L3901 Miss Firing / white exhaust

[ZzLowrider]

L3901 with 140 hours started misfiring with white exhaust between 1500 and 1800 rpm
At other RPM speeds no problem
Fuel cap off same problem
Water separator has no water visible. Fuel filter has been changed with no positive results

 

[Roadworthy]

White exhaust frequently indicates moisture but should clear after a few minutes of run time. Water in fuel is a possibility as is a defective head gasket or cracked head. You really need to do some troubleshooting.