Another engine question

[david viewing]

Since we seem to be on a roll here, perhaps someone could offer an idea about the following:

The O-320-D3G in our 1981 Warrior has become picky about starting. It won't pick up first try and needs 6 primes from cold to get it to fire at all. When it fires it kicks out the Bendix. However, it will start on subsequent crankings without further primings, but needing up to 4 attempts. It has a modern lightweight starter.

The engine is well proven (1850 Hrs) but has good compression and minimal oil consumption. The engineers have checked plugs, mags, timing, manifold for air leaks, primer (3 cylinders) and anything else they know about without any change at all.

I don't pump the throttle after someone else set it on fire doing that. I appreciate that the starter might be too fast for the mag., but the problem has developed since that change.

I don't see why it shouldn't start first time and would like to know the true cause of the problem. Ideas anyone?

[shortstripper]

Edited away as I didn't read your question properly before answering :\

SS

[LowNSlow]

I assume that the engineers have checked the carburettor? Maybe the mixture control isn't going to fully rich even with the mixture control fully forward?

[david viewing]

LowNslow: good point. I'll add it to the list for the forthcoming check. Thanks

[homeguard]

Ask the engineers to check the number of cylinders that the Primer is feeding. If the aircraft starts well when primed with the throttle then it is possible that not all the cylinders are primed using the priming system.

If the aircraft primer is only primeing one or two cylinders that could be your problem. As to priming using the throttle, we start all our aircraft that way - it prevents over priming from too much enthusiasm. If an engine is primed when the engine is hot and rich you will have the possibibilty of a fire whatever.

[Speedtape]

Knackered Impluse coupling on left Mag? Mag P leads crossed over?(this is the usual suspect) Blocked Primer Nozzles? Carb accelerator pump diaphragm ruptured. Knackered camshaft or cam followers upsetting the valve timing (quite possible on a high time engine) Spark plug gaps incorrect? Have you had the prop off lately and it's been refitted out of phase? (Shouldn't be possible with the master dowel on the flange but you never know). Starter ring gear and flywheel incorrectly fitted making a nonsense of the timing?
Three or squirts with the primer followed by a couple of pumps with the throttle should start any Lycoming from cold without risk of flooding.

[Lomcovaks]

Extract below taken fromt the Lycoming key reprints web site:

http://www.lycoming.textron.com/main.jsp?bodyPage=support/publications/keyReprints/index.html

Have you also considered as well as below how long you leave the engine after priming and before starting? Try leaving it 30-40 seconds to allow the fuel to fully vaporise in the cylinder and see if that makes a difference.


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Probably the most important factor in starting an engine is achieving a fuel/air mixture that is satisfactory for combustion. Since the engine usually starts very easily, many pilots are unaware of or ignore the change of starting procedure needed to successfully start under varying temperature conditions. In warm weather the air is less dense and therefore must be mixed with a lesser amount of fuel than in cold weather. In addition to this, in warm weather the fuel will vaporize readily and make starting easier. Simply stated, as temperatures go down it becomes more and more important that we have a plan for priming that will achieve the correct fuel/air mixture.

When priming a carbureted engine, the pilot’s plan must consider the temperature, the number of cylinders which have priming lines installed, and the number of strokes of the primer needed to produce the correct fuel/air mixture. The primer lines are ordered or installed by the airframe manufacturer and not all aircraft are configured the same. Some aircraft have actually been produced with only one cylinder being primed, and these engines are extremely hard to start in cold weather. The number of cylinders that are primed must be considered since the total fuel delivered by the primer will be divided and sent to these cylinders. As the air becomes colder and denser, the amount of prime used must be increased, but the number of strokes to be used should be planned as a result of some trial and error experimentation for each aircraft a pilot flies. When the correct number of primer strokes for each temperature range has been established, the engine will usually start very quickly. We may find that an engine starts easily when one stroke of the primer is used in the sixty-degree range, two strokes in the fifty-degree range, three strokes in the forty- degree range, etc. This is an example of the trial and error we might use to establish the number of primer strokes to use under any particular temperature condition.

While discussing the priming of an engine, there have been situations where primer lines become clogged. This makes engine starting difficult and negates any trial and error experimentation that may have been done. When maintenance is done on an aircraft before the start of winter, it may be wise to have those primer lines checked to insure that fuel will flow through them.

The amount of fuel needed to achieve the correct fuel/air mixture for starting a fuel injected engine is controlled by timing rather than number of primer strokes. With the electric fuel pump on, moving the mixture control to the rich position allows fuel to flow to the cylinders. For cold weather starting, it may be necessary to keep the mixture control in rich somewhat longer than in warm weather.

The fuel part of the fuel/air mixture may be the part we have the most control over during the engine start, but keep in mind that the amount of throttle opening does have an effect on the air that is pumped through the engine. Just as we compensate for cold/dense air by adding more fuel for start, it may also be appropriate to reduce the air part of the mixture when the temperature is very cold. For example, if the throttle is normally set open one half inch for warm weather starting, it may be helpful to reduce this to one quarter inch in cold weather. Again, it will require some experimentation to determine what is needed to achieve the correct fuel/air mixture for any particular aircraft at any temperature range.

[tmmorris]

Re priming with the throttle, not all Lycoming installations have a primer anyway (e.g. Slingsby Firefly - the one I fly has an O320 but no primer, so you pump the throttle 4 times or so to prime: never had any trouble starting it).

Tim

[david viewing]

Sincere thanks for all the ideas, chaps or chapesses. The engine primes on 3 cylinders and the lines and nozzles are OK. Interesting about leaving it after priming - won't the fuel just condense in the inlet? Other advice is to prime while starting!

Anyway, it's just had 150 Hr check and the engineer told me it started fine! So we'll see.

[Sensible]

I know it's not kosher to start an engine from cold this way but it works for me and that is: Leave the primer locked. Throttle full then return to start up position. Turn over the engine and if it doesn't start on the first two complete turns, briskly advance the throttle to full then back to start up position. If it doesn't start after another two revolutions, repeat. If it doesn't start after that, it's time to rethink since continuing to pump the throttle may result in a fire! Personally, I never ever use the primer.