Q: I own a 1963 PA24-250 Comanche. Over the past two years I have replaced all six cylinders, both mags, harnesses, primer lines, new fine wire plugs, fuel lines, engine fuel pump, both electric boost pumps, and I am baffled. During cruise settings at 2,400 rpm and 23 inches of manifold pressure, my engine still feels as if I’m flying in rough air. There’s a constant roughness that is minor in nature, but still there.
I’m supposed to be able to lean it back to 13.5 gallons per hour, but if I go below 14.5 the engine wants to cut off, so I’m normally left to fly it at 15 gallons per hour.
The shop I use did find a primer line that was leaking a few months back and that was replaced. I did notice a difference in the smoothness, but it did not take the minor vibration or miss out completely.
The compressions are excellent, all in the high 70s, and oil consumption is about one quart every 12 to 14 hours. I did have the shop replace the rubber intake hoses, but I’m not sure if the gaskets were replaced holding the intake pipe to the engine.
I would appreciate any suggestions you might have to solve this problem.
Larry Keener, North Myrtle Beach, S.C.
Q: Larry, I must say you’ve certainly invested some money in your Piper 250 Comanche recently, so let’s see if I can solve this mysterious minor roughness you are experiencing.
After reviewing the list of components you’ve replaced, I’d say there isn’t a whole lot more that I can think of.
When I look at your comments regarding your inability to lean the mixture to what you expect, that sounds like a fuel management issue to me. My concern here begins with the age of the aircraft.
Aging aircraft are notorious for having cockpit instrumentation that can be inaccurate. So, my question to you is have you had your tachometer calibrated recently? We may have a condition here where the tach is reading a couple hundred rpm low which, in turn, means the engine is drawing more horsepower and not getting the proper amount of fuel to support that extra power.
If your carburetor has a flow setting to support 250 hp at 2,575 rpm and your engine is actually turning 2,650 or 2,700, which is not indicated on your tach, the engine is not getting the proper amount of fuel to support the horsepower being taken out of the engine. This would explain why you are unable to lean because if the engine isn’t getting the fuel it’s asking for to support the power, and you try to take more fuel away by leaning, the result is the engine wants to quit because it’s starving for fuel.
Let me back up and mention one other thing. What is your manifold pressure at engine idle around 650-700 rpm? On a normally aspirated Lycoming engine the manifold pressure should be around 10 to 12 inches at most. If the manifold pressure is higher, say 13 to 14 inches at idle, this may be a result of an induction leak. This could have a negative impact on mixture leaning, too.
You mentioned having the rubber induction hoses replaced, but you weren’t certain if the gaskets on the intake pipe flange were replaced. I’m sure you’ve checked that by now, but I’d also encourage your maintenance facility to closely inspect the intake pipes at the flange end for any possible cracks. This could also cause an induction leak. Also, look for any fuel staining in the area, indicating a leak.
If we remove the leaning issue from the equation, there is another thing that may cause a constant minor roughness and that is the engine mount rubbers. Please check to see when they were last replaced. Since they live in a very hostile environment and are often overlooked, this also may lead to some strange things happening while the aircraft is in flight.
The age of the aircraft has little to do with the problem. There are lots of items that can suggest a rough engine, including turbulence, and I’ve seen many owners have this problem, all of which were dealt with.
A recent one was a vibrating fuel hose to a firewall mounted fuel totalizer which was misinterpreted as a rough engine.
You don’t mention which shop you are using, and if they know Comanches (which most don’t but pretend that they do).
My point being – the problem that you have may not be a rough engine at all. We’ve had reports from people with minor items causing air flow turbulence which hits the stabilator and causes vibration that resembles a rough engine.
If you are a member of the International Comanche Society you have access to their technical advisers and that could help.
My take on this is that you need to look at different power settings and see if the roughness occurs at all power settings. If not, determine when the roughness occurs and at what airspeeds. My guess (and it is merely a guess) is that you need to look past engine roughness and determine where the roughness is coming from. It could well be an accessory.
This is not a unique Comanche problem, so let’s not blame the Comanche 250 because it is a fine aircraft. However, I suggest that you look beyond the engine itself. You haven’t described the problem, besides saying that you have engine roughness, and when the roughness arises (aka not enough information).
Did anyone check the prop balance? There’s a good reason for roughness!
The description that caught my attention is “my engine still feels as if I’m flying in rough air”.
If it “feels” like you’re in rough air, and the audible component has been suppressed to a minor level, then I would begin to suspect balance / bearing vibration issues, and not combustion issues. You have been chasing combustion problems so long that you may still be fixated on that.
I’m guessing purely from a verbal description, but it may be worth an examination.
For vibration – are there prop balance issues? Engine isolator mount issues?
Are they all the correct isolator partnumber and are they all the same brand? Are they still in good shape?
Is the engine mount / mount points / attachment hardware / firewall damaged?
How about cowling / baffling / accessory/component security? There’s a lot more than just an engine in front of the firewall.
As suggested, be sure to check the calibration of the instruments. If you have been flying at erroneous tach settings then you may have crankshaft and crank bearing issues by now.
If the plugs don’t fix it, start looking at everything else that can move and rattle up there.
I second Lee Ensminger’s reply. Several years ago a friend with an A36 started having a rough engine, at the last very rough, after installing a new set of Champion fine wire plugs. A resistance check, recommended by Cliff Orcutt, showed the best one of the bunch at 12k ohms and two at infinity, open.
Almost a $1000 worth of spark plugs with less than 50 hours that weren’t worth a d***!
Has the prop balance been checked, dynamically if possible?
Also check the calibration of the fuel flow for the same reasons he mentions checking the tachometer calibration.
I read recently about an issue with the resistors in Champion spark plugs. Apparently, there was an issue, denied vehemently by Champion for years. Here’s a link to the article:
http://blog.aopa.org/opinionleaders/2015/03/19/champion-from-denial-to-acceptance/
If you have Champions, I’d be looking at them as a potential source of your trouble.
Lee I agree with you 100% about the poor quality of Champion plugs and particularly the fine wire Champion plug. Those are the most expensive plug on the market and also the worst. The first thing I would do is replace all of them with Tempest, either fine wire or massive, and you will be a lot better off for sure!
In addition to Champion plugs having a resistance problem with high OHMS readings their fine wires plugs develop cracks in the white insulation part of the plug. This happens sometimes in less then 100 hours of use. If left in service the cracks can get to the point where a piece of the insulation comes lose and goes into the engine. This can cause engine damage. I can’t say enough bad about Champion plugs. I ferry planes all over the world and if any plane has Champion plugs installed they will be replaced with Tempest for the ferry.
As a former Comanche 250 owner, I’d also check the fuel tank caps to make sure they are properly venting. Had the opposite problem one sub-zero day when the rubber seals lost their plasticity and fuel began venting overboard, considerably shortening my flight by an unscheduled landing.