Here's my recent odyssey with a similar issue on my 2018 Bonneville Speedmaster. Maybe there was a bad batch of spark plugs at NGK back in 2018?
TLDR: Keep it simple, silly. Replace your spark plugs cuz, it's easy and cheap.
I've seen many posts on this idling, surging, misfiring, snatchiness, etc etc. I read many trying to solve mine. I wanted to post my embarrassing but educational journey so others can benefit.
Here's the whole story:
Early ownership, autumn 2022 to winter 2022/23
50-70 Fahrenheit. 500-2500 miles
No issues with running. Occasionally I smelled burning rubber when stopped during or after a ride.
Winter changes to the bike
- Unlikely but possiblyrelated electrically:
- Replaced turn signals with custom LEDs
- Removed / unplugged speedo housing and re-installed (to install custom bezel)
- Removed / reinstalled headlight (to access speedo)
- Almost certainly unrelated:
- Replaced seat
- Installed passenger backrest
- Installed pannier racks
Spring 2023
60-80f. 2500-3500 miles
Early rides I felt some jerkiness / not-smooth take-offs. I initially thought it was me being sloppy with the clutch – growing complacent with the slip-assist clutch. Later I somewhat suspected the clutch itself. After noticing and paying more attention to the clutch’s behavior I ruled this out and, given the very infrequent nature, forgot about it.
Summer 2023
70-90f. 3500+ miles
Noticed infrequent but distinct / obvious symptoms:
- Severe lack of power when taking off from stop or slow movement (clutch pulled engine idling, but still coasting).
- Lower frequency of cylinder firing noise – sounds like running on one cylinder.
- Sometimes popping / back-fire sound during this behavior
- Preceding this behavior while stopped/idling: the engine idles slows (from it’s usual 1000) to 750-850 briefly then bounces up to 1000 or higher (1200-1400). Sometimes would then smooth itself out. Blipping the throttle would sometimes resolve this. I could hold the throttle open a little so that I could take off smoothly. If I did not (attempting to take off during this idle issue), the above issues presented themselves with certainty)
Later in the summer – I seemed to be able to replicate this issue reliably by going for longer rides. 40minutes+ seemed to when this issue would start. The idle dip issue would sometimes seem to occur briefly upon cold-start, but nearly immediately resolved itself when the cold idle was much higher - and this would NOT be accompanied by lack-of-power/misfire symptoms.
Later in summer, seems to come on perfectly reliably once the bike up to temp ~100-103. After even shorter rides of 5-10min
Attempts / experiments:
- Remove and inspect air filters
- Slightly dirty – seemed appropriate for mileage
- Remove and inspect spark plugs
- Slightly white, even coloring/condition between sides
- 12 Minute Tune
- Reset Adaption with TuneECU, performed quick re-adapt procedure
- Using TuneECU, measured throttle body balance
- Usually dead on when at idle cold and up to temp but not ridden. Occasional fluctuations to +/- 10-20 hPa.
- Use TuneECU to Observe Idle Adaption
- O2 Sensor 1 Range: -21.9
- O2 Sensor 2 Range: -43.8
- Why difference between cylinders?
- Removed and inspected airboxes Included: Connections to throttle bodies, Air Temp Sensor (confirmed accurate in TuneECU), Connections to crankcase recirculation tubes.
- Confirmed TPS voltage
- .58 volts. Within spec: .6 +/- .04
- Idle Adaption should compensate
- Installed BoosterPlug.
- Obvious impact on reported AirTemp in TuneECU
- Issue Still Present
- (removed BoosterPlug)
- Installed DNKTuneWorks Tune (proprietary, actual map change unknown)
- Installed DNKTuneWork Tune v2. Increase Target idle to 1100 minimum.
- Issue Still Present
- Reverted to OEM Map.
- Unplugged O2 Sensors, rode with MIL light.
- Unplugged O2 Sensors and disabled in TuneECU, also running BoosterPlug
- Throttle does seem livier, quicker to rev
- Issue Still Present
- Recorded Ride Telemetry: Throttle Position, RPM, Fuel Trim, Coolant Temp, Battery Voltage, Coil Dwell, One cylinder MAP (missed selecting both), Barometric Pressure.
