Air Filter & Carb – Heat System Check – The 100 hour Radial Engine Annual Inspection Series

This is the fifth topic in the series about the 100 hour / annual inspection. This series focuses on maintenance performed during an effective 100 hr. inspection on R1340 and R985 engines.

  1. Oil Change with filter/screen & sump checks.
  2. Valve adjustment – Positive or compression.
  3. Ignition timing check – Spark plug servicing. 
  4. Compression check – differential. 
  5. Air filter and carb – heat system check.
  6. Fuel System Screens.
  7. Cylinder Head Checks.

Carb-heat: The carb-heat system on an aircraft is more important than most people give credit. This is not because the R-1340 / R-985 (short carburetor-to-engine adapter equipped) engines are ice prone, they’re not. The reason is that most systems have notoriously leaky valve-bodies at best, and a substantial amount of performance can be lost through a valve that is open just 1/2″ due to worn or loose linkages. This is especially true if the exhaust system is worn or in otherwise marginal condition!

Carb-heat air is unfiltered! Dirt ingested through a sloppy or slightly open carb-heat valve isn’t likely to cause engine failure, but can, in very dirty operating conditions, result in completely worn out piston-rings and high oil consumption in as little as 150 hours of operating time.

How important is carburetor-heat system maintenance? I’ll relate a true story: The excited pilot on the phone asked me: “can a failed governor cause an engine to loose RPM from 2250 RPM to 1900 RPM at take-off power?” I answered him “I suppose so, depends on how it fails”. He indicated that he was going to replace the governor and go back to work and, without giving me opportunity to ask more questions, hung up. He called back in a couple of hours to inform me that he had put the aircraft down in a muddy field and asked if I could bring another governor!

r-1340 radial

I loaded a governor in the Cessna 206 (the tools, a ladder and the support parts were already on-board) and took off. As I was taxing up to the customer’s hangar the pilot rushed out to the Cessna and asked me if a 2” hole in the exhaust pipe of the #8 cylinder could cause a loss of power! It could on his particular aircraft! The #8 exhaust pipe enters the carb-heat muff on the left side of the aircraft. The Carburetor heat chamber is situated just a few inches forward of the carb-heat air inlet box! The carb-heat valve was found to be open about ¾ inch from the fully closed position and raw exhaust was being fed to the carburetor. That much exhaust gas in any carburetor heat muff is a fire hazard and could damage the aircraft even if it didn’t cause a significant loss of power! We replaced the exhaust pipe, readjusted the carb-heat air valve to the closed position and, when he found a spot in the field solid enough to get his wheels rolling, he took off.

Looks like the ole #AgCat won this round against the #Cessna! Can you name the yellow plane type?

A photo posted by Covington Aircraft (@covingtonaircraft) on

The 1340 and, agricultural installations of the R-985 aren’t very prone to carburetor icing problems, a fact that sometimes places the system in an “Out of sight, Out of mind” category. However, carburetor icing can develop in just the right circumstance. And, the system is an aircraft certification requirement! Keep it in good shape and it’ll remain in the background!

Regarding air filters; there are cleanable air filter elements available for many of the aircraft we service. I have yet to see anyone get in trouble using these filters, turbine or radial, if manufacturer directions for servicing them are followed to the letter! If you keep your air filters clean you won’t have to burn as much fuel to produce the manifold pressure necessary! We all know how cheap fuel is these days!

Ya’ll Fly Safe!

Ron Hollis

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