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This is another long, long post from bgk.
I have posted this because I know that there are many other GSR members who, like me, have been battling with GS carb-tuning issues, and in particular, a few who have had a chronic symptom with just one dark (rich) spark plug.
I have gotten bits and pieces of ignition and carb diagnostic info by reading through lots of individual questions and answers on the Technical Forum, but I never got the ?big picture? all at once in one place. So, I decided to try and put a logical sequence together in one post.
This is NOT intended to be a definitive step-by-step diagnostic procedure, but rather a conceptual guide to diagnostic steps to be taken in logical order, with references to info already carefully and excellently explained elsewhere on the GSR
This post assumes that your bike at least starts and runs, although it may barely start and may run poorly.
At this point in time, and development of the GS Resources, I?m reasonably convinced that almost every routine GS ignition and carburetor servicing issue question has been asked and answered (probably numerous times) in the Technical Forum. When in doubt, use the Search feature with keyword(s) to find pertinent info in the Technical Forum. All the info I?m posting here has already been posted at various times by different GSR members. In particular, excellent carb info on the Technical Forum is typically contributed by Keith Krause and Earl Fornes, who are generally considered to be the ?carb gurus? on this website. Also, Brian Wringer (bwringer) has put together some very nice step-by-step instructions with photographs of each step on his website (do a Search, to link) for a few GS bike maintenance procedures. And of course, John Bloemer has contributed the Carb Cleanup Series of photographs and instructions, which can be linked to from the Home Page of the GSR website. Many others have contributed on the GSR Technical Forum, too many for me to list (so don?t feel slighted if I didn?t print your name here!).
The bottom line is that you really have to take a logical, sequential approach to solving ignition and carb problems. Simply installing new spark plugs or twisting mixture screws or spraying carb cleaner into the carb fuel and air inlets to cure symptoms will never solve the problem.
I have also found that two critical components of getting things right are a voltmeter (multimeter) and a carb synchronization tool. You can buy a decent digital multimeter from Radio Shack on sale for around $20 and a decent home mechanic quality Motion Pro synch tool from Dennis Kirk or Ron Ayers or other internet sources for about $40. Most everything else can be accomplished with a few well-fitting Standard and Phillips screwdrivers, a metric socket wrench set (about $20 on sale at Sears) and metric combination wrench set (about $20 on sale at Sears). Maybe add a set of feeler gauges, Allen wrenches, a manual impact driver/wrench (the type you hit with a hammer) and a small battery charger. If you shop carefully and wait for sales you can outfit yourself with damn near everything you need for routine GS bike maintenance for around $125. Compare that to $70/hour or so for professional motorcycle mechanic labor, and you?re way ahead doing the work yourself. Also, buy yourself a Suzuki factory repair manual, or a Clymer repair manual.
Cylinders/Rings/Valves
Ideally, the first things you should do (and which most people skip over) is to do a compression check (requires a compression gauge, about $30) and to check the valve clearances (which at the very least will require buying a new valve cover gasket). If any of the valve clearances are out of spec, you?ll also need a valve depressor tool, shim removal tool (Suzuki dealer, Dennis Kirk, Bike Bandit, etc.), and replacement shims of varying thickness. Sometimes, you get lucky and you can just move different shims around from valve to valve, but chances are that you?ll have to buy one or a few new shims from your friendly Suzuki dealer. For this post, I?m going to skip the explanations of compression check and valve shim adjustment procedures.
But the important message here is that you should at least know if you have decent and consistent compression across all four cylinders, and you should know when the last valve adjustment was done on the bike. Therefore, you should be reasonably confidant that your compression and valve clearances are OK before proceeding with ignition and carb diagnostics. If not, do a Search for ?compression test? and ?valve clearance? and learn to do it yourself, or take the bike to a motorcycle shop and have them do it (probably around $200 in labor and parts charges).
Electrical
Most motorcycles, and our GS bikes in particular, really need to have at least 12 volts consistently to run properly. If you?ve got low voltage through your ignition, you should fix it before proceeding further with diagnostics.
