My motor mower speed fluctuates - the engine revs fluctuate from just above idle to full revs

Rev Counter?? Do you mean tachometer? Do the engine RPM's fluctuate when returning to idle? If so, its probably time for a valve adjustment... Intake valves that are TOO TIGHT will cause the rpm's not to settle back to idle quickly.

Specifications

Engine HD45

Engine Model Number(Kawasaki FC420V
Horsepower (10.5 kW (14 hp)
Displacement (423 cc (25.8 cu.in.)
Crankcase Capacity w/o oil filter (1.3 L (1.4 qt.)
Crankcase Capacity with oil filter (1.6 L (1.7 qt.)
Spark Plug-Gap (0.64 mm (0.025 in.)
Spark Plug-Torque (20 N·m (15 lb-ft.)
Cylinders (One
Stroke/Cycle (Four
Lubrication (Full-Pressure
Oil Filter (Standard Single Element
Air Cleaner (Dual Stage

Engine HD75

Engine Model Numbe(Kawasaki FC540V
Horsepower (12.7 kW (17 hp)
Displacement (535 cc (32.6 cu.in.)
Crankcase Capacity w/o oil filter (1.6 L (1.7 qt.)
Crankcase Capacity with oil filter (1.9 L (2.0 qt.)
Spark Plug-Gap (0.64 mm (0.025 in.)
Spark Plug-Torque (20 N·m (15 lb-ft.)
Cylinders (One
Stroke/Cycle (Four
Lubrication (Full-Pressure
Oil Filter (Standard Single Element
Air Cleaner (Dual Stage

Fuel System

Fuel (See Fuel in the Service Miscellaneous section
Fuel Filter (Single Stage, 60 Micron
Fuel Pump (Vacuum Operated-Diaphragm
Fuel Delivery (Gravity Feed to Fuel Pump)
Fuel Tank Capacity (19 L (5.0 gal.)

Drive Train Mower Deck (36-inch)

Blades (2
Blade Bolt Torque (81 N·m (60 lb-ft.)
Cutting Heights (25-114 mm (1-41/2 in.)
Weight (86.7 Kg (191 lb)
Overall Width (chute up) (927 mm (36.5 in.)

Mower Deck (48-inch)

Blades (3
Blade Bolt Torque (68 N·m (50 lb-ft.)
Cutting Heights (25-127 mm (1-5 in.)
Weight (83.6 Kg (184 lb)
Overall Width (chute up) (1308 mm (51.5 in

ower Deck (54-inch)

Blades (3
Blade Bolt Torque (68 N·m (50 lb-ft.)
Cutting Heights (25-127mm (1-5 in.)
Weight (86.7 Kg (187 lb)
Overall Width (chute up) (1460 mm (57.5 in.)

Recommended Lubricants

Engine Oil (John Deere PLUS-4
Grease (John Deere MOLY HIGH Temperature EP
Transmission Oil (HY-GARD-J20C (TURF-GARD-SAE10W-30 (PLUS-4-SAE10W-30 (TORQ-GARD SUPREME-SAE30
(Specifications and design subject to change without notice.)

Engine stalls cold or RPM fluctuates at idle up and down or dies near idle.

This problem is commonly caused by a dirty automatic idle speed control valve and throttle valve but always run a OBD2 fault code as well as the cleaning procedure. Buy a can of throttle valve cleaner (do not use carburetor spray cleaner!) from NAPA or Carquest (made by CRC chemicals) and spray it into the air intake while the engine is running, use up about 1/2 the can, engine will try to stall hold the speed up, shut it down and let it soak for 30 minutes, restart and blow out the remaining fluid, shut it down and disconnect the negative battery cable for 5 Min's to reset the base idle control. If the above cleaning doesn't cure your issues you need to have the computer controlled automatic idle speed control system diagnosed and repaired by a dealer or electronic engine control repair shop.

This problem is commonly caused by a dirty automatic idle speed control valve and throttle valve. Buy a can of throttle valve cleaner (do not use carburetor spray cleaner!) from NAPA or Carquest (USA) (made by CRC chemicals) and spray it into the air intake while the engine is running, use up about 1/2 the can, engine will try to stall hold the speed up, shut it down and let it soak for 30 minutes, restart and blow out the remaining fluid, shut it down and disconnect the negative battery cable for 5 Min's to reset the base idle control

check fuel filter and check air filter

You need to clean the engine throttle body and the automatic idle speed control motor, they are likely full of cylinder backwash deposits and this causes the problem. Use throttle body spray celaner form the auto parts store and after cleaning disconnect the battery for 5 minutes to reset the system.

You need to have OBII scan performed. The fluctuating idle sinus like a MAF sensor, Temp & fan are separate issues. Get scan done, that should identify exact issues.

The oil pressure guage will fluctuate with engine RPM. At idle it should be approximately 20 lbs./sq.in, at high revs about 50-60 lbs./sq.in. Hope this helps any concerns you may have. Good luck!

1. vacuum leak; check all rubber hoses for cracks
2. idle control valve; remove and clean with carb cleaner

I would suggest idling your high idle speed up a quarter turn while air conditioner is on full .