Hi All,
I’m hoping to get some thoughts on an idle issue I’ve been investigating on my 1985 Honda VT500C with Keihin VD-series carbs. I know most of the bikes here are the 600cc or higher, but thought it might be a common issue across the 1980s VTs.
At true idle, the front carb consistently shows an extremely lean / misfire condition on a wideband, while the rear behaves normally. I’ve reproduced this across two sets of carbs (with their own jets, etc). When the carbs are rotated 180°, the lean condition follows the carb, not the cylinder, which strongly suggests a design or calibration asymmetry rather than an engine-side fault. Above idle both cylinders are running great.
Additional observations:
- Pilot circuits are clean and functional; pilot screw adjustment on that carb has little effect at true idle but becomes effective just off-idle.
- This behaviour is repeatable and consistent across two different carb sets, which makes me doubt blockage or wear.
I’m trying to determine whether this is an intentional Keihin/Honda idle bias (e.g. throttle plate angle, idle port exposure, air bleed geometry), or whether I’m missing something subtle in the pilot circuit design.
Any thoughts? For those with one of the bigger capacity VTs, have you also observed this?
Cheers,
Donster