So, before you start screwing around with carburetors, do some quick diagnostics on the electrical system. Start with the battery, and work your way through to the spark plugs. Check the electrolyte level in the battery and if any cells are low add some distilled water. (Not tap water you lazy bastard. Drive down to CVS or Walgreens and buy some distilled water!)
Clean the battery terminal connections and charge the battery overnight with a 1 or 2 amp charger, rather than trying to charge the battery in an hour with some ?quick-charge? charger rated at 10 amps or more. The next day, check the battery with a voltmeter. A good healthy battery with a full charge should read approximately 13.2 volts (approximately 2.2 volts per each of the six cells totals 13.2 volts).
Then, turn the ignition key to the ON position (which should activate the headlight, taillight and gauge lights) and check the battery with the voltmeter to make sure that the voltage does not drop to below about 12 volts. Then, (make sure the tranny is in neutral!) start the bike (with choke if required) and check voltage at the battery again, making sure you?re still getting at least 12 volts at idle on choke. Then, after a minute or so of fast idle to warm the oil and engine, shut off the choke (actually the ?fuel enrichener?) and rev the bike up to about 3000 rpm and then up to 5000 rpm. You should get increasing voltage up in the 13 to 14 volt range. If not, you?ve got a charging problem and you?ll have to go read the Stator Pages (go to home page on GSR website) and trouble shoot your charging system accordingly.
In general, it?s always a good idea to check your whole wiring harness, looking for chafed insulation and tight bends, and unplug and check and clean the connectors and put a little dab of dielectric grease on the connector terminals before plugging them back together.
Then, wait for night and/or get the bike into a dark garage and pull all four spark plug caps off the spark plugs and unscrew all four plugs from the cylinder head and re-attach the caps on to the plugs. With tranny in neutral, ground the threaded section of one spark plug at a time to the engine head cooling fins, turn the ignition key ON, press and hold the starter button for a few seconds, and look at each plug sequentially to make sure you?ve got a nice fat blue spark at each spark plug. If not, do a Search for ?test coils? and follow the directions. (You might have to loosely cover the headlight or pull the headlight fuse to get your surroundings dark enough to observe the spark.) While you have the plugs out, observe the color of the ceramic insulators and the condition of the electrodes. Check the air gap between the electrodes (should be around 0.7 mm or 0.027 inch for most GS bikes).
Also, it can?t hurt to pull the plug caps off the plug wires and inspect the ends of the wires to see if you have a nice clean-contact copper wire end. If not, cut about ?-inch off the ends of the wires with a pair of scissors or knife to expose fresh copper and re-attach the plug caps to the wires (some push-in, some screw-on). Look inside the plug caps and unscrew the little brass screws that are recessed inside. Carefully pull out the screws (over a clean, flat table or workbench surface) and a small ceramic cylinder (which is a resister) should fall out. If the resister looks ?cooked? replace your plug caps with new plug caps (do a Search for ?NGK caps?). If you have a pre-1980 model GS bike, you?ll also have to check the points and condenser and ignition timing. (Do a Search for ?points? or ?timing? and follow the directions.)
Fuel
Now, we finally get to the carbs, or more generally the fuel systems. First, check that the choke rail connected to the carburetor end of the choke cable fully closes all four choke plungers in the carbs when you push the choke knob or lever closed. Also check the throttle cable(s) for proper slack and operation from fully closed to fully open (VM carbs have two throttle cables, CV carbs just one). If your plugs look OK and are gapped properly, re-install them. If the plugs are fouled, soak them in regular white vinegar overnight and then clean them with a toothbrush, or simply buy new plugs (always check gap, even with new plugs).
Then, get a small shallow fuel-proof container that will fit underneath the carbs and above the transmission housing. Usually you can find something in your kitchen cabinets that will work. Also, get a small funnel and a baby bottle or other transparent container that has graduated volume markings stamped on the side (liquid ounces, cc?s, etc.). Open the drain screws on the bottoms of the carb float bowls one at a time and carefully drain each carb individually into the shallow container, and pour the contents of each carb individually into the graduated container. Observe the contents of the fuel from each carb (dirt, silt, rust, water, etc.) and record the fuel volume on a piece of paper. Repeat this individually for each carburetor. If the fuel in any or all carbs is dirty, and/or if the volume of fuel drained from each/all of the carbs is not the same, you will surely have to pull the carbs off the bike and clean them and re-set float levels. For this, go to the excellent Carb Cleanup Series on the Home page of the GSR (lower left corner of page under ?Technical? heading) and also do a Search for ?float height? on the Technical Forum.
