Runs ROUGH at higher altitude...where do I start?

My 6 has an O360 with Ellison TBI, CS Hartzell, and mags. It starts fine and runs well at what I consider normal cruising around altitudes (8-10K). EGTs/CHTs are pretty even. However, at altitudes starting around 11K up to 14K it will initially be ok, but after 10 minutes or so it is progressively rougher with temps getting farther apart. Adjusting mixture, throttle, or RPM doesn't help, although in the initial stages of the problem I sometimes find a spot that is briefly better. When I come back down the engine begins to run smooth again. Now, I'm severely lacking in anything resembling wisdom or expertise in this area. But it's as if the ignition can't fire the mixture at higher altitudes. Maybe like plugs fowling, then clearing when I come down. I'm not saying that's what it is, just how it "seems". Do mags get "weak" eventually? Is mixture harder to ignite at altitude?
If it matters, the lowest place around here is the airport, which is at 7100'. My typical cross country (PNA-SHR) requires clearing some pretty tall rocks. Thus the need to get up high for awhile.
Any suggestions? I'll probably pester aerhed about this when I get the chance, but thought I would post here to start. Thanks!

I don't have a lot of aircraft engine experience, but raced and built motorcycles for a while. I know that in that application, more compression results in a harder-to-ignight mixture and vice versa. I would think that higher altitude would make the mixture easier to ignight as the pressure in the cylinder will be less (for a given throttle and rpm setting). So, it is probably not ignition related. Otherwise I'm not much help but good luck!

There can be a failure mode with Slicks, where the mags start arcing over internally at higher altitudes due to the thinner air being less resistance inside the mag body.

How long since the mags were inspected? The service interval is 500 hours for them, and the Slicks are pretty sensitive to that... much more so than the Bendix.

That said, 11k isn't that high for this problem to be occuring, and I'd suspect the Ellison first before the mags.

David,

What kind of shape are your mags, ignition harness, and plugs in?

At higher altitude there is less air, as we all know. That air works as an insulator down low and a small ignition leak due to an old harness may show up as you climb. That is the reason aircraft certified for flights up high usually have pressurized ignition systems.

Just a thought.

Are you...

... running mogas or avgas?

Just as I suspected...

...That is: I don't know enough to know what I don't know.

The ignition system was tested about 50 hrs ago and reported all ok.

What would be suspect about the Ellison?

I'm running 100LL.

I think I better fly it this evening. More research, you know. I'll pull the plugs this week to see what they look like.
Thanks for the help!

Runs rough

I have 655 hours on my RV-8, O-360A1A, Ellison TBI, Whirlwind prop, Slick mags. Also use the standard Van's airbox for the float carb.
The Ellison has been problematic in my aeroplane. #2 cyl always runs hotter than 1,3 and 4. 4 is almost always way colder- it runs rich. The plugs tell the tale.
I have occasionally had similar symptoms. What plugs are you using? I used Unison's for a while since the price was right. Since switching back to Champs I've had no problems. Last time I tried the Unisons I tested them before installation - 2 out of the 8 had a really weak spark.
Before you do anything expensive I'd try testing your plugs.
O.B.
RV-8 N60B

Runs rough

Had a Senior moment-
Add to my previous post that the temps I refer to are EGT's. CHT's in my installation are always very close.
O.B.

Ref the Ellison, I don't know what to check... thats the problem. I've seen some installations work out amazingly well, and others than can just never get the tuning to work out on them. Problem is, i've never had to tweak with one on a customers airplane, so I don't know what the failure modes are for them.

Dig up your logs and see if you can figure out how many hours your Mags and plugs have on them. Could be something simple like that, if they are fairly recent, call up Ellison and explain to them what happens. They might have some insight as to whether it could be the TBI or not.

Sounds like a Mag problem - arcing sounds likely.

Replace one with the electronic ignition of your choice. You will be happy.

breister said:

Sounds like a Mag problem - arcing sounds likely.

Replace one with the electronic ignition of your choice. You will be happy.

Click to expand...

I agree.

I flew with Ellison and Jeff Rose's EI in the Cozy and that airplane just loved cruising at 13-15000'. The Ellison is a stone simple single point injection system and did not have a problem except it would dribble fuel sometimes after shut down if the fuel valve was on.

Thanks for the advice!

