Twin Comanche engine surge

[Bryan Rose]

Still baffled....

Right engine is 350 SMOH. I've been chasing a 100 rpm surge for a year now.

Engine runs fine on climb out. Pull the prop back and it may run normal for a few minutes, but then starts a +50-100 RPM surge. Fuel flow does not surge, nor does MAP, not sensitive to mixture position, not sensitive to in flight mag check, does not run rough.

Here's what's been done:

Prop gov overhauled
Checked crank shaft plug
Fuel Controller overhauled
Spark Plugs Tested
Spark Plug harness tested
Fuel injectors cleaned
Mags checked
Compressions are excellent

[Ray B]

How long has it been since the Tach cable was checked? Ray B

[Bryan Rose]

Not sure how long the cable has been there, but the surging is yawing the aircraft when it jumps to 100. So, I know that it's not just an indication error. Good thought though

[N3322G]

Bryan,

So sorry to hear that you've been having this difficulty.

Clearly the obvious components are/have been worked on. Couple of thoughts.

My twin was flying pretty well except for some tiny and nearly unnoticeable things. Went to scheduled annual and engines ran up perfectly. Mechanic noticed a tiny seepage so the left engine fuel servo removed and checked for a leak. None found and was fine on the bench.Only technician comment was the butterfly valve opened only 8 thousandths of an inch and the engine shouldn't be able to run so he changed the idle to middle of the spec range. 8130'd and it was returned to service. Ran up the engine. The IA and I had never had an engine perform in the way this one did.

Sent the servo back and at first the tech said it ran fine on the bench again. He diddled with it and found not one but two different problems. I've been told there is a shoe that closes the mixture and it was not flat and had to be lapped. The gasket was also replaced. He looked further and found the same problem with the throttle shoe valve.

On the right engine a few drops of fuel would fall on the floor between flights. Chased for 3 years now. Finally a number 3 spring was replaced with a #4 spring and the drip has stopped.

Weather was bad so haven't flown yet. Run-ups were perfect. I expect good tests flights tomorrow.

Another thought might be transducer interference that is masked at full power. They only last so long. Just changed ours.

Several years ago after an annual, run-up and normal take-off, turned-off the aux fuel pumps and bad engine stumbles. Pumps back on and landed safely. Found contaminants in freshly cleaned injectors. Years before, had a spec the size of a pepper grain that clogged one injector partially and intermittently.

Don't know if you've already checked engine driven fuel pumps.

So, don't have any definite symptom problem correlation for you but perhaps the experiences may give you some thoughts on what to pursue next.

Can't recall - was it this way when you got it? If the problem happened after some work, then change management might piton to a source. Regular wear is harder to find.

[Kristin Winter]

Do you have an engine monitor or fuel flow totalizer? These might be able to eliminate fuel fluctuations as the root. It has to be either in the combustion or the prop system. Does the engine have a steel line from the governor to the front of the engine? Has this either been replaced or cleaned out? When was the prop last overhauled?

I am sorry that I have more questions than answers.

[Zach Grant L1011jock]

Check the oil suction screen as well as the gasket screen on the gov. If you have a non C/R airplane, swap gov side to side. If the problem doesn't follow, then you may be looking at something more serious. I would definitely say that if you see rpm fluctuations, it is most definitely prop or gov related. If the surging does not affect RPM, then it is probably combustion related as the CONSTANT speed prop will be working normally.

Lets look at the potential issues causing prop surging. First is obvious. The gov or prop are bad. If those items have been exonerated (just because they were overhauled doesn't mean they are right), then you need to look at the oil path. Lack of oil supply to the gov will cause inconsistent pressure output. As Kristin said, the problem could also be in the output oil run. The transmission tube that runs from the gov to the forward crank gallery must be clear. The crank holes and crank must also be free of sludge and crap. Further, the forward gallery must be oil tight so that the pressure can be transferred to the prop as opposed to dumping back to the case through a spun or excessively worn crank or bearing. This will be most noticeable after the oil warms and becomes less viscous, and sometimes at lower thrust. There is a Lyc service instruction that addresses how to check for the possibility of this before cracking the case. If the forward gallery has excessive leakage, the gov will not be able to pump enough oil to control the prop. If the leak is still within the capacity og the gov, then control continues but it will be sloppy and surges may occur.

Zach

[N3322G]

I've also seen surges with out of spec dome pressure.

[Kristin Winter]

Good point Pat. I have seen that too. Always best to check the simple stuff first.

[Bobbi Boucher]

well u did the gov frist so time to look at the prop dome seal and the charge on the prop dome
a surge is one or the other the dome seal may be bad the plane doctor 540-220-8138 Ms bobbi

[Bryan Rose]

Thanks to all who commented. Zack, you're the winner with, "just because they were overhauled doesn't mean they are right". Swapped the gov...(Holy cow that's a big job!)...surged followed the gov.

Thanks for everyone's help, if you been following the board, I've had a LOT of trouble with this little Twinco.
I had the gov overhauled a year ago! In the middle of fighting that battle, the left engine developed a problem where it was feathering the engine and killing it if you brought the power back to idle. That proved to be a small chunk of the cam shaft that found it's way to the front main! Overhauled the left engine!

While it was in the shop for the overhaul, I decided to overhaul both fuel controllers...surely that would fix the surg problem on the right engine.

I don't want to be a hater, but I think people need to know about a very serious issue that I had with Kelley Aerospace. After they overhauled the fuel controllers and my shop (Rocket Aviation formerly Miller Flying Serv, Plainview, TX) put them back on, neither engine would run. They were pulled off again and sent back, they shipped them back and said everything was correct on them. This time the airplane would run, but you had to have the mixtures at just before idle cutoff to keep them running. Pulled them off and sent them back again, they reported that "somehow" they got changed. Put them back on the airplane and the engines ranup fine, flew fine, but once the engines were hot, you had to pull the mixtures all the way back again to keep the engines running. All this cost about $6,000! Rocket, then pulled them off again and sent them to Mike's Fuel Metering Service in Tulsa. Pete is the guy there that reported that they were not even close to being setup properlly. He re-overhauled the units and the aircraft ran fine. Rocket is stuck with trying to get the money back out of Kelley Aerospace, but more importantly I flew the airplane twice with them like this, and that was very dangerous. I would be very leary of sending anything to Kelley.

I've been very frustrated with the little Twinco. But she's such a nice bird and I think we've got it whipped now! This time the prop gov is going to Quality Aircraft Serv for the overhaul. Tim Talley down at Clifton Aero is standing behind his work too and is fixing that for me. So, I think I've just about got her whipped into shape! Wow cowls will be here for her in January! She's a 2800 TTAF NDH airplane with excellent avionics and all the Lopresti mods except the wow cowls. On the flight to Clifton on Fri she ran 178 knots TRUE at 16 GPH @7,500 MSL. THAT is economical!

Merry Christmas!

[Kristin Winter]

I have seen problems with governors. I have had good luck with Southwest Aero up in Washington State. They are very reasonable and stand behind their work. Others here put me onto them for governors, which I think is all they do. However, I would go with Tim's advise as you are down in Texas. I trust Tim as well.

Good luck with your plane in the coming year.

[N3322G]

Bryan, ditto on the fuel servo/controller. took twice to get it right. See above. Makes me wonder what makes them so different.

FYI. It took Hartzell 4 whacks to get the governor right in 2009. It was overhauled and installed and wasn't right. I flew to Piqua and it still took three more. Fortunately I was at Piqua. I flew and returned twice. Finally, the third try worked. I was surprised and so were they.