Engine
Rough Idle
Idle surging, engine dying at a stoplight.
Harley-validated
Checklist
- Spark plugs — always the first item
- TPS sensor out of adjustment
- Intake air leak
- Weak coil
- ECM needs a reset after changes
Safety & cautions
- Always work with the engine COLD — cylinders reach 450°F
- Disconnect the battery before touching sensors or ECM
- Wear nitrile gloves: motor oil is a carcinogen (CA Prop 65)
- Never crank without oil, even for a second — kills cams
Average US cost (USD)
- DIY oil + filter change: $45-70 (SYN3 + Harley filter)
- Dealer oil change: $180-260
- Cam chain tensioner (pre-2007 Twin Cam): $900-1,400 installed
- Full Stage 1: $1,200-2,400 (parts + tune)
- M8 top-end rebuild: $1,800-3,200
When to see a mechanic
- Knocking, metallic knock under load — stop immediately
- Oil pressure below 10 psi at hot idle
- Continuous blue smoke from exhaust (burning oil)
- Check Engine flashing + power loss
Quick questions
Can I mix mineral and synthetic oil?
In a pinch, yes, but you lose the synthetic's lifespan. Change at the next opportunity.
How long does a Milwaukee-Eight last?
With regular maintenance, 150,000+ miles without opening the engine is common.
Do I need to warm it up before riding?
30-60 seconds at idle, then ride gently for the first 3 miles. Don't rev it cold.
Where to dig deeper
- Official Harley-Davidson Service Manual (PDF by model/year)
- Forums: HDForums.com, V-Twin Forum
- YouTube: Kevin Baxter (Pro Twin Performance), Lawless Harley