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Too much time on my hands recently waiting on parts and came up with a simple solution for checking mixture settings. All air going into the carburetors enters at the intake in front of box on left side under the top filler with a hot engine. With a cold engine air is brought up from headers by way of the hot air door inside of intake.
Cut a piece of cardboard 3-1/4"h x 4"w and use it as a shutter up against the intake opening. By limiting airflow we can artificially richen the mixture and any increase in RPM's confirms a lean condition. Starting at the front slowly slide cardboard back closing off intake opening. If the engine is running rich any amount of cardboard in front of intake will decrease RPM's. If the engine is dead nuts on at idle (mixture wise) a 3/4 blockage will make the engine begin to stumble. Will work at all RPM's.
The cardboard will seal the entire intake opening blocking all airflow quickly killing engine. If the engine continues to run with full blockage air is being drawn in elsewhere, likely the hot air door vacuum temperature system malfunction or missing hoses on bottom of air box. Check that the clip is secured bottom left of picture holding intake to air box with a good seal.
AND SOME MORE TESTS WHICH USUALLY SHOULD BE DONE FIRST
The shade tree mechanics way to engine diagnosis:
Will start off on the premise of an otherwise well running and properly maintained engine and has developed a driveability concern. Filters and plugs have been changed at regular intervals. This is a first check of basic systems that anyone can do.
First off is to check the ignition system. With the bike warmed up and idling remove and replace one plug wire at a time from the spark plugs. What we want to observe here is a uniform idle RPM drop when the wire is removed providing no ignition to that cylinder. At this same time we are also testing the plug wires. While slowly removing the wire from plug you will hear a distinct snapping noise of the spark jumping to the plug. It should jump from 1/2-1 inch, this tells us the wires are good with no excess resistance. In rare cases the plug will only fire with the wire partly disconnected adding RPM and is likely due to a fouled plug. Pulling the plug wires can be done at higher RPM's by cranking up the idle screw when needed.
Next up is to remove the spark plugs noting their condition and location within the engine. Note the condition of the plugs below which were taken from my 1500 when acquired with no driveability concerns except a release start type of turning over. Plug gaps were on the wide side. These could have been original twenty two year old plugs as the bike had only thirty thousand on the odometer and I was still going through it with frequent starts and idling. Note the two inner plugs (cyls.3&4) look like they have been running slightly hotter or leaner than the others. This is due in part because inner cylinders on an engine will typically run hotter than the ones on the corners. Note these same cylinders are the ones the vacuum hoses are attached to in the runners which could also contribute to the slight difference. All white or all black plugs on one side usually indicate carburetor problems. If you suspect a bad plug you can swap it with another and repeat to see if the problem follows the plug.
When pulling plug wires and none to little RPM drop we must then find if it is lean, rich or compression related. A compression test is always recommended on an unknown or new to you ride.
AND SOME MORE SPECIFIC TESTING
A good quality carb clean spray (Cyclo) non foaming with straw is a valuable diagnostic aid and I keep a can in the bag. Do not mash on the spray valve, just a lite touch with increasing pressure if needed.
Bike dies on the road: While cranking spray a little in air intake, if bike wants to fire, fuel problem confirmed.
Vacuum leaks?: Spray a little around suspect area, if idle increases or smooths out found source of leak. Pinch off vacuum lines at source to confirm for downstream leaks.
Lean carbs?: Spray a little directly into its individual intake, idle increases or smooths out lean carb confirmed.
Rich carbs?: Spray a little directly into its individual intake, idle decreases
rich carb confirmed within reason.
Quick confirmation of pilot adjustments: Consistent idle (RPM) response when a little spray is introduced to each individual carb intake.
High speed miss?: Spray a little into intake stream, if smooths out and picks up lean condition confirmed.
Avoid overuse around composite parts. (1500 slides for sure) Starting fluid is too volatile and messes things up. When revving the engine you want to confirm vacuum slides are moving in tandem on the 1500 and that all air bleeds are free and clear with good accelerator pump shot.
Other bikes can use sync gauge ports for rich or lean checks. Control the uncovering of ports when screw removed idles up it is rich, spray directly in for lean check while sealing area around straw with your fingers.
