Pilot makes precautionary landing because of a rough running engine.

Attributes
ACN	242049
Time
Date	199305
Day	Sun
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : gjt
State Reference	CO
Altitude	agl bound lower : 1000 agl bound upper : 1000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tower : gjt
Operator	general aviation : personal
Make Model Name	Small Aircraft, High Wing, 1 Eng, Fixed Gear
Flight Phase	cruise other
Flight Plan	VFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : private
Experience	flight time last 90 days : 5 flight time total : 674 flight time type : 250
ASRS Report	242049
Person 2
Affiliation	government : faa
Function	controller : local
Qualification	controller : non radar
Events
Anomaly	aircraft equipment problem : critical
Independent Detector	other flight crewa
Resolutory Action	other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

Prior to flight, the aircraft spark plugs were pulled, cleaned, gapped, and reinstalled by the owner/operator as allowed under far part 43, appendix a, (C)(20). Also complied with mfg service bulletin on carburetor heat valve gasket inspection. Aircraft engine runup checked ok. Aircraft taken on maintenance check flight. Aircraft performed adequately. Flight altered to survey local flood conditions. Contact made with local tower for flight along fringe of air traffic area. Contact maintained and traffic called and acknowledged. 35 mins into flight, aircraft engine roughness occurred. Pilot elected to make precautionary landing on abandoned road near river. All necessary tools, including a torque wrench, were on board the aircraft. Leaning mixture and high power runup burned off plugs. Pilot prepared for takeoff and was stopped by highway patrol. California highway patrol officer called away and turned situation over to county sheriff officers. After much discussion, pilot was allowed to takeoff with admonishment. An autogas stc is currently not available for this engine/air frame combination. Use of 100 'low lead' will continue to cause fouled plugs and rough engines. Tcp AVGAS additive is used routinely in this aircraft with no noticeable results.

Original NASA ASRS Text

Title: PLT MAKES PRECAUTIONARY LNDG BECAUSE OF A ROUGH RUNNING ENG.

Narrative: PRIOR TO FLT, THE ACFT SPARK PLUGS WERE PULLED, CLEANED, GAPPED, AND REINSTALLED BY THE OWNER/OPERATOR AS ALLOWED UNDER FAR PART 43, APPENDIX A, (C)(20). ALSO COMPLIED WITH MFG SVC BULLETIN ON CARB HEAT VALVE GASKET INSPECTION. ACFT ENG RUNUP CHKED OK. ACFT TAKEN ON MAINT CHK FLT. ACFT PERFORMED ADEQUATELY. FLT ALTERED TO SURVEY LCL FLOOD CONDITIONS. CONTACT MADE WITH LCL TWR FOR FLT ALONG FRINGE OF ATA. CONTACT MAINTAINED AND TFC CALLED AND ACKNOWLEDGED. 35 MINS INTO FLT, ACFT ENG ROUGHNESS OCCURRED. PLT ELECTED TO MAKE PRECAUTIONARY LNDG ON ABANDONED ROAD NEAR RIVER. ALL NECESSARY TOOLS, INCLUDING A TORQUE WRENCH, WERE ON BOARD THE ACFT. LEANING MIXTURE AND HIGH PWR RUNUP BURNED OFF PLUGS. PLT PREPARED FOR TKOF AND WAS STOPPED BY HWY PATROL. CALIFORNIA HWY PATROL OFFICER CALLED AWAY AND TURNED SIT OVER TO COUNTY SHERIFF OFFICERS. AFTER MUCH DISCUSSION, PLT WAS ALLOWED TO TKOF WITH ADMONISHMENT. AN AUTOGAS STC IS CURRENTLY NOT AVAILABLE FOR THIS ENG/AIR FRAME COMBINATION. USE OF 100 'LOW LEAD' WILL CONTINUE TO CAUSE FOULED PLUGS AND ROUGH ENGS. TCP AVGAS ADDITIVE IS USED ROUTINELY IN THIS ACFT WITH NO NOTICEABLE RESULTS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.