There are a
couple of things you'll need to do to tune the carbs to your engine's specific
characteristics if you expect to realize the best performance possible.
The first thing is setting the idle mixture adjustments. 99% of all the
carb rebuids I do are for modified setups such as pod, breadbox, or velocity
stack intake, and open header or drag pipe exhaust. Both of these change
the how the engine breathes. Free flow intake or air cleaners tend to affect
everything from idle to wide open throttle where open headers or exhaust
with only mild baffling affect the throttle from about 1/3 to wide open
throttle.
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Setting
the idle mixture: It is very imortant to bring the bike to Normal Operating
Temperature BEFORE setting the idle mixture or doing ANY carb tuning
for that matter! There is no way around this, and shortcutting this step
will result it a poorly running engine when it does reach normal operating
termperature. Once the bike is up to temp set the RPMs to around 800-900
and one at a time turn the idle mixture screws in until the engine begins
to stumble or until the screw gently seats (DON'T FORCE IT!!!). 71-76 carbs
run rich at this point, 77/78 carbs will lean out at this point. SLOWLY
turn the idle screw back out in about 1/8 turn increments and then wait
30 seconds or so for the change to take effect.
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If the
engine will not idle at all on the later style carbs or chokes out on the
early style carbs bump the screws another 1/8th turn at a time until it
will idle long enough to make changes. You may have to increase the idle
speed with the idle set screw at first, but adjust back down to 800-900
RPMs again between each change since as the mixture gets closer the idle
speed will increase and hold there so you will have to adjust accordingly.
Continue tuning one step (1/8th turn) at a time unit the idle no
longer increases in RPM but then begins to drop again. Listen very closely
for RPM changes and take your time, it will save you trouble and frustration.
If you do not have a tachometer on the bike most dwell meters have a tach
option. If neither of these options are available you will have to use
your ears.
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Now that the
idle screw has gone past the point where the bike idles the fastest turn
it back in 1/8 turn at a time unitl it returns to the spot where it idles
fastest. IMPORTANT! As you make each change and when you move to
the next carb be sure the idle is set BACK to 800-900 RPM before moving
on to the next. Once this is done for all four carbs you can drop the idle
down to a little more reasonable RPM.
.
NOTE: The reason
I mention 800-900 RPM instead of the 900-1100 RPM most manuals recommend
is that their number are for a STOCK bike, not one which "breathes easier".
An engine that breathes easier will begin to use more than just the idle
circuit at stock recommended RPMs as the air velocity through the carb
is increased with free flow air cleaners and no back pressure.
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Fine tuning
from there is something that will take a little time as you ride. I keep
a small screwdriver in my kit/pocket and listen to how the bike is idling
and make changes as I go. If things are close it should have a little bit
of a lope but not load up or starve out and die. If it does starve out
or load up after a while: To determnine which (load up or starve) give
all four screws just a 1/16 turn in or out, doesn't matter. If it then
behaves better at idle or just off idle you're about as close as you will
ever get. If it makes the idle and just off worse turn the idle mixture
screws two 1/16 turns (1/8 turn) in the OPPOSITE direction you turned last
that made the idle worse.
.
NOTE: These
engines will only idle so low without causing problems, so keep this in
mind. If the idle is too low snapping the throttle too fast will allow
too much air through the carbs at once before the fuel can rise to the
top of the mains and out the main nozzle, and plop, flat on it's face it
will go. The 77/78 carbs are less prone to this since they are equipped
with an accelerator pump that loads the carbs with a short blast of raw
gas as you snap the thottle. Another symptom of an idle that is set too
low is the engine will die with a quick blip of the throttle. The slides
open, a blast of lean mixture flows through, and just as the fuel reaches
the nozzle the slides come down and the spurt of fuel coming out of the
main enters the engine as it's coming back down to idle and actually floods
it.
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Tuning the
mids and top end is a matter of trial and error, but doing proper plug
chops can eliminate a good bit of guess work and fewer adjustments. Doing
a plug chop the right way makes all the difference in the world. Bring
the engine to the desired speed under normal riding conditions and hold
it there for 2 to 3 minutes. Without changing engine speed hit the kill
switch or shut off the ignition and pull in the clutch as close to the
same time as possible and coast to a stop (driveway, parking lot, off ramp,
etc. so as not to be in the roadway if possble for safety's sake). Pull
one outside plug and one inside and make note of the color of the strap
and tip of the insulator in the middle. Don't worry about the outside ring
too much or deep inside on the insulator. Just the strap and tip of the
insulator.
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I would suggest
tuning the midrange first (35 to around 50 mph in 5th gear depending on
your sprocket ratio). If the plug chop reads white or very light grey you
will probably have to move the clip on the needles one notch down, raising
the needle itself and allowing a little more fuel to pass through the needle
jet nozzle. Dark brown or almost black move the clip up one notch loweing
the needles allowing a little less fuel to flow past the needle jet nozzle.
Once you are getting a consistent tan in the mids do a plug chop at higher
rpms such as normal highway speeds. Unfortunately you will have to static
synchronize the carbs after each needle change since getting to them undoes
the previous static sync.
If possible
pick a stretch of highway that will run you 2 to 3 minutes such as between
exits where you will be able to maintain a consistent highway speed (65-70mph).
This way you can coast up the off ramp out of harm's way. Again pull an
inside and outside plug. If the plugs read white or very light grey you
will probably need larger main jets. A quick and dirty test on mains is
to choke just a little (about 1/3 choke or so) while you are at highway
speed then roll the throttle on. If it responds better chances are the
mains need to be bumped up.
.
There is no
such animal as a bolt and go set of carbs no matter what sellers on eBay
claim. Each bike runs differently. If you run into difficulties with upper
midrange and highway plug chops or the chops don't seem to be consistent
with the exact same setttings a couple things you want to check for are
leaks at the carb boots and cross drafting/wind turbulence around the pods
or bread box air cleaners. Most types of free flow air cleaners can be
affected by wind flow at even slower speeds. Things such as tank shape
and wind flow around it, where your legs are while riding, type of pods
or stacks, how air build up behind the intakes, and whether they are open
enough on the sides to be subjected to side drafts (passing a semi at 70
for instance) can all have an appreciable impact.
.
To eliminate
the possibility of air flow issues try wrapping a piece cardboard across
the tops and sides of the pods or stacks so they can only draw air from
between or below. You can substitute anything as long as it defelects the
wind so the pods must draw air from the back or bottom only but not so
much you actually end up choking them. Breadbox you can temporarily tape
the sides and top to accomplish the same thing. One hint air flow issues
are affecting tuning is if the carbs seem to demand mains larger than say
130 or less than 120 on a stock engine running pods/breadbox and unrestricted
exhaust. You should be able to do consistent plug chops then, and tell
you if you need to make a pod shield.
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To eliminate the
possibility of air leaks around the boots make sure there are no cracks,
the clamps are not worn out and don't clamp well, and it also helps to
re-cinch the clamps once the bike has been running for a while and the
boots are warm and pliable. |
