Remember the rule of thumb: if the engine is hot you’ve probably over-primed and if the engine is cold, you haven’t primed it enough.

Taxiing with a full rich mixture is like driving a car with the choke on, it causes fouled plugs, contaminated exhaust valves, and dilution of the oil film on cylinder walls. 

Absence of RPM drop when checking magnetos may indicate a malfunction in the ignition circuit.

There are two good reasons why you should take care of the lump of metal that turns your prop and these are; your pocket and your ongoing existence. If either (or both) of these is important to you, then stick around as we take a look, this month, at how to baby the big-bore Contis. A lot of these ‘care tips’ apply to other makes of engine as well, so, if you want to keep your prop(s) turning smoothly – read on.

Big-bore Continental engines like the IO-520, IO-550, their turbocharged counterparts and the GTSIO-520 are perhaps the most popular high performance piston power plants in general aviation. They power Beech Bonanzas, Barons, Cessna 200, 300, 400 series, Cirrus, Mooney and many other models.

All the big-bore Contis share a common design, many common parts and maintenance practices. A -520 is simply a -470 fitted with larger cylinders with a quarter-inch wider bore, and a -550 is nothing more than a -520 modified to have a quarter-inch longer stroke.

The big-bores come in two basic flavours. Those used in most Beech Bonanzas, Barons and twin Cessnas use a permold crankcase configuration characterised by a front-mounted gear-driven alternator and a rear-mounted oil cooler. In contrast, the big Cessna singles use the “sand cast” case configuration with a rear-mounted belt-driven alternator and front-mounted oil cooler. Although they differ from each other in important ways, from a pilot’s point of view, there’s not much difference in the way they are operated.


It’s good practice to inspect the engine compartment carefully and often – I would say every 10 hours or so during your pre-flight inspections. Some aircraft like the Bonanzas, Barons and Twin Cessnas have big hinged cowlings for easy access, while other models make it a lot harder to gain access – and you end up peering through the oil filler hatch.

Look for fuel, oil and exhaust stains, which are generally an indication that something is wrong. If a small oil leak is coming from a rocker box cover it will probably be okay to fly, but an oil leak coming from a cracked oil cooler, oil filter adapter, or from a cylinder would suggest that you should stay on the ground.

Hidden exhaust leaks can be killers, particularly in turbocharged aircraft. It’s a good idea to use a torch to look up the tailpipe of non-turbocharged engines for damage to exhaust baffles. I’ve found cracked and broken exhaust baffles which have come adrift and blocked the exhaust gases, causing partial engine failure. Also look for signs of chafing where hoses, wire bundles and control cables are in proximity to the engine or each other. If you see two things rubbing, isolate them with a tie-wrap or clamp before you fly.

Pay special attention to fuel contamination, especially if the aircraft has been standing outside in rainy conditions, refuelled from underground tanks or where aircraft has been in a hot hangar with the tanks not completely full.


Fuel injected Continentals are typically very easy to start. The aux pump and injector nozzles provide optimum priming right at the intake port of each cylinder. If you have difficulty starting the engine, remember the rule of thumb: if the engine is hot you have probably over primed and if the engine is cold, you have not primed it enough.

If you do have difficulty starting the engine, give the starter motor time to cool down and don’t crank for longer than 30 seconds or you will burn out the starter. Starting an engine with a flooded intake system can result in a hydraulic lock causing serious damage to the engine.

Hot starts are the most challenging; some pilots are clueless as to the technique to follow. Some intentionally flood the engines and then crank with the mixture at the idle cut-off position until its starts. While this commonly works for Lycomings, it’s an invitation for a stack fire on the Continentals. The fuel injector lines on the continuous-flow injection system run over the top of the cylinders and after shutdown these lines are heated by the hot cylinders, causing the fuel that is inserted during priming to vaporise. This is the reason for the engine firing up and then dying on you during hot starts.

The following hot start technique has always worked well for me: Throttle fully open and the mixture at idle cut-off, run the electric fuel pump on high for a full 30 seconds. This circulates cool fuel through the fuel lines, fuel pump and fuel control unit, purging them of vapour. After half a minute, turn the pump off. Advance the mixture to full rich, retard the throttle, and crank the engine, using no prime or just a touch.

By the way, if you flatten the battery, it’s good practice to remove a dead battery for a proper charge rather than to hook up a battery cart or GPU for starting. If you’re going to fly with a low or dead battery, one of two things may happen: You won’t be able to get the alternator on-line or the alternator will charge the battery too rapidly and damage or destroy it. Starting engines with low battery power can also cause starter contactor points to burn stuck or a shorted-out battery.


Once the engines are running and it’s time to taxi out, perhaps the most common mistake made by pilots is not to lean the mixture. Continental fuel injection engines are set up with rich idle mixture to facilitate cold starts. Taxiing with a full rich mixture is like driving a car with the choke on, causing fouled plugs, contaminated exhaust valves, and fuel dilution of the film of oil on the cylinder walls. 

