RB30-TB42 Nikki carby tuning procedure(temporary) [Archive]

dom14

10th November 2016, 12:24 PM

Above tuning procedure only tunes the AFR at idle rpms(hence the reason it's called "idle mixture screw")

Above idle rpms, jets, airbleeds & accelerator pump, etc takes more control of the AFR(Air to Fuel ratio).
Therefore, if the symptoms of "bad tuning"(see below for the list) continues, then the carby may need a rebuild.
Before rebuilding a carby, it's IMPORTANT to rule out any electrical issues that may be causing issues with the carby.
These include petrol pump issues, weak or broken connections with the wires/connectors to petrol pump or the solenoids that control the valves inside the carby.

Symptoms of Lean and Rich

Here are some basic symptoms, some duplicate

Lean

1)Reduced Power - Sluggish at certain RPM's. Wide open throttle yields no power. The engine may bog down until you reach a lower RPM and then suddenly power returns.
2)Difficulty Starting
3)Spark plugs are clean - No residue. Insulator may be white.
4)Runs better at higher altitudes - AFR becomes normalized due to reduced atmospheric pressure allowing fuel into the venturi easier.
5)Backfires - Popping on deceleration for a lean idle circuit or backfires in general.
6)Runs on choke - The vehicle may run on choke when it's warm but stalls if the choke is turned off.
7)Idles poorly - Fluctuations in idle RPM, stalling
8)Engine runs hot - Due to more oxygen than fuel combustion temperatures are hotter reflecting on a temp gauge.
9)Hanging idle - The engine idles high and then drops and stalls. Typically a lean condition caused by an air leak between the butterfly and the head or a vacuum line that is not attached.
10)Sharp Odor - The exhaust smell may be sharp and burn your nose. This is NOx or Nitrogen Oxide. It's created by high temperatures in the combustion process between nitrogen and oxygen. Although NOx is present in all exhaust it can be more pronounced in higher combustion temperatures associated with lean conditions with more oxygen. More oxygen=higher burn temp

Rich

1)Reduced Power - It has less power but seems to be OK and runs.
2)Reduced Fuel Mileage - Your using more fuel per cycle so your mileage becomes worse.
3)Rough Idle - Combustion takes a just a bit longer with more fuel and at lower RPM's the engine can idle rough. Unlike a miss-fire on a spark plug this idle seems fine if the idle is turned up just a bit.
4)Spark Plugs Black - Carbon buildup on insulator and electrodes of plug. Wipes off your finger as sooty and back. Carbon is a conductor of electricity and getting considerable carbon buildup from a rich condition with ground the inner electrode down the positive insulator. When this condition occurs there will be no spark. You can clean the sparkplug with carburetor cleaner and re-use.
5)Exhaust Exit is Black and Sooty - Excessive carbon buildup in the exhaust is thick and overly grimy. A properly jetted carburetor will leave a nice grey scale color in the end of the exhaust pipe.
6)Odor of unburned fuel - In cases of a very, very rich mixture you may smell some unburned fuel coming out of your exhaust. This could be due to a grounded out sparkplug from too much carbon buildup preventing the combustion process in a cylinder or so much fuel that combustion is not occurring.
7)Runs better with more air - If you remove your air filter and the engine begins to run a bit better with more oxygen in the mix it is likely you have a rich condition.
8)Runs worse at altitude - If the bike starts to run worse at higher altitude with reduced oxygen, it's likely a rich condition.
9)Black Smoke - If you have black exhaust when you rev it, it's rich. You can get a light colored rag and place it over the exhaust lightly and rev it to see if you get carbon/black residue on the rag.

Sources(meant for motorbikes, but the principles are the same) :
symptoms-of-lean-or-rich-carb-settings (http://mechanics.stackexchange.com/questions/732/symptoms-of-lean-or-rich-carb-settings-on-a-motorcycle)
lean-vs-rich/ (http://www.randakksblog.com/lean-vs-rich/)