+-

Author Topic: bouncing before returning to idle.  (Read 717 times)

Offline bigbird337

  • Trade Count: (0)
  • Newbie
  • **
  • Posts: 10
bouncing before returning to idle.
« on: July 31, 2018, 12:08:04 pm »
I was out this weekend and my quad idle would bounce up and down for several seconds before returning to normal idle after winding it up. then I had the dreaded diesel run when I got low on fuel and had to choke it out to turn off. any ideas on a cause for both or could it be two separate problems.

Share on Facebook Share on Twitter


Offline Rainman56

  • Trade Count: (0)
  • Elite Member
  • **********
  • Posts: 1185
  • Location: Ontario,Great White North
Re: bouncing before returning to idle.
« Reply #1 on: July 31, 2018, 02:51:23 pm »
Sounds like you're maybe running lean...Possible air leak somewhere?
Can,t fix stupid.:)

Offline Q2W

  • Trade Count: (0)
  • Forum Moderator
  • Elite Member
  • *********
  • Posts: 1318
  • Gender: Male
  • Location: NWA
Re: bouncing before returning to idle.
« Reply #2 on: August 01, 2018, 07:03:42 am »
Sounds like you're maybe running lean...Possible air leak somewhere?

+1 air leak or lean carb circuit.

Offline MotorGeek - Jerry Hall

  • Trade Count: (0)
  • Pro Member
  • ********
  • Posts: 851
Re: bouncing before returning to idle.
« Reply #3 on: August 02, 2018, 09:11:18 pm »
What type of carburetor are you running and what is your carburetor jetting specs?

Offline bigbird337

  • Trade Count: (0)
  • Newbie
  • **
  • Posts: 10
Re: bouncing before returning to idle.
« Reply #4 on: July 14, 2019, 09:16:47 am »
Sorry for the late reply.running a stock mikuni tm38. Tore the carb down. 32.5 pilot 410main needle clip 3rd from top. Just did pressure check no leaks.

Offline MotorGeek - Jerry Hall

  • Trade Count: (0)
  • Pro Member
  • ********
  • Posts: 851
Re: bouncing before returning to idle.
« Reply #5 on: September 03, 2019, 12:05:57 pm »
Is information that I want to know. Thank you for sharing.

You did not give the needle jet number and the needle number.  The fuel flow area between the needle jet and the needle at closed throttle has a huge effect on the ring ding dings, when the RPMs are high, and the engine RPMs are heading toward the stabilized idle RPM.

You will not find this information in ANY of the Mikuni, Keihin, Delorito, Bing, Amal Etc. tuning manuals.  It took me 30 years of tuning high performance two strokes before testing indicated there is more fuel flowing between the needle and needle jet, at closed throttle, than there is from the pilot jet at high RPM.

The fuel flow from the needle jet diminishes to zero as the engine RPM approaches an idle RPM.  The crankcase surfaces "dry out" at high RPM.  Adequate fuel flow into the air entering the crankcase must be present when the throttle is closed and the RPMs are returning to an idle RPM.  The crankcase surfaces must be "wetter" for a stable idle RPM.

 The optimum crankcase surface "wetness" changes with RPM and throttle position.  When the RPM is low enough that there is not any fuel flow from the needle jet, the pilot jet has to supply all of the fuel to the engine.  The surfaces in the crankcase will rob fuel from the air fuel mixture traveling through the crankcase until these surfaces have reached their stabilized "wetness".  During this short time period when the crankcase surfaces are "rewetting" the mixture is lean causing the ring ding dings.

A large enough pilot jet will stop the ring ding dings as it approaches an idle speed, but after a few seconds or less, the engine wants to load up and die.  The pilot jet is too rich to allow the proper idle mixture once the crankcase surfaces have "rewetted" and will not accept any more fuel.  Once the crankcase's surfaces "wetness" has stabilized all of the fuel from the pilot jet will end up in the combustion chamber, creating the idle mixture that is solely dependant upon the pilot jet size and air mixture setting.  . 


 

+- Suzuki Quadracer HQ Store Zone

http://www.suzukiquadracerhq.com/index.php?action=store