Avoid long periods of ground operation with these engines. A common taxi mistake is carrying too much power and riding the brakes. Throttle back to 800 or even 700 rpm for taxi. Big bore Contis are supposed to idle smoothly at 600 RPM if everything is adjusted properly. If your engine runs rough at low RPM, the unmetered fuel pressure, idle mixture, and magneto timing could be out of adjustment. As soon as temperatures are in the green arc you can continue with your run-up as per the aircraft operating handbook. Continental recommends power checks at 1700 RPM on most of their engines. The two magnetos operated individually should not differ by more than 50 RPM with a maximum drop of 150 RPM for either magneto. Watch for engine roughness during this check. Absence of RPM drop when checking magnetos may indicate a malfunction in the ignition circuit. This must be corrected before you fly.

Check propeller operation and other procedures as per the Pilots’ Operating Handbook.


Takeoff is an especially hazardous phase of flight, which doubles if you’re flying a twin. Checks must be carried out as per the Pilots’ Operating Handbook and pay particular attention to the position of the fuel pumps.

Next would be to apply full power for takeoff and it’s now that things really start happening., To go from idle to the best it can give in a matter of seconds is a significant thermal event for the power plant. PLEASE DON’T SLAM THE THROTTLE OPEN! For optimum engine life, we must minimize the gradient of this thermal event by throttling up as slowly as possible. Never allow cylinder head temperatures to exceed the specified limitations.

Near maximum temperatures should occur only when operating under adverse conditions such as high power settings, low airspeed, extreme ambient temperature, etc. Reduce the high temps as soon as possible. On turbocharged engines monitor manifold pressure carefully to avoid over boosting. Mixture settings on turbocharged engines should be full rich, and ‘as required’ on normally aspirated big bore engines; however, allow sufficient fuel flow for the high power demand.


Once the after takeoff checks have been completed and the aircraft is climbing through circuit altitude, reduce power to cruise power settings. In normally aspirated engines it’s fine to leave the throttle fully open. On turbocharged engines the pilot must reduce the power. Cruise-climb is normally 75% power, this occurs at top-of-green manifold pressure at top of green RPM. Once the power reduction is made, progressively lean the engines for the climb.

With a turbo, the automatic wastegate system should hold your manifold pressure more or less constant as it climbs. Some shortfall is normal however, especially if you use single-weight oil. Check and maintain the cylinder head temperature (CHT) in the normal parameters not exceeding 400 °F.

Keeping your Conti cool can be a problem when climbing out in very hot weather or at high altitudes. If you notice your CHT’s are getting warmer the best way to bring them down is by trimming nose-down to trade reduced rate-of-climb for increased airspeed. Cowl flaps aren’t terribly effective at low airspeeds but must be set as required. Not all cowl flaps have only an open and closed setting, so use the variable adjustment to control temperatures. Enriching the mixture can help bring down CHT’s but if settings are too rich, this is at the expense of contaminating the engine with unburned fuel..


When you reach cruise altitude, level off and don’t be in a hurry to reduce power. Leave the 75% cruise-climb setting until your airspeed has accelerated to cruise speed and stabilised there. Adjust cowl flaps as necessary.  Big-bore Continentals are generally rated for continuous cruise at 75% power. Adjust manifold pressure, RPM and fuel flows according to the Pilots Operating Handbook.  A major advantage of using cruise power settings of 65% or lower is that the engine may be leaned more aggressively at these power settings. I would also recommend lower RPMs and High manifold pressures as opposed to the other way around.

If you can’t get full takeoff power at a turbo aeroplane’s rated critical altitude, then you have a problem that needs to be found and fixed. Aircraft service manuals give detail requirements for test flights after MPIs that should be carried out. Should big-bore Continentals start running rough at flight levels, you may be experiencing a high altitude misfire. Try descending a few thousand feet and if the problem goes away, the diagnosis is confirmed. Ask your AMO to adjust your spark plug gaps to the low end of the allowable range (generally 0.016”) and clean the distributor caps of your magnetos. A high altitude problem is that you may exceed the turbocharger’s turbine inlet temperature (TIT) limit before reaching peak EGT. Published TIT red line is usually 1650 °F but for optimum turbocharger and exhaust system life, it’s a good idea to limit TIT to 1600 degrees. If your TIT is higher than that, you can either reduce cruise power or run richer, I recommend the former.


As you approach your destination, your engine needs to be cooled down gently for maximum longevity, and this is even more important on turbocharged engines. The key is proper descent planning and temperature management. For most high performance aircraft with big Continental engines that cruise in the 180-200-knot range, you can calculate your airspeed to be about 3 NM per minute, and the maximum comfortable descent rate is about 1000 feet a minute. Thus, if you have X thousand feet to descend, you should start down at X miles or X minutes out from the airfield pattern. You also need to cool the engine down by reducing 1 inch of manifold pressure every minute, and the ideal would be to arrive at your destination circuit with around 20” of manifold pressure. Gradually richen the mixture setting during the decent.


A turbocharger must be given the opportunity to ‘cool down and spool down’ at idle for at least 3 to 5 minutes before shutdown. In many cases the landing roll and taxi gives enough time to do just that and therefore no additional time is needed once you’re parked. Remember to lean the mixture for taxi after landing.

After shutdown, if the aircraft has a three blade prop(s), check that the V (Two blades) is pointing downwards and one blade upwards. If this is not so, your propeller is installed incorrectly which will cause excessive stress on the counter weights of the crankshaft and reduce the life of your engine.

Unless you’re a glider pilot, you need the engine and you need it to run smoothly and safely from chock to chock. Be considerate to your Conti and it will reward you in the short, medium and long term.


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