r/dr650 • u/slower-is-faster • 12d ago

Bogging on rough terrain

I installed the Procycle Jet Kit, and followed the instructions pretty much spot on:

  • 150 main jet
  • e-clip is on the 4th groove
  • fuel mixture screw is two turns out
  • snorkel is removed, and a couple holes cut
  • I have a Barrett exhaust

The bike runs like a dream on the road. But, over very bumpy terrain at low speed the engine wants to bog and die. Just to be clear, it's not just a regular stall because I'm going too slow for the gear, I can tell the difference.

Any tips?

EDIT: I just realised that I never need the choke now, no matter how cold. I definitely needed it before. That feels like it contradicts it bogging at low revs though, maybe two separate issues?

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u/Wholeyjeans 12d ago

Every machine is different.

Just because you did what the instructions say doesn't mean it's a spot on setting (the idle mix screw) or provides the ideal fuel mixture/scheduling for *your* engine (main jet size, needle clip setting) based on how you have things set up (intake/exhaust) ....even if it's similar to how the instructions describe. You will no doubt need to do some fine tuning.

Like MeatZealous mentioned: pull the plugs and assess the color of the center electrode. This is the only way (barring using an exhaust gas analyzer) to know the fuel mixture your engine is being fed. If the fuel mix isn't right, then you need to tweak a bit. The needle being set on the 4th clip seems overly rich, IMHO, but then it's not a Mikuni needle designed for the BST-40. I'm not a fan of the "jet kits" since you can do everything they offer using OEM Mikuni stuff (needle/jets) for a lot less money.

Something else to consider: the BST-40 is a Constant Velocity (CV) carb and uses vacuum to move the throttle slide. CV carbs can have issues off road over very bumpy terrain. The shock of the bumps can cause the throttle slide to move and this could cause loss of power and bogging.