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.