Q: I have a PA-25-260 ag plane. When I let it idle 640 rpm, it makes a popping noise, however after takeoff, when I reduce power (slowly), the loader says he can hear a popping noise.
Both mags check good and I recently replaced the spark plugs due to a drop in one mag. This fixed the mag problem. A day after I replaced the spark plugs, one mag sounded rough, which caused my mechanic and I to pull and look at the bottom plugs. Several already had lead forming. Cleaned, no problem.
The ag pilot I worked for read that it’s not good to idle Lycomings at low rpm (less than 1,000 rpm).
Any suggestions you can provide will be greatly helpful! Thanks!
In a second email, Herb noted that he “just read another article from you regarding an increase in rpm at idle cut-off. One other thing I’ve noticed is that there’s no increase in rpm at idle cut-off just prior to the engine quitting. Is this an indication of a lean mixture setting? If so, how would I adjust?”
Q: Herb you’ve presented me with an interesting question. After reading your first email, I was almost convinced that we were just looking at a lean mixture condition. This lean condition at low rpm is usually indicated by the popping out the exhaust stacks.
After reading your follow-up email where you mentioned that when checking the mixture at idle rpm you see no rise in rpm just prior to the engine quitting, this is also an indication of a lean mixture and may be the result of a misadjusted idle mixture on the carburetor or may be the result of an induction leak.
An easy way to confirm a suspected lean mixture, regardless of the cause, is to check the manifold pressure gauge reading at idle rpm, usually at 650 to 700 rpm. On a normally aspirated engine with a carburetor, we would expect to see something like 10 inches of manifold pressure. If the manifold pressure reading is, let’s say 12 inches, then I’d suspect we either have an induction leak or a need to readjust the idle mixture adjustment screw on the carburetor.
If an induction leak is suspected, the best troubleshooting method is begun with a very close visual inspection of the entire induction system. It may be something as simple as a bad, leaking intake gasket where the intake pipe flange attaches to the cylinder head.
On some occasions, if this gasket is leaking, you may see a slight fuel stain on the intake pipe, which will confirm the problem or at least part of the problem.
You should also check closely for the possibility of a cracked flange on all of the intake pipes at the cylinder heads. This problem does show up on occasion and is a result of improper installation of the intake pipe flange where both attaching bolts are not tightened equally during installation.
The next area that deserves a close look is the intake pipe rubber connectors between the intake pipe and the oil sump. Check for the integrity of the hose clamps and also the overall condition of the rubber hose. You should also look for any fuel stains in this area.
There is one more area to closely inspect, which is often overlooked when an induction leak is suspected, and that is the engine primer system. There is always a possibility that one or more of the engine fuel primer lines has cracked or broken. This is also a safety issue, so close inspection of this system is very important.
If you find that the induction system is sound, then the next hurdle to get over is making a proper adjustment to the idle mixture screw on the carburetor. This adjustment is rather easy and should always be done with the engine at normal operating temperature.
You will notice that the idle mixture screw is marked with the direction for rich indicated by an arrow. Adjusting this in small increments will allow you to achieve the proper idle mixture adjustment, assuming all else is as it should be. Your final adjustment should allow you to acquire about a 25 to 50 rise in engine rpm when moving the mixture control from full rich into idle cut-off at engine idle rpm mentioned previously.
Now, all this being said, I am a little confused about your statement that several of the spark plugs showed signs of lead fouling. Usually with a lean mixture this is not a condition I would expect, so I’m not sure how to comment about this.
I can’t argue with the ag pilot who mentioned not idling less than 1,000 rpm, but this may not be possible if you’re doing a “hot” reload, so I’d suggest you consider idling during a “hot” reload, then once the loading is complete and you are taxiing out, increase the engine rpm to 1,000 to 1,200 rpm and slightly lean the mixture so that the core nose temperature of the spark plug becomes hotter and self cleans the possible lead build up.
CAUTION: Always remember to return the mixture control to full rich prior to takeoff!!
Hopefully these suggestions will relieve you of the popping and may also provide some operating tips for better operation in the future.