Increased "mag" drop troubleshooting - ROTAX 912ULS2

[FastEddieB]

I recently noticed a definite change in my rpm drop when testing IGN 1 vs. IGN 2.

 

My tach is analog, with hash marks every 200 rpm, making it hard to see the exact drop.

 

But it used to drop just under 50 rpm on each ignition, and both were equally smooth. I use 3,800 rpm for the check - my checklist calls for 3,850 but, again, it's hard to discern a 50 rpm difference.

 

Now, when I ground IGN 1 to test IGN 2 I get about a 200 rpm drop and it's noticeably rough. Grounding IGN 2 I still get the prior, smaller drop. My checklist calls for maximum drop of 300 rpm and 120 rpm difference, so I'm still close to OK, but close or slightly over on the difference.

 

I pulled all the plugs, which were recently cleaned, inspected and gapped, and they looked OK.

 

My plan is to first replace the plugs, as the easiest/cheapest place to start. My local O'Reilly's had 5 in stock and will get the rest in today. I guess I could have swapped them top/bottom to see if the problem migrated (that would work, right?), but they're cheap enough to just go ahead and replace.

 

My next step would be to take my BullyHawk SoftStart module out of the loop, in case it has an issue.

 

My spark plug wires and caps are 7 years old. Is it worth replacing those if the other things don't work, and how expensive is that?

 

Oh, and the nature of the problem makes me think it has to be ignition related, not fuel - right?

 

Any thoughts beyond that if none of that works? My ignition modules are pretty hard to reach, and it might be time for a professional.

 

Roger, you headed to points east any time soon?

[Anticept]

First, have a person use an infrared scanner on the exhaust stacks. Look for a cold(er) cylinder/stack. That will help you find which cylinder is the problem. Then you can consider switching things top to bottom, left to right, etc.

 

It is practically guaranteed to be ignition related. If you move the plugs and it moves with it, it's obviously a plug. If it doesn't move, it's either the harness (more likely, since you said it's jumpy) or module (unlikely, usually failing ignition module will cause a large rpm drop).

[Tom Baker]

First I'll throw my thoughts on spark plugs out. I never pull plugs and clean them and reinstall. I always install new plugs, because the gasket is designed to be a onetime use gasket. If you have one bad plug I think the drop would be higher.

I really doubt that you have an ignition problem, even though that is how it appears. Knowing that you just had the carbs off for disassembly and inspection that is where I would look. I know I had one run rough like yours after a carb inspection because one of the idle mixture screws was not screwed all the way in. I'm not sure why it didn't get set it right, unless I was interrupted when I was putting the screw in.

[Roger Lee]

Hi Eddie,

 

Don't look for some exotic problem. This has a simple answer.

 

From your description it is ignition caused. With this small mag drop your problem is absolutely after the ignition modules and the coils. My best guess right now is a plug or plug wire. Up to a 150-300 rpm drop is usually only 1 plug or wire. I would unscrew each plug cap. Trim the cable back 1/4" and then screw the cap back on. Replace the plugs. Make sure no thermal paste is on the last third of the plug near the tip. If one electrode gap is closed down compared to the others it will cause this drop. 

 

p.s. I highly doubt it involves the soft start. The drop is way to small. I too never reuse plugs. I would never go past 100 hrs. and we usually recommend 75 in class.

 

I can't come out there. You have too many bugs and humidity.

[Tom Baker]

Like I said I have seen these exact symptoms with a carb problem.

[Jim]

What Tom said.  Never re-use a crush gasket, especially when the plug and all only costs $3.

[FastEddieB]

Thanks, guys!

 

Roger, I never thought about the plug caps. Are they simple "screw-on" type with a little "sheet metal screw" in the center? Easy enough to check them all and re-dress them once I see if the plug change takes care of it.

 

Tom, since the plugs are evenly distributed L/R, it's hard for me to see how a carb problem could manifest itself in this way. Though if it can, I guess it can!

 

As an aside, full power still results in 5,300 rpm on takeoff, and cruise speed, climb rate and engine smoothness seem unaffected.

[FastEddieB]

What is different from before?  Carb rebuild.

