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1981 Suzuki GS850GL – bogging/stuttering under load

M

Moarpower

Forum Apprentice
I’m chasing a gradual power loss and bogging issue on my 1981 GS850GL.


Setup:


  • Dynojet kit
  • Pod filters
  • 4-into-1/open exhaust
  • New Rick’s stator
  • New regulator/rectifier
  • New Dyna-style coils
  • New copper-core leads, resistor caps and plugs

The bike previously ran very well with this exact carb/jetting setup and had strong power. The issue has developed gradually over the last couple of months. It now feels roughly 10–15% down on power overall.


Symptoms:


  • First and second will still accelerate fairly cleanly to around 9,000 rpm.
  • Third will pull cleanly if I shift into it at high rpm.
  • If I’m cruising at lower rpm in third and suddenly open the throttle hard, it may stutter/bog like it is running out of fuel.
  • If I ease the throttle on until around 5,000 rpm and then go wide open, it generally takes off properly.
  • Fourth is similar but does not recover or pull as strongly.
  • In fifth, even at 100–110 km/h, opening the throttle fully causes it to bog/stutter and lose power.
  • Above roughly 7,000 rpm it can start jerking and feel like the power is returning.
  • If I cruise in fifth at very low rpm, around 40 km/h, the bogging gets progressively worse the longer I do it.
  • Once it has become bad, changing down to fourth does not immediately clear it. I need to get down into third or second, raise the rpm and eventually it clears and starts running properly again.

It sounds/feels like a repeated “bop, bop, bop” bog or misfire under load.


Already checked/replaced:


  • Coils and HT leads replaced – no change.
  • New plugs – all four looked very similar.
  • Fuel cap removed during a test ride – no change, so tank vent appears ruled out.
  • Petcock on PRI – no change.
  • Choke cable is permanently disconnected and the enrichener is fully closed.
  • Pilot-jet rubber plugs are new and installed.
  • The Dynojet needle/spacer arrangement has not been changed and previously worked very well.

I’m not looking to change jet sizes at this stage because the bike ran strongly with the existing setup before this fault developed.


Current possibilities I’m considering:


  • Incorrect or unstable wet float levels
  • Float needles/seats or restricted seat screens
  • CV diaphragm leak or sticking slide
  • Petcock diaphragm leaking fuel through the vacuum hose
  • Ignition pickup/igniter issue
  • Mechanical advance sticking
  • Intake boot/O-ring vacuum leak

Has anyone had this exact type of load-dependent bogging on a GS850 with BS32SS carbs? Particularly interested in faults that worsen after prolonged low-rpm running but then clear once the engine is revved hard.
 
Something you said caught my eye: when run at higher rpm plugs eventually 'clear.'

Sounds like a review of a Contach or some such Italian super car decades ago. They fired it up, it smoked and ran like shyte, so they called the owner. "That's normal," he told them. "The plugs are just fouled. Drive it a while and it will start clearing up." And it did.

Yours sounds like a multitude of problems; so bite the bullet and adjust only one thing at a time.
 
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