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Yamaha f40betl 2000 yeae model bogging down

Z

zonez

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After driving at low rpm or idle for longer durations the motors idle starts to slow down and engine might shut down and when trying to accelerate it hesitates for a while and if i push it to full gas straight the engine shuts down. But if i drive it faster and dont idle or drive at low rpm for long time the motor works perfectly. I checked the spark plugs and they are black and smutty. So is there something to do or is it normal in carburetor engine to do this if driving at low rpm for longer periods. I have cleaned carbs, changed new spark plugs, float chamber seals ,fuel line from tank to motor, fuel filter and tried adjusting the carbs. I have put the carbs to 1.75 turns open at leanest
 

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2.5 turns from being lightly seated is the base idle mixture screw setting.

How were the carburetors cleaned? If not fully dismantled and run through a heated ultrasonic cleaner, using a good cleaning fluid, and then carefully inspected to ensure all holes and passageways are free and clear, it might be a waste of time.

If and when the throttle is advanced quickly, are you seeing a spray of gasoline coming from the accelerator nozzle?
 
boscoe said:
2.5 turns from being lightly seated is the base idle mixture screw setting.

How were the carburetors cleaned? If not fully dismantled and run through a heated ultrasonic cleaner, using a good cleaning fluid, and then carefully inspected to ensure all holes and passageways are free and clear, it might be a waste of time.

If and when the throttle is advanced quickly, are you seeing a spray of gasoline coming from the accelerator nozzle?
Click to expand...
Yes i dismantled the carburetors and have ran them trough ultrasonic cleaner many times and after that cleaned them by hand and used wire to ensure every hole is open. Accelerators work too. This problem i have happens only after low rpm driving. I have to use the throttoe carefully to get the motor to high rpm but when it does it works perfectly again until long period low rpm driving
 
How about the fuel pumps? Perhaps they are not working well at low RPM's.

If you use the primer ball to keep the carburetors full of gasoline just before you go to accelerate after a long period of low RPM operations, does the problem go away or stay the same?
 
boscoe said:
How about the fuel pumps? Perhaps they are not working well at low RPM's.

If you use the primer ball to keep the carburetors full of gasoline just before you go to accelerate after a long period of low RPM operations, does the problem go away or stay the same?
Click to expand...
The problem stays the same. I put new diaphgram in the fuel pump too.
 
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