At this point in the diagnosis, if you had crap in the carb float bowls, you will now have to pull the fuel tank off the bike, drain it, inspect the fuel that comes out of it (dirt, rust, etc.) pull the petcock off the bottom of the tank, inspect and clean the petcock screen, and inspect the inside of the tank to the best of your ability with a flashlight. If your tank needs cleaning, it will be self-evident. If it does need cleaning, you can take it to a local radiator shop and have it cleaned, or do it yourself with a product called POR-15 (do a Search).
Now, at this point in the diagnosis, there could be multiple problems inside the carbs, or none at all. You might need new float valve assemblies, new gaskets, new diaphragms, springs, etc. Maybe some pecker-head Previous Owner might have drilled out the pilot jets or main jets, changed needle-height settings, etc. For this post, I?ll assume that your carbs are stock and the components within the carbs are stock and not worn out or broken.
The critical factors before proceeding to the next step in this post is to make sure that the carbs are clean inside (float bowls, float valves, emulsion tubes, pilot fuel jets, main jets, air jets, etc., and that all the floats are at the correct height (do a Search to find out correct float height for your bike), and that the floats all operate with the same pressure and ?travel distance? when you turn the carbs upside down on your workbench and push down on the float arms gently with your finger to compress the float valve springs and close the float valve assemblies. I bring this up because one of my carbs had a bent float valve pin that increased friction on the float arm which caused the float valve to ?hang-up? and close slowly, resulting in high fuel level in the bowl, even though the float height was set correctly. This was the cause of my ?one dark spark plug? symptom, which drove me absolutely nuts for about a month until I discovered the bent float pin problem. Also, on one of my GS bikes with BS constant velocity (CV) carbs, all my floats were set too high by a Previous Owner, and I had been compensating for this by screwing-in my mixture screws much farther than the ?standard default? setting, which is about 1 and ? to 2 full turns out from lightly seated.
There are hundreds of carb posts on this website, and the answer to your specific carb problem question is probably in there somewhere.
Assuming clean carbs and proper float height and proper friction-free float pin and float arm operation (and assuming the carbs are back on the bike), carefully turn your carb screws in to full closed (very gently, not tight), and keep track of the number of turns for each screw on a piece of paper. (Mikuni VM round slide carbs have separate air pilot screws and fuel pilot screws on each carb. Mikuni BS constant velocity (CV) carbs have a single combined pilot mixture screw on each carb.) If the number of turns in for screws on one carb compared to others are wildly out of whack from one carb to others, that?s a signal that you?ll have to either pull your carbs apart (if you didn?t do this already in the step listed above) and clean them and reset float heights, or that you will have to do a dynamic carb synchronization, or both.
Carb Synch
There are some slight differences of opinion amongst the GSR carb gurus on this site, about how to set up fuel/air/mixture screws before a carb synch.
The opinion I follow for VM carbs is to first set all four pilot fuel screws to a ?default? position, which is 1 and ? turns out from lightly seated, and to then adjust the air screws by the ?best idle method?. This is done by starting and fully warming up the bike (go for a short ride), setting the bike on the center stand, running the bike in neutral, setting the idle to 1100 rpm with the main idle adjusting knob (under the carbs, between carbs #2 and #3). Then, slowly and carefully turn one carb airscrew in and out to obtain the highest rpm idle. After adjusting the first carb airscrew, then set the idle back to 1100 rpm with the main idle screw (under carbs). Then repeat the process for the other three carbs, always returning the idle to 1100 rpm with the main idle knob after adjusting for highest idle with the individual carb airscrew.