Picture of a good burning plug with 20k miles.
Cut a piece of cardboard 3-1/4"h x 4"w and use it as a shutter up against the intake opening. By limiting airflow we can artificially richen the mixture and any increase in RPM's confirms a lean condition. Starting at the front slowly slide cardboard back closing off intake opening. If the engine is running rich any amount of cardboard in front of intake will decrease RPM's. If the engine is dead nuts on at idle (mixture wise) a 3/4 blockage will make the engine begin to stumble. Will work at all RPM's.
The cardboard will seal the entire intake opening blocking all airflow quickly killing engine. If the engine continues to run with full blockage air is being drawn in elsewhere, likely the hot air door vacuum temperature system malfunction or missing hoses on bottom of air box. Check that the clip is secured bottom left of picture holding intake to air box with a good seal.
AND SOME MORE TESTS WHICH USUALLY SHOULD BE DONE FIRST
The shade tree mechanics way to engine diagnosis:
Will start off on the premise of an otherwise well running and properly maintained engine and has developed a driveability concern. Filters and plugs have been changed at regular intervals. This is a first check of basic systems that anyone can do.
First off is to check the ignition system. With the bike warmed up and idling remove and replace one plug wire at a time from the spark plugs. What we want to observe here is a uniform idle RPM drop when the wire is removed providing no ignition to that cylinder. At this same time we are also testing the plug wires. While slowly removing the wire from plug you will hear a distinct snapping noise of the spark jumping to the plug. It should jump from 1/2-1 inch, this tells us the wires are good with no excess resistance. In rare cases the plug will only fire with the wire partly disconnected adding RPM and is likely due to a fouled plug. Pulling the plug wires can be done at higher RPM's by cranking up the idle screw when needed.
Next up is to remove the spark plugs noting their condition and location within the engine. Note the condition of the plugs below which were taken from my 1500 when acquired with no driveability concerns except a release start type of turning over. Plug gaps were on the wide side. These could have been original twenty two year old plugs as the bike had only thirty thousand on the odometer and I was still going through it with frequent starts and idling. Note the two inner plugs (cyls.3&4) look like they have been running slightly hotter or leaner than the others. This is due in part because inner cylinders on an engine will typically run hotter than the ones on the corners. Note these same cylinders are the ones the vacuum hoses are attached to in the runners which could also contribute to the slight difference. All white or all black plugs on one side usually indicate carburetor problems. If you suspect a bad plug you can swap it with another and repeat to see if the problem follows the plug.
When pulling plug wires and none to little RPM drop we must then find if it is lean, rich or compression related. A compression test is always recommended on an unknown or new to you ride.
AND SOME MORE SPECIFIC TESTING
A good quality carb clean spray (Cyclo) non foaming with straw is a valuable diagnostic aid and I keep a can in the bag. Do not mash on the spray valve, just a lite touch with increasing pressure if needed.
Bike dies on the road: While cranking spray a little in air intake, if bike wants to fire, fuel problem confirmed.
Vacuum leaks?: Spray a little around suspect area, if idle increases or smooths out found source of leak. Pinch off vacuum lines at source to confirm for downstream leaks.
Lean carbs?: Spray a little directly into its individual intake, idle increases or smooths out lean carb confirmed.
Rich carbs?: Spray a little directly into its individual intake, idle decreases
rich carb confirmed within reason.
Quick confirmation of pilot adjustments: Consistent idle (RPM) response when a little spray is introduced to each individual carb intake.
High speed miss?: Spray a little into intake stream, if smooths out and picks up lean condition confirmed.
Avoid overuse around composite parts. (1500 slides for sure) Starting fluid is too volatile and messes things up. When revving the engine you want to confirm vacuum slides are moving in tandem on the 1500 and that all air bleeds are free and clear with good accelerator pump shot.
Other bikes can use sync gauge ports for rich or lean checks. Control the uncovering of ports when screw removed idles up it is rich, spray directly in for lean check while sealing area around straw with your fingers.
Picture of a good burning plug with 20k miles.