 

I did stumble across a video I took last September when I first started noticing the drop:

 

 

Very hard to make out, even in full screen HD (tach is to the left of the 496), but the problem did start to rear its ugly head long before the rebuild, then kind of resolved itself, and now its back.

[FastEddieB]

Life is always more interesting if there are stakes involved!

[Roger Lee]

Hi Tom,

 

The reason I'm looking at the plugs and wires first is this was an acute issue right after maint. to that area. I do agree that a carb can cause this he didn't maint. the carbs before this happened, but he did work the plugs and had to pull on the plug wires to remove the caps. So this is where I would put my attention towards first then work outward for the most common causes.

[FastEddieB]

To clarify...

 

...this started some time ago, with no maintenance performed prior.

 

On my post-annual flight, things were normal.

 

Now the drop is back, identical to what it was last September, before it cleared itself up.

 

Same plugs and wires in use throughout.

 

Stopping by another O'Reilly's to see if they have 3 more plugs. If they do I may futz with this this afternoon, before the forecast bad weather hits.

[Roger Lee]

I thought this was an acute onset right after the plug work.

Then if this is intermittent and didn't just occur after the plug work then I would still do plugs first and if that doesn't fix it follow Tom's advice on the carbs. I piece of debris could cause the issue. If you get to the carb level pop the bowls a look for debris and check with a a set of gauges during a sync.

[Tom Baker]

Roger, the time I saw this was right after carb maintenance. I don't know why, but for some reason I didn't get one idle mixture screw screwed in to where it should be.

[Anticept]

What I find odd for the carb group guys, is why would an idle needle cause one mag drop to drop 200 rpm, while the other only 50 rpm? That's what fasteddie said in his first post, and I don't see how an improper idle jet setting would not affect both mag drops in such a small engine.

[Jacques]

have a look at ''operational checks' in this document

912 carb troubleshooting.pdf

[Anticept]

have a look at ''operational checks' in this document

Yes, but fasteddie has a RPM drop spread. If it was carb, then both mag drops should be uniform.

[Tom Baker]

The difference in drop is due to the difference in mixtures between the carbs, and the difference in fuel burn patterns with the top or bottom sparkplug firing.

[Anticept]

Mixtures between the carbs should not matter, the left and right ignition positions still fire all four cylinders, and the same fuel air mixture is being pulled into the engine, the only difference is if it is top or bottom plugs turned off on the respective sides of the engine. If a carb was having a problem, then top or bottom plug firing, he should see the same drop across the board, and it would be quite significant.

 

Fuel burn patterns should not matter in such a small bore engine. This usually only matters in large bore engines where they offset the timings for one set of plugs (heat off of the exhaust valve changing the density of the air near it). Of course, a rotax engine runs as a higher RPM, so I can kind of see that playing a factor, but I do need more information to understand why a carb would cause an RPM drop spread.

[FastEddieB]

The O'Reilly's in Copperhill had three more plugs, so I went over to the hangar and replaced them.

 

Viola!

 

 

Hoping that did it. Unless it performs differently with a warm engine, I think I must have just had an iffy plug.

 

Thanks, everyone - every fix should be a $20 proposition!

[Roger Lee]

Fixes are usually simple. So it's always the best place to start.

[Ian]

Fixes are usually simple. So it's always the best place to start.

I'm sure William of Occam would have made a great mechanic!!

[S3flyer]

I'm sure William of Occam would have made a great mechanic!!

And he'd be 'Razor' sharp 

[Roger Lee]

http://en.wikipedia.org/wiki/William_of_Ockham

[Doug G.]

"Contact!" - that was the movie I first remember hearing of Ockham's Razor. Seems fitting for aviation.

[FastEddieB]

Comes up all the time on the Skeptical sites I frequent.

 

Important that it is not really the "simplest" solution that's most likely correct, but the one with the fewest assumptions.

 

From Wikipedia:

 

"It states that among competing hypotheses, the one with the fewest assumptions should be selected. Other, more complicated solutions may ultimately prove correct, but—in the absence of certainty—the fewer assumptions that are made, the better."

 

In relation to physical phenomena, "God did it" seems a lot "simpler" than pages of equations. But in that apparently simple statement lie many, many assumptions.

 

How is that for a derail?

 

BTW, Contact was an OK movie, but I really liked the book better.