The opinion I follow for Mikuni BS carbs, is to simply set all the mixture screws to a ?default? position, which is 1 and ? turns out from lightly seated.
Then, I hook up the Motion Pro carb synch tool, follow the directions that come with the tool (and/or directions posted on the GSR or Brian Wringers website) and get all four mercury columns as close to the same level as possible (some people have #2 and #3 slightly lower than #1 and #4, I just try and get ?em near even across all four).
Then, after the carb synch, I go for a ride of at least 20-minutes, trying to keep my throttle settings below ? throttle the whole time. I find a safe place, pull in the clutch lever, chop the throttle, turn of the ignition, coast to a stop off the road, pull out my spark plug wrench, and check all four plugs. The ?plug reads? will indicate what you need to do. If a plug is dark, you need to turn the fuel screw (on VM carbs) or mixture screw (on BS carbs) IN (clockwise) to lean the mixture. If the plug is white, turn the fuel or mixture screw OUT (counterclockwise) to richen the mixture.
On my 1980 GS1000G with BS carbs, I just adjust the mixture screws in (leaner) or out (richer) depending on the plug insulator color I see after the ?chop throttle? test. Then I put the plugs back in, ride some more and repeat the ?chop throttle and read plugs? test.
On my 1979 GS1000L with VM carbs, it?s slightly more complicated, but the same general idea. During the chop throttle and read plugs? test, I only adjust the fuel mixture screws (under the carbs) and repeat the test and pilot screw adjustments a few times until the plugs look generally evenly tan. Then, I repeat the ?highest idle? test with the air mixture screws.
I am not sophisticated enough yet to screw around with changing jets and changing jet needle clip positions. This is mostly because my VM carbs were cleaned and rebuilt and set up with a Dynojet kit by one of the carb gurus on this website, and there?s no way I?m gonna screw around with that expert setup! This is also because my BS carbs are bone stock on a bone stock bike, so I have no reason to be screwing around with the factory jets and needle jet clip settings.
So anyway, I hope this post helps some of my fellow GSers to diagnose and fix some ignition and carburetor problems.
Happy GS?n to you all.
I have posted this because I know that there are many other GSR members who, like me, have been battling with GS carb-tuning issues, and in particular, a few who have had a chronic symptom with just one dark (rich) spark plug.
I have gotten bits and pieces of ignition and carb diagnostic info by reading through lots of individual questions and answers on the Technical Forum, but I never got the ?big picture? all at once in one place. So, I decided to try and put a logical sequence together in one post.
This is NOT intended to be a definitive step-by-step diagnostic procedure, but rather a conceptual guide to diagnostic steps to be taken in logical order, with references to info already carefully and excellently explained elsewhere on the GSR
This post assumes that your bike at least starts and runs, although it may barely start and may run poorly.
At this point in time, and development of the GS Resources, I?m reasonably convinced that almost every routine GS ignition and carburetor servicing issue question has been asked and answered (probably numerous times) in the Technical Forum. When in doubt, use the Search feature with keyword(s) to find pertinent info in the Technical Forum. All the info I?m posting here has already been posted at various times by different GSR members. In particular, excellent carb info on the Technical Forum is typically contributed by Keith Krause and Earl Fornes, who are generally considered to be the ?carb gurus? on this website. Also, Brian Wringer (bwringer) has put together some very nice step-by-step instructions with photographs of each step on his website (do a Search, to link) for a few GS bike maintenance procedures. And of course, John Bloemer has contributed the Carb Cleanup Series of photographs and instructions, which can be linked to from the Home Page of the GSR website. Many others have contributed on the GSR Technical Forum, too many for me to list (so don?t feel slighted if I didn?t print your name here!).
The bottom line is that you really have to take a logical, sequential approach to solving ignition and carb problems. Simply installing new spark plugs or twisting mixture screws or spraying carb cleaner into the carb fuel and air inlets to cure symptoms will never solve the problem.
