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503 Everest Single Carb Mikuni Won't Idle

Rotax 503 Mikuni Carb Dies at idle lean

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#1 bjp

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Posted 19 April 2020 - 07:30 PM

I have an '81 Everest 503 with a single Mikuni carb that won't stay idling, it dies after about 10 seconds if I don't give it a partial primer pump or put my fingers over the carb throat.

  • Dies from being too lean.
  • Holding palm over carb throat or giving it a pump of the primer it starts right up. 
  • Playing with throttle and keeping RPM's above 3000 and it will run all day. 
  • Holding fingers across carb throat or putting a towel over carb throat and it runs all day.
  • Giving it a little primer shot when it starts to die keeps it running all day.
  • Adjusting idle air screw from closed to open 2 turns has no effect.
  • Put new seals on both ends, same symptom as prior.
  • Tried different carb, same symptom.
  • Disconnected fuel pump pulse line at crankcase and plugged fitting, no change.
  • Put on new carb boot, no change.

 

Any thoughts or tips?

Thanks,

BP


Edited by bjp, 20 April 2020 - 05:42 PM.


#2 70-335

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Posted 19 April 2020 - 07:38 PM

Sounds like pilot jet in carb plugged not allowing fuel for idle circuit



#3 bjp

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Posted 19 April 2020 - 11:27 PM

Thanks for replying.  I did clean the pilot, but can check again. I thought pilot controlled off-idle to mid range where the needle takes over, and idle air screw controlled actual slow speed idle mixture?  



#4 Skidoogrampa

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Posted 20 April 2020 - 04:46 AM

Thanks for replying.  I did clean the pilot, but can check again. I thought pilot controlled off-idle to mid range where the needle takes over, and idle air screw controlled actual slow speed idle mixture?  

Hello bjp.

 

You may want to first check and make sure your carb boot is not cracked and letting in any air between carb and intake manifold.

 

The pilot is the sole source of fuel at idle. It also DOES provide some fuel at off idle. It's a fine dance between enough fuel supplied at idle and enough/too much air from the air screw. Re-clean the pilot jet and start with your air screw turned in to richen the mixture. Try starting at 1/2 turn out from bottoming and if it starts and runs rich, lean it out from there by unscrewing. Set the final air adjustment once engine has warmed up for a few minutes.

 

If all intake air leaks are eliminated and the air screw needs to be turned in more than a half turn, the pilot jet needs to be changed or drilled out to the next size up (careful...it's real easy to drill out too much...ask me how I know!). You can see the number on the face of the jet.  

 

When you get good at this you'll be ready to take on the next challenge...dual carbs! (then it gets to be more like a square dance with multiple partners!).

 

Let us know how it turns out.

 

Rene


Edited by Skidoogrampa, 20 April 2020 - 04:48 AM.

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#5 john170950

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Posted 20 April 2020 - 08:40 AM

  • Tried different carb, same symptom.


You may want to first check and make sure your carb boot is not cracked and letting in any air between carb and intake manifold.


Edited by john170950, 20 April 2020 - 08:40 AM.

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#6 Skidoogrampa

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Posted 20 April 2020 - 01:23 PM

 

  • Tried different carb, same symptom.


You may want to first check and make sure your carb boot is not cracked and letting in any air between carb and intake manifold.

 

Good point on having tried a different carb. The different carb may have a smaller pilot than needed or it may also be plugged. Many old carbs are prone to plugged jets especially if previously run with ethanol gas. If I'm out in left field, please let us know.  


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On a Stratos, you're neither leader or follower...you simply make your own tracks.


#7 bjp

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Posted 20 April 2020 - 11:15 PM

Swapped pilot jet and it purrs like a kitten, thanks for the advice, especially to Grampa!  Case closed!



#8 Skidoogrampa

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Posted 21 April 2020 - 06:07 AM

I'm glad that 70-335 and I could help you out.  :D

 

Rene 


On a Stratos, you're neither leader or follower...you simply make your own tracks.






Also tagged with one or more of these keywords: Rotax, 503, Mikuni, Carb, Dies at idle, lean

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