check
for flow through the
two little holes either side of the edge of the throttle
slide nearest the engine. With the extension tube up the pilot jet hole you should
get a jet of cleaner through both these holes (credit Steve Winterton)