I have also found that two critical components of getting things right are a voltmeter (multimeter) and a carb synchronization tool. You can buy a decent digital multimeter from Radio Shack on sale for around $20 and a decent home mechanic quality Motion Pro synch tool from Dennis Kirk or Ron Ayers or other internet sources for about $40. Most everything else can be accomplished with a few well-fitting Standard and Phillips screwdrivers, a metric socket wrench set (about $20 on sale at Sears) and metric combination wrench set (about $20 on sale at Sears). Maybe add a set of feeler gauges, Allen wrenches, a manual impact driver/wrench (the type you hit with a hammer) and a small battery charger. If you shop carefully and wait for sales you can outfit yourself with damn near everything you need for routine GS bike maintenance for around $125. Compare that to $70/hour or so for professional motorcycle mechanic labor, and you?re way ahead doing the work yourself. Also, buy yourself a Suzuki factory repair manual, or a Clymer repair manual.
Cylinders/Rings/Valves
Ideally, the first things you should do (and which most people skip over) is to do a compression check (requires a compression gauge, about $30) and to check the valve clearances (which at the very least will require buying a new valve cover gasket). If any of the valve clearances are out of spec, you?ll also need a valve depressor tool, shim removal tool (Suzuki dealer, Dennis Kirk, Bike Bandit, etc.), and replacement shims of varying thickness. Sometimes, you get lucky and you can just move different shims around from valve to valve, but chances are that you?ll have to buy one or a few new shims from your friendly Suzuki dealer. For this post, I?m going to skip the explanations of compression check and valve shim adjustment procedures.
But the important message here is that you should at least know if you have decent and consistent compression across all four cylinders, and you should know when the last valve adjustment was done on the bike. Therefore, you should be reasonably confidant that your compression and valve clearances are OK before proceeding with ignition and carb diagnostics. If not, do a Search for ?compression test? and ?valve clearance? and learn to do it yourself, or take the bike to a motorcycle shop and have them do it (probably around $200 in labor and parts charges).
Electrical
Most motorcycles, and our GS bikes in particular, really need to have at least 12 volts consistently to run properly. If you?ve got low voltage through your ignition, you should fix it before proceeding further with diagnostics.
So, before you start screwing around with carburetors, do some quick diagnostics on the electrical system. Start with the battery, and work your way through to the spark plugs. Check the electrolyte level in the battery and if any cells are low add some distilled water. (Not tap water you lazy bastard. Drive down to CVS or Walgreens and buy some distilled water!)
Clean the battery terminal connections and charge the battery overnight with a 1 or 2 amp charger, rather than trying to charge the battery in an hour with some ?quick-charge? charger rated at 10 amps or more. The next day, check the battery with a voltmeter. A good healthy battery with a full charge should read approximately 13.2 volts (approximately 2.2 volts per each of the six cells totals 13.2 volts).
Then, turn the ignition key to the ON position (which should activate the headlight, taillight and gauge lights) and check the battery with the voltmeter to make sure that the voltage does not drop to below about 12 volts. Then, (make sure the tranny is in neutral!) start the bike (with choke if required) and check voltage at the battery again, making sure you?re still getting at least 12 volts at idle on choke. Then, after a minute or so of fast idle to warm the oil and engine, shut off the choke (actually the ?fuel enrichener?) and rev the bike up to about 3000 rpm and then up to 5000 rpm. You should get increasing voltage up in the 13 to 14 volt range. If not, you?ve got a charging problem and you?ll have to go read the Stator Pages (go to home page on GSR website) and trouble shoot your charging system accordingly.
In general, it?s always a good idea to check your whole wiring harness, looking for chafed insulation and tight bends, and unplug and check and clean the connectors and put a little dab of dielectric grease on the connector terminals before plugging them back together.
Then, wait for night and/or get the bike into a dark garage and pull all four spark plug caps off the spark plugs and unscrew all four plugs from the cylinder head and re-attach the caps on to the plugs. With tranny in neutral, ground the threaded section of one spark plug at a time to the engine head cooling fins, turn the ignition key ON, press and hold the starter button for a few seconds, and look at each plug sequentially to make sure you?ve got a nice fat blue spark at each spark plug. If not, do a Search for ?test coils? and follow the directions. (You might have to loosely cover the headlight or pull the headlight fuse to get your surroundings dark enough to observe the spark.) While you have the plugs out, observe the color of the ceramic insulators and the condition of the electrodes. Check the air gap between the electrodes (should be around 0.7 mm or 0.027 inch for most GS bikes).