Unfortunately I have not had time to pursue this problem, yet. After a couple of leisurely months, work got busy again. The energy industry is kind of feast or famine. So you take it when you can get it.
The arcing in thinner air makes sense to my simple mind. When I can get time I'll pull all the plugs. I'll also see if I can get the whole ignition system tested. One electronic sounds like a good idea. But I'ld like to identify the culprit before replacing anything.
I don't think I mentioned it initially. But when it was running rough, the temps on 2 cylinders stayed more or less normal, other 2 got cold and erratic, then back to normal as I descended to around 11k. I don't know if that fits the arc'ing scenario In the one local flight since then it ran fine at 8-10k.
Again, thanks for the help! The education I've got from this site has been a huge help through the whole journey!

98% sure it's mags... The distributor is arching over between two of the cylinders (it's called carbon trailing) which is causing those two to misfire. Get one mag overhauled, and get a EI for the other side... You will be very happy.

don't overlook the plug wires

Just a note from an old timer, don't overlook the plug wires and thier routing. If the plug wires are leaky, they run near some metal baffles or a grommet is cracked, or they run next to each other and bleed over to the other cylinder, the wires could arc robbing the plug of the energy or misfiring. Plug wires are cheap and may be worth an investment in a new pair anyway. Also, make sure the plug wires are firmly plugged into (or screwed onto) the spark plugs. I would bet a whole quarter it is an arcing problem in the mags, wires or plugs. Good luck.

wires and plugs, ditto

You can buy a wire tester fm various sources, i.e.

http://www.chiefaircraft.com/airsec/Aircraft/Tools/SparkPlugService.html

Cheaper than a new set of wires in the blind. $77+-

Also, carefully check your plug gaps, AND the resistance of the plugs. I had one open which did not manifest until a couple of sustained 8-10k' flights, not normal flying for my airplane. It crudded up the rotor and associated stator to the point of bad run up mag check. That open plug and the others hooked to that mag had exceptionally larger gaps than the plugs on the good mag.

As I also had a couple plugs up near 5k ohms thru the center, and new should be about 1500 ohms, so I replaced the lot of them.

My wires tested good. Ditto for condenser and points.

Airplane runs fine now. Slick mags both sides.

Good luck.

Carb icing??? Try a little carb heat

#N72CR said:

Carb icing??? Try a little carb heat

Click to expand...

Good question. Ellison TBI's usually are not susceptible to carb ice. Wyodave?

...no, i haven't fixed it yet...

I figured everyone had forgotten about this by now. When I got busy at work I got really busy, so not much trouble shooting yet. But on recent local flights up to 10k every thing is normal. The Ellison TBI has been mentioned by a few. But the temps are all very even below 11k. #3 is the 1st to drop, and it is sudden and dramatic when it does. It is followed by #4, though it isn't as dramatic. It seems to me that if it were a fuel distribution problem it wouldn't happen in this way.
Ice was mentioned. I did try heat the 1st time it happened. But there was no improvement, just a little loss of power.
On the one recent Xcntry I climbed to 15.5k for awhile to see what it would do. It still exhibited the problem, though not quite as bad as before. As before, it starts very suddenly on #3, clears up just as suddenly when I come down.
I had hoped to do a little more trouble shooting on my own so I could hopefully learn something. But I think I'm going to have to break down and see if I can get aerhed to help me go through the ignition. He's in the neighborhood (well, by Wyoming standards anyway)
Hey, aerhed! Are you out there?
Thanks again for the help! And for prodding me to get on this...

Try removing

the two vent plugs compleatly and go for a fly. I am operating a set of bendix regularly at 14K and up that are not pressurised, but do have fresh air plumbed to run through the vent plugs.
Fast, cheap troubleshooting.
Good luck,
Andrew

Dave,
I had a similar problem 2 years ago returning from the LOE flyin. Like you I run an Ellison with a vans filtered airbox and fly at high altitudes. I personally think the Ellison suffers a little at high altitude but I dont think thats the problem. When this happened to me I was flying in formation with some one that I flew with regularly so I knew how my airplane should be running relative to his. I had to ask him to pull back because my airplane was not performing right (still smooth though). I checked LH mag Vs RH mag and had a pretty bad drop on the LH. I turned it off and we continued to Sante Fe. I fully planned on repairing a mag but on the ground it checked normal. Once climbing through 12,000 the problem turned up again. We opeted for IFR (I follow roads) and limped it home. Once again ground checks ok. I took the mag appart and discovered a cracked distributor housing.
I think you to will find your problem in the ignition system. BTW visited my son in BIL thurs and stoped in SHR for lunch with a freind, great place but they need to do something about those fuel prices. good luck!!
Ryan Bendure
RV4 131RB
Rocket 75%