Question for Paul McBride, the General Aviation News engines expert: We have a Piper PA28-235 with a Lycoming O-540-B4B5 engine. Total time: 1,977.7 hours; time since major overhaul 510.3.
The engine runs OK at 2,000 rpms but runs rough at 1,500 rpms. If we switch to either mag the engine seems to smooth out somewhat.
Troubleshooting so far includes:
- Compression check: All 70 or above.
- Mags: 30 hours since overhaul
- All new spark plugs
- Checked prop balance: .4-.5
- No indication of backfire or after fire
Can you give us some possible insight into the problem?
Pat Moore, via email
Paul’s Answer: Hi Pat, and thanks for your question.
As you can imagine, your first email didn’t give me much to work with and that was the reason our editor reached out to you for additional information. I appreciate that you responded, but after reading that information, I have to admit that the additional details still left me with the reminder that I failed my Mind Reading 101 Class.
For instance, it would have been nice to know just when this problem began or when it was first noticed. Could it have been connected to a recent maintenance event?
However, I’ll try to offer you a couple of troubleshooting suggestions that I’d begin with if I were working on this problem in your hangar.
My first suggestion would be to run up the engine until you reach the normal operating temperature, just as you would for an upcoming flight.
With the mixture in the full rich position and at idle RPM, you should have the engine RPM around 650.
Now, very slowly pull the fuel mixture towards the idle cut-off position watching the RPM. As you approach full idle cut-off, you should see a 25 to 50 rise in the engine RPM. If this is not the case, then I’d suggest you adjust the idle mixture control on the carburetor.
Allow me to back up just a bit and ask you to check the manifold pressure at engine idle. Normally, we would expect it to be around 10 inches to 11 inches, but if it reads higher, it may indicate an induction leak.
So, if the above is found to be okay, let’s move to the next thing.
I’d be curious to learn what happens when you are in the 1,500 rpm range. What would happen if you slowly leaned the mixture? If the engine is running rich in that RPM range, by leaning the mixture you should possibly see an increase in RPM and the engine smooth out. If using the same check, the engine wants to die, then that could indicate that the mixture is too lean in that RPM range and not getting enough fuel to support the power.
At this point, I’d pull carb heat on to see if it enrichens the mixture enough to smooth out the engine. If that helps, then we may have to look more closely at the carburetor.
As you’ve heard me mention many times before, carburetor engines suffer from poor distribution of fuel/air mixture compared to a fuel-injected engine. For that reason, I’d suggest a close visual inspection of the entire induction system, looking for indications of fuel stains, loose clamps, etc.
I know you mentioned that you had the prop balanced, but how about the age of the engine mounts? I’m trying to think of things that you can do to induce the situation on a repeatable basis that may offer us some additional clues.
Pat, I’m going to stop here, but feel that what you learn by trying the above-mentioned things will lead you to a resolution of this situation.
And your reliance on 30 hour mags and new spark plugs may not be warranted. Then there’s the plug harnesses. Sticking valve. RPM covers up a lot of things.
When I purchased a PA-28-140/180hp, the engine always ran a little rough at idle. The cause was a very small hole in one of the 4 primer lines. It was hard to see unless actually priming the engine when checking the engine induction and priming systems…..
Bore check the cylinders. Check for a broken valve. Had same problem in my 182. It ran smooth at high RPM but rough at 1500. Turned out to be a broken valve.