Question for Paul McBride, General Aviation News’ engines expert: My engine was purchased new from Lycoming. I’ve put 289 hours on it and it hasn’t missed a beat.
I’ve noticed that if I reduce the throttle quickly I get a popping noise from the exhaust for a few seconds that then quits. A slow reduction in power to idle doesn’t cause it. I’ve had an A&P look at it, check the magnetos, mixture, etc. Thoughts?
Thank you for sharing your expertise with those of us who fly for the sheer pleasure of flight. Wish we could bottle your knowledge.
Rich Hrezo
Paul’s Answer: Rich, it’s so good to learn that your Lycoming hasn’t missed a beat in the 289 hours since new. However, I don’t think anyone would expect anything less and, hopefully, with frequent operation and proper operating procedures, it will continue operating that way for many more hours.
Of course, good routine maintenance, such as frequent oil and filter changes, will contribute to that as well.
Regarding your question as to what might be causing the popping sound from the exhaust when reducing the throttle quickly, I think I can offer some thoughts that may help you solve the problem.
First, if I have to guess, my suspicion would be that the fuel mixture might be a bit on the lean side. I’d suggest that you check the idle mixture setting at idle.
Run the engine until you reach the normal engine operating temperature, then bring the throttle back to idle, which should be between 650-700 rpm. Slowly lean the mixture control towards the idle cut-off position and closely observe the RPM rise on the tach right before the engine shuts down. You should see between a 25-50 rpm rise.
If little or no rise is noted, then I’d recommend having your maintenance facility adjust the little star wheel on the side of the fuel injector body a couple of clicks towards the rich side.
Increase the RPM to clean out the mixture and then try the test once again or until you achieve the 25-50 rpm rise.
Also, if you have a manifold pressure gauge, as you adjust the mixture, you may notice a slight difference in the idle manifold pressure. Typically, a lean mixture at idle indicates a higher manifold pressure, but in your particular case, I’m not certain that the slight adjustment will cause any noticeable difference in what you observe on the MAP gauge.
Getting back to the situation of not experiencing the popping when slowly reducing the throttle, I think the closing of the throttle slowly just allows the system to accommodate the change in the mixture a little bit better. This is why I believe that any adjustment that you make will be minor in nature.
Boy, this is alot about nothing. When reducing power rapidly, the inertia in the prop will cause a momentary rich mixture which ends up popping/ igniting in the exhaust system. Slow throttle reduction insures a proper mixture throughout and does not “pop” . If the engine is popping at idle during an approach this could be a rich idle mixture setting causing the same symptoms, but the post said that the symtoms occurred during rapid throttle reduction. So be smooth. Don’t do that.
Ditto, I worked on IO-540s in Twin Commanders for years. It’s never a good idea to make rapid, large throttle changes. On Twin Commanders it’s also an excellent way to shock cool the engine on descent. The second time one of our pilots damaged an engine due to this behavior, they were looking for another job. I’m a bit disappointed the Lycoming expert didn’t advise rapid, large throttle changes are not recommended period.
This is the same exact problem I have with my C172M ! Very disappointed that I have an A&P that couldn’t figure it out. I’ll ask him to try this!
Hello Selby, Please see Neil G’s comment. I would add operation that causes popping in the exhaust is also an excellent way to damage the exhaust system. Plan ahead when flying and avoid rapid, large throttle changes all together.
Thanks to everyone- I have a few things to check. Being a fuel injected engine eliminates the carb part. I am instrumented for four cylinder digital CHT and EGT so running these tests should be easier. Many thanks again! Rich
Try Changing the Nana Worley Flange. That may Solve the Problem.
Great reply by Paul. I know when the mixture is set when the weather is warm , becomes to lean when the weather gets cold….I wonder if this occurs on warm day versus cold? Air density does have an effect on our engines….
Great answer.
This is a normal occurrence on my O-300-A, and really any carb’d engine I’ve had.
1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected —- the exhaust then pops or backfires.