- There seems to be correlation between a [higher positive values in O2 Sensor 2 Short Term Fuel Trim 2] combined with [very warm engine (for a ~30min or more period of riding)] and [the occurrence of issue (you can see the erratic idle in the data)]
- Ramifications. The issue still occurred when O2 Sensors were disabled: I theorize this means the high O2 Sensor 2 Short Term Fuel Trim occurring here is another symptom of the same underlying cause. This what led to investigating MAP sensors. However, the Bonneville Speedmaster has one MAP for both cylinders (an another for Barometric Pressure), so a failing MAP seems unlikely to cause a cylinder fuel/air imbalance.
- Removed Tank, visually inspected coil spade connections, spark plug wires, various electrical connectors, and vacuum lines
- Removed fuel rail and inspected injectors
- Swapped injectors between cylinders
- Removed and inspected MAP sensor. Swapped Barometric and MAP sensors
- Sensor mount very very slightly loose.
- Vacuum lines seemed tight generally – the end connecting to the sensor was low enough to be touching the valve cover. Pushed further onto nipple.
- By installing it in place of Barometric pressure confirmed consistent values with Barametric sensor WHEN COLD.
- Issue Still Present
- I DID reset adaptions and failed to do a full 12 min tune after the fuel and MAP sensor experiments.
- When testing riding after previous reassembly I inadvertently less one crankcase recirc hose off the airbox.
- The issue seemed to appear much more quickly this test ride. And it seemed more severe.
- Reset Adaptions on hot bike already exhibiting issue
- Issue seems to resolve very briefly.
- New theory: bad O2 sensor causing bad learning?
- Disabled O2 sensors and reset adaptions – preventing any re-learning on Cyl 2 Trim
- This was done right after the above. It seemed to idle a bit higher and with NOTICABILY reduced dipping/stumbling. I idle for 5-10 min, and seems perhaps 80-90% better.
- Road beautifully when cold then it warmed up and...
- Issue Still Present
- Remapped to enable 02 sensors: observed Idle Adaption Range was at standard (it does not show in TuneECU when 02 disabled. Theorize: 02 sensors do not impact ecu when disabled even if still plugged in.)
- New Map setting:
- Fan on Temp: 100
- Idle minimum: 1200
- O2 sensor disabled
- Issue seems to still be present, but idle won’t dip as drastically so it’s less impacts to riding experience.
- New Map settings and unplugged o2
- Fan on Temp: 100
- Idle: stock 1000
- O2 disabled AND unplugged
- Issues Still Present
- Re tried BoosterPlug alone
- Smoother, but issue still present
- Checked spark visually on each plug (classic method grounding on the engine headd)
- Checked primary and secondary resistance on ignition coils
- Randomly encountered while futzing with plugs: bike would run on Cyl 1 (left) alone but not Cyl 2 (right). However, I did not replicate or attempt to keep it running on Cyl 2. (Bike was warm-to-hot)
- Swapped plugs between cylinders
- Bike could start and run on either cylinder. (bike was warm-to-hot) But now seemingly more reliably on Cyl 2.
- Did not spent much time attempting to replicate as I didn't want to run on one cylinder very long.
- Checked resistance of spark plugs
- Original Cyl 2 plug: 3900k ohms
- Original Cyl 1: 4800k ohms
- spec is 5k
- Replaced Spark Plugs. Connected and re-enabled o2 sensors. Stock tune. with 100deg radiator fan activation.
- Reset Adaption and did full 12 minutes idle learning cycle. Then 10 minute ride
- Issue Resolved!!!!
- 1.5 hour ride including varied RPMS and speed and plenty of time idling at lights.
- YES!! RUNNING PERFECT!!
- I now understand the real meaning of “snatchiness” the bike is confident and clearly running 100% as intended. Theory: When misfiring previously the O2 sensors were seeing A LOT of oxygen sometimes and richening. This, when not misfiring, acted like a booster plug or richened tune.
- Will ride on stock tune for a bit – may switch to DNK now that I’m comparing “properly operating” to “maybe-better?”
Previously planned Future Experiments:
Gather Telemetry with swapped side-to-side spark plugsAnother session of recording. If possible, capturing…MAP by cylinder02 voltage by cylinder and/or O2 reading by cylinderRe-check throttle body balance while issue is occurringConfirm TPS voltage when issue is occurring
