Air pollution is a growing environmental and public health concern in rapidly urbanizing cities such as Phnom Penh, Cambodia, where motorcycles account for over 80% of registered vehicles. Despite their dominance, localized data on motorcycle emission characteristics particularly under cold-start conditions remain limited, hindering evidence-based air quality management. This study quantifies particulate matter (PM) emissions from motorcycles during idle engine operation in a semi-enclosed university parking environment and examines the influence of fuel type, engine size, vehicle age, and model on emission levels. Field measurements were conducted on 150 motorcycles at the Institute of Technology of Cambodia, with 135 included in final analysis. PM concentrations (0.3–10.0 µm) were recorded using a ZJSJ-G portable laser particle counter over a three-minute idling cycle immediately after ignition. Descriptive statistics, Welch’s t-tests, one-way ANOVA, and two-way ANOVA were employed to assess differences and interactions among categorical variables. Results show that PM concentrations peaked sharply during the first minute of idling (mean: 564.50 µg/m3) before declining to 159.01 µg/m3 by the third minute. Fuel type had the strongest influence: motorcycles using higher-grade Super gasoline emitted significantly less PM than those using Simple or mixed fuels (p < 0.001). Vehicle age also significantly affected emissions, with mid-aged motorcycles (4–7 years) producing the highest levels. Engine size was not a significant predictor, but model-specific differences were observed, suggesting the role of design and technology in emission performance. These findings highlight the cold-start phase as a major but under addressed source of urban PM pollution. Policy recommendations include upgrading fuel quality standards, implementing targeted inspections for mid-aged motorcycles, introducing model-specific emission benchmarks, and improving ventilation in high-density parking areas. The study provides the first localized evidence on motorcycle cold-start emissions in Cambodia, directly supporting the National Clean Air Plan (2022–2030) and offering actionable pathways to reduce PM emissions and protect public health in rapidly motorizing Southeast Asian cities.