Also, it can?t hurt to pull the plug caps off the plug wires and inspect the ends of the wires to see if you have a nice clean-contact copper wire end. If not, cut about ?-inch off the ends of the wires with a pair of scissors or knife to expose fresh copper and re-attach the plug caps to the wires (some push-in, some screw-on). Look inside the plug caps and unscrew the little brass screws that are recessed inside. Carefully pull out the screws (over a clean, flat table or workbench surface) and a small ceramic cylinder (which is a resister) should fall out. If the resister looks ?cooked? replace your plug caps with new plug caps (do a Search for ?NGK caps?). If you have a pre-1980 model GS bike, you?ll also have to check the points and condenser and ignition timing. (Do a Search for ?points? or ?timing? and follow the directions.)
Fuel
Now, we finally get to the carbs, or more generally the fuel systems. First, check that the choke rail connected to the carburetor end of the choke cable fully closes all four choke plungers in the carbs when you push the choke knob or lever closed. Also check the throttle cable(s) for proper slack and operation from fully closed to fully open (VM carbs have two throttle cables, CV carbs just one). If your plugs look OK and are gapped properly, re-install them. If the plugs are fouled, soak them in regular white vinegar overnight and then clean them with a toothbrush, or simply buy new plugs (always check gap, even with new plugs).
Then, get a small shallow fuel-proof container that will fit underneath the carbs and above the transmission housing. Usually you can find something in your kitchen cabinets that will work. Also, get a small funnel and a baby bottle or other transparent container that has graduated volume markings stamped on the side (liquid ounces, cc?s, etc.). Open the drain screws on the bottoms of the carb float bowls one at a time and carefully drain each carb individually into the shallow container, and pour the contents of each carb individually into the graduated container. Observe the contents of the fuel from each carb (dirt, silt, rust, water, etc.) and record the fuel volume on a piece of paper. Repeat this individually for each carburetor. If the fuel in any or all carbs is dirty, and/or if the volume of fuel drained from each/all of the carbs is not the same, you will surely have to pull the carbs off the bike and clean them and re-set float levels. For this, go to the excellent Carb Cleanup Series on the Home page of the GSR (lower left corner of page under ?Technical? heading) and also do a Search for ?float height? on the Technical Forum.
At this point in the diagnosis, if you had crap in the carb float bowls, you will now have to pull the fuel tank off the bike, drain it, inspect the fuel that comes out of it (dirt, rust, etc.) pull the petcock off the bottom of the tank, inspect and clean the petcock screen, and inspect the inside of the tank to the best of your ability with a flashlight. If your tank needs cleaning, it will be self-evident. If it does need cleaning, you can take it to a local radiator shop and have it cleaned, or do it yourself with a product called POR-15 (do a Search).
Now, at this point in the diagnosis, there could be multiple problems inside the carbs, or none at all. You might need new float valve assemblies, new gaskets, new diaphragms, springs, etc. Maybe some pecker-head Previous Owner might have drilled out the pilot jets or main jets, changed needle-height settings, etc. For this post, I?ll assume that your carbs are stock and the components within the carbs are stock and not worn out or broken.
The critical factors before proceeding to the next step in this post is to make sure that the carbs are clean inside (float bowls, float valves, emulsion tubes, pilot fuel jets, main jets, air jets, etc., and that all the floats are at the correct height (do a Search to find out correct float height for your bike), and that the floats all operate with the same pressure and ?travel distance? when you turn the carbs upside down on your workbench and push down on the float arms gently with your finger to compress the float valve springs and close the float valve assemblies. I bring this up because one of my carbs had a bent float valve pin that increased friction on the float arm which caused the float valve to ?hang-up? and close slowly, resulting in high fuel level in the bowl, even though the float height was set correctly. This was the cause of my ?one dark spark plug? symptom, which drove me absolutely nuts for about a month until I discovered the bent float pin problem. Also, on one of my GS bikes with BS constant velocity (CV) carbs, all my floats were set too high by a Previous Owner, and I had been compensating for this by screwing-in my mixture screws much farther than the ?standard default? setting, which is about 1 and ? to 2 full turns out from lightly seated.
