Looked for holes in exhanger don't see any ,all air flow is good ,no roll out at burners ,high limit keeps triping changed it , temp on limit is right ,what else can cause this??

High limit? With out knowing make and model of the system its hard to know what you call a high limit. If it's the temperature switch for air flow-- Check for a dirty evaporator coil, dirty cups on the blower wheel, loose duct liner in the duct work. If its reaching high limit trip on air flow it's a reason-- Restriction on intake or supply air.