Q: I have a Beagle Pup 150 (the father of the Scottish Aviation Bull Dog) fitted with a Lycoming O-320 engine. When the weather is cold — in the UK this is anything less than 5° C — it has become an absolute pig to start, but once started it runs perfectly.
The engine is primed by using the accelerator pump. To check the accelerator pump, I looked up into the venturi using a boroscope while someone pumped the throttle. Fuel was pumped into the venturi but it appeared to be in droplets as opposed to a spray. Is this spurting of droplets normal or should the fuel be spraying into the venturi?
A: I must say it’s been many years since I’ve heard anything about Beagle Pup aircraft. Both the Beagle Pup and the Bull Dog were great aircraft and you should be proud to be an owner of a Pup.
With regard to your problem, I believe the problem lies with the accelerator pump itself. When the accelerator pump is functioning properly, it will squirt a concentrated stream of fuel towards the throttle shaft. When the fuel hits the throttle shaft, it is somewhat dispersed or broken up, allowing a finer spray to enter the intake system. If you remove the carburetor and hold it in your hand and point the venturi away from you, then actuate the accelerator pump, you should see a strong stream of fuel squirt from the carburetor (be sure to wear eye protection). I suspect this will not be the case with your carburetor.
That being said, I would suspect there may have been contributing factors that have caused this lack of normal operation. Typically, I’d expect this lack of proper operation from an accelerator pump to be caused by extended periods of inactivity of the aircraft. I don’t think we can overlook the possibility of just plain old age either, which I personally can identify with. Another factor may have been the type of fuel used over the life of the carburetor.
One other thing I’m curious about is if the Pup incorporates an engine primer system? This system, in most cases, was installed by the aircraft manufacturer, but may not have been included in your particular installation. These primer systems were useful for starting, but the aircraft manufacturer, because of the added cost of such a system, may have decided to let the carburetor accelerator pump do all the work.
Lastly, I’d recommend you also make certain your spark plugs are cleaned and properly gapped. I’d also check the magneto to engine timing to make certain it’s on specification and not a degree or so off.
Leighton, I don’t think you’ve got any serious issues here, but suggest you take some simple troubleshooting steps and I’m confident you’ll resolve your problem.
C. David Buchanan says
When using the “Airframe” pimer, priming is done prior to engaging the starter. If using the accelerator pump I would expect to prime after engaging the starter.
I am a CFI & A&P and I totally agree with your response. I find in cold weather
and possible low battery that manual or hand priming helps a lot.
Caution: Make sure that the mags are grounded ie OFF when doing this.
Pump the throttle 6 or 7 times and close it all the way and then pull the propellor
through two or three complete rotations.
Lee Beery says
I had the same problem with our Varga 2150 which we have owned for 22 years, same engine, no primer system. An Alaska pilot provided the cure. Wheels chocked, mag switch OFF, fuel boost ON, mixture rich, stroke throttle 6 times then turn boost pump OFF.. Then pull prop through about 8 to 10 times.. Remove chocks and with throttle set about 1/4 open crank engine.. Should start within 5 seconds,,,Give it a try!
Jack Thompson says
It took a long tome for me to find a good explanation of why we use priming and choking measures on cold engines. As you may know, gasoline is a broad cut of hydrocarbons from light to heavy. Only the fraction of gasoline that will evaporate to a gaseous state at the temperature of the air and induction system contributes to the useful mixture to be fired at the sparkplug. Liquid fuel lying in the intake manifold, or adhering to the metal parts of the cylinder and combustion chamber aren’t available to the instant and place of combustion at the plug. In order to get a combustable mixture at the sparkplug electrodes, you need maybe 5 to 10 times as much fuel as you will need when violent mixing and a warm induction passage physically mixes fuel and air and rapid compression of the mixture in the warm cylinder heats the charge adiabatically, without losing heat of compression to cold surroundings.