There are hundreds of carb posts on this website, and the answer to your specific carb problem question is probably in there somewhere.
Assuming clean carbs and proper float height and proper friction-free float pin and float arm operation (and assuming the carbs are back on the bike), carefully turn your carb screws in to full closed (very gently, not tight), and keep track of the number of turns for each screw on a piece of paper. (Mikuni VM round slide carbs have separate air pilot screws and fuel pilot screws on each carb. Mikuni BS constant velocity (CV) carbs have a single combined pilot mixture screw on each carb.) If the number of turns in for screws on one carb compared to others are wildly out of whack from one carb to others, that?s a signal that you?ll have to either pull your carbs apart (if you didn?t do this already in the step listed above) and clean them and reset float heights, or that you will have to do a dynamic carb synchronization, or both.
Carb Synch
There are some slight differences of opinion amongst the GSR carb gurus on this site, about how to set up fuel/air/mixture screws before a carb synch.
The opinion I follow for VM carbs is to first set all four pilot fuel screws to a ?default? position, which is 1 and ? turns out from lightly seated, and to then adjust the air screws by the ?best idle method?. This is done by starting and fully warming up the bike (go for a short ride), setting the bike on the center stand, running the bike in neutral, setting the idle to 1100 rpm with the main idle adjusting knob (under the carbs, between carbs #2 and #3). Then, slowly and carefully turn one carb airscrew in and out to obtain the highest rpm idle. After adjusting the first carb airscrew, then set the idle back to 1100 rpm with the main idle screw (under carbs). Then repeat the process for the other three carbs, always returning the idle to 1100 rpm with the main idle knob after adjusting for highest idle with the individual carb airscrew.
The opinion I follow for Mikuni BS carbs, is to simply set all the mixture screws to a ?default? position, which is 1 and ? turns out from lightly seated.
Then, I hook up the Motion Pro carb synch tool, follow the directions that come with the tool (and/or directions posted on the GSR or Brian Wringers website) and get all four mercury columns as close to the same level as possible (some people have #2 and #3 slightly lower than #1 and #4, I just try and get ?em near even across all four).
Then, after the carb synch, I go for a ride of at least 20-minutes, trying to keep my throttle settings below ? throttle the whole time. I find a safe place, pull in the clutch lever, chop the throttle, turn of the ignition, coast to a stop off the road, pull out my spark plug wrench, and check all four plugs. The ?plug reads? will indicate what you need to do. If a plug is dark, you need to turn the fuel screw (on VM carbs) or mixture screw (on BS carbs) IN (clockwise) to lean the mixture. If the plug is white, turn the fuel or mixture screw OUT (counterclockwise) to richen the mixture.
On my 1980 GS1000G with BS carbs, I just adjust the mixture screws in (leaner) or out (richer) depending on the plug insulator color I see after the ?chop throttle? test. Then I put the plugs back in, ride some more and repeat the ?chop throttle and read plugs? test.
On my 1979 GS1000L with VM carbs, it?s slightly more complicated, but the same general idea. During the chop throttle and read plugs? test, I only adjust the fuel mixture screws (under the carbs) and repeat the test and pilot screw adjustments a few times until the plugs look generally evenly tan. Then, I repeat the ?highest idle? test with the air mixture screws.
I am not sophisticated enough yet to screw around with changing jets and changing jet needle clip positions. This is mostly because my VM carbs were cleaned and rebuilt and set up with a Dynojet kit by one of the carb gurus on this website, and there?s no way I?m gonna screw around with that expert setup! This is also because my BS carbs are bone stock on a bone stock bike, so I have no reason to be screwing around with the factory jets and needle jet clip settings.
So anyway, I hope this post helps some of my fellow GSers to diagnose and fix some ignition and carburetor problems.
Happy GS?n to you all.
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