Article concludes this thread.
The accelerating pump nozzle is on the right, just inside the carburetor throat. Twist the throttle and a squirt of gas will be injected back into the carburetor venturi.
The slow air jet is also at the right front, just inside the carburetor throat but behind the accelerator pump nozzle. The float bowl has to be removed for access to the slow air jet. The slow jet sits near the top of the float bowl reservoir. Above it extends the slow jet bleed tube that feeds gas through a channel to the top of the carburetor’s idle transfer ports and idle port.
The main air jet is to the left of the accelerating pump nozzle. The float bowl needs removal for access to the main air jet. The main jet nozzle extends up from the float bowl and into the venturi. The main jet itself is located in the float bowl in the gasoline reservoir. Above it extends the main bleed tube, which becomes the main nozzle extending into the venturi.
Troubleshooting
Most problems occur because of rider neglect and inadequate daily or weekly maintenance. The reader will quickly observe many common problems and solutions. Gas cap venting, float levels, dirty air filters, sediment in the gas tank, and carburetor air leaks top the list. In short, many times troubleshooting is necessary because of inadequate maintenance. I have listed the causes and remedies in regard to difficulty and expense, starting with the easiest and cheapest. Consult your various manuals for troubleshooting charts, tips, and a better understanding of the Keihin carburetor that has been updated almost constantly since its inception in 1976.
If your gas mileage is high, check the gas cap vent. If it’s plugged, remove the dirt with a wire or compressed air, or else replace the gas cap. All tanks that use a single gas cap have a vented cap. The right-side gas cap vents on dual-cap tanks. The next item to inspect is the air filter. If it’s dirty, clean or replace it. See if your engine is idling too high. If so, adjust it to specifications. Look at the choke butterfly plate. If it’s partially closed when it should be fully open, adjust the choke cable wire. Inspect the float level. If it’s too high, adjust the float level as described earlier in this article. Inspect the carburetor jets and tighten them if they’re loose. Inspect the bleed tubes to see if they’re clogged and clean them if necessary.
If the engine is idling rough, check the idle speed as per spec and adjust as needed. Check the gas cap vent; if it’s plugged, proceed as you would if your mileage was high. You may fix the idling problem and pick up a few more miles per gallon. Inspect various sealing components for an air leak, which may include a worn throttle plate or choke plate shaft where it goes through the carburetor body. Look for a worn insulator block, which is between the carburetor and intake manifold, loose mounting nuts, or, the most likely culprit, a leaking manifold seal. Replace worn parts, retighten loose fasteners, and always use new rubber seals. Check if the low speed jet is loose and tighten if needed. If it’s clogged, clean it. Determine if the idle port is clogged and clean if needed.
If the engine is hard to start, inspect the carburetor and clean if dirty. Inspect the carburetor mounting hardware for tightness. Loose hardware causes an air leak, which can severely affect the air/fuel ratio. Inspect the choke for proper operation and repair or adjust as needed. Inspect float level and adjust if needed. Inspect the float bowl and/or clean out any sediment.
A lean air/fuel mixture has only three possible causes, if the carb was properly jetted for the engine. Inspect the carburetor and clean if dirty. Inspect the fuel line for internal dirt, blockage, and kinking and clean or replace as needed. I always replace the line if problematic. Inspect various sealing components for an air leak, as with rough idling.
If fuel is overflowing, check for loose float bowl screws. (There are five shorter screws and one long one.) Check the gas cap vent to see if it’s plugged, which allows the fuel tank to pressurize and force fuel into the float bowl. Service the gas cap as you would with poor mileage. Check for dirt in the float inlet valve (needle) seat. Clean with compressed air. Check for a worn inlet valve (float needle) and replace if necessary. Inspect the float level. If it’s too high, adjust the float level as described earlier in this article. Check the float gasket for wear and replace as needed. I replace the gasket and seals every time I work on a carburetor. Check for a worn float pin. Check for a loose float retaining screw. Also check the float mounting tabs for wear. Determine if the float is deformed. If so, replace with a new float.
Now, for a favorite topic, improving poor performance. Check the gas cap vent to see if it’s plugged. Check the air filter and wash or replace if dirty. Inspect the throat of the carburetor and clean if dirty. Check the spark plugs to see if there’s an intake leak. If the plugs are blistery-white, find the air leak, which may be the intake manifold seals. Check the idle for proper adjustment. If the idle is not within specifications, adjust it. Inspect the main jet; if loose, tighten. Inspect for gas overflow as previously stated.
There are two possible causes for a loss in power: inadequate gas or inadequate air. If your power loss is caused by gas issues, check the gas cap vent and spark plugs, using the methods and solutions listed earlier. Then inspect the fuel line for internal deterioration (due to ethanol), obstructions, or kinking. If you find any problems with it, always replace the fuel line because the expense is minimal. Inspect the carburetor for cleanliness. Inspect accelerating pump operation (squirt) into the venturi. If it’s not working, repair it. The cause will most likely be a damaged diaphragm. Inspect the external aftermarket fuel filter, if you have one. Clean out the sediment and/or replace the filter. Inspect the fuel petcock internal fuel strainer (filter). Remove any sediment from the tank. Flush the fuel tank if necessary or the problem will certainly recur. Inspect the fuel tank. If dirty with sediment (oxidation, micro paint chips, dirt from gas filling, etc.), clean the tank and reseal if necessary. For a power loss from an inadequate air supply, inspect the air filter. If dirty, clean or replace. Inspect the throttle/idle cables. Correct them if they aren’t working properly or are not properly adjusted. High altitude riding will require changing the carburetor jets to balance the air and fuel mix. Inspect jets for looseness and tighten if needed.
Starting with the very basics, if the engine has poor acceleration, check for low fuel (fill the tank!). Check that the accelerating pump is squirting fuel into the venturi. If not, clean the system and inspect the accelerating pump diaphragm for damage. If damaged, cut, or worn, replace with a new one. Inspect the low-speed jet and bleed tube for obstruction or clogging.
If you have problems once you get up to highway speed, check the gas cap vent, the air filter, and any external aftermarket fuel filter, in that order. Inspect the fuel petcock internal fuel strainer (filter). Remove any sediment from the filter and flush the fuel tank. Check the float level as a low level may cause performance problems. Adjust the float level, if necessary, to 0.63"-0.67" (16-17mm). Inspect the main jet for tightness. Check the main jet air passage for clogs.
As stated at the beginning, most problems occur because of rider neglect and inadequate daily or weekly maintenance.
Conclusion
Next month we’ll finish with the butterfly Keihin and start on the CV Keihin carb and its more refined ways of delivering the proper air/fuel mixture to the engine.
Donny Petersen
Tattoo Tony’s Heavy Duty Cycles
Toronto, Canada
www.HeavyDutyCycles.com 
Jan. 2010-Techline-EVOLUTION FUEL DELIVERY Part IV: (concludes) 
Linear Mode
