Rough Running Engine

My engine in my RV-12 is running rough and so far, I haven't been able to
determine the problem. At first I thought I just needed new sparkplugs, but
after installation of new plugs, engine still ran rough.

Next, I spoke with Lockwood and they thought it might be carb related, since
I was right on the 200 hour mark and a carb overhaul is due at that point. Had the carbs overhauled, engine still ran rough. But, after shutting engine down,
I felt the exhaust pipes coming from each cylinder and the pipe from No. 1
cyl was cool to the touch while the other three were hot.

Next I did a compression check and cyl No. 1 was normal 75/80 cold. I also
pulled the valve cover from cyl 1 and the valves opened and closed as the
prop was rotated.

Called Lockwood and they advised to try and snip 1/8" off both No.1
sparkplug leads (sparkplug end) and reinstall on plug connector. They
also advised to ohm out the spark plug leads and check trigger gap for
.012".

Started engine after snipping the plug leads, still runs rough. Haven't done
the ohm checking yet.

From this information, do any of you have a suggestion as to the cause of
my problem? It seems almost inconceivable that I can be having a problem
with both ignition modules at the same time.

Thanks for any help you might give.........

Walt Shipley

If you have a non-firing cylinder it's not the carburetors. Since the ignition fires two cylinders at a time (one on power and another on exhaust) each time it triggers it seems that the problem must be between the ignition modules and the spark plugs unless you have two cylinders not firing.

Is it running rough at high RPM's, low, or both?

My experience with 912's running rough was due to unbalanced carbs. With the cowling off and using due care, manually work the throttle on one carb versus the other and see if it smooths out. Maybe check the grounds on the ignition mod's?

If anyone can help you, it is Lockwood. I've been there and done that and and paid the piper.

As already mentioned, the 912 engine uses a waste spark ign system where each ignition coil (and its associated trigger coil and ign module) fires two cylinders. If you only have one cyl not firing at all, I would be looking at something specific to only that cyl (like you said, the likelyhood of both ign failing at the same time is remote, and even if they did it should effect two cyl).

You said compression check was good so we no that both valves are closing and sealing.

Have you done the valve lifter clearance check on the bad cyl?

Some automotive stores sell a module you can plug in between the spark plug and the plug lead. It lights up if the spark plug gets hi-voltage. You can use it safely since you don't have to stand by the prop to see the light. You might try that to see if the plug is getting a spark. If it isn't I'd suspect the plug lead. However, since each cylinder has two plugs and each is fired by a different ignition module it seems very unlikely that at least one plug isn't firing.

Thanks so much for the responses to my post about my rough running rotax.

Today I removed the top plugs and set No. 1 piston on TDC. I did the valve check by pushing on the valve rocker to see if I had excessive clearance. I did not. Both the exhaust and intake valve clearance was nice and tight.

Now comes the really strange part: I put the plugs back, reconnected the plug wires and started the engine and ran it for 2-3 minutes then shut down and felt the No. 1 cyl exhaust pipe, and it, along with the other three were hot!

I restarted the engine and, at idle, I experimented with the ignition switches by switching off the B side. The engine continued to run as smooth as it had been. Then I switched off the A switch and the engine would have died had I not caught it by flipping the A switch back on.

After reaching operating temp, I advanced the throttle very slowly and the engine began to shake so violently I had to shut it down.

Lockwood had previously advised to ohm out the ignition system, and that's
what I plan to do tomorrow. At this point my problem definitely seems to be
ignition related. I have a terrible feeling this is going to be costly.

Walt Shipley

Bummer, Walt. Those ignition modules cost just south of a grand.

You should swap the ignition module plastic connectors A to B and B to A to see if the problem swaps to the other ignition switch. That would tell you it's the ignition module.

Checked module grounds?

Walt, Before you spring for a new ignition module, check the little hex bolts that ground each module to the intake manifold.

Your initial report of a dead cylinder indicated something else but your most recent post sounds like what I experienced a few years ago.

Pull each bolt and clean up the contact surfaces. The casting for the manifold on my engine had not been cleaned up properly IMHO on one of mine and the contact was intermittent.

Jim
315 hours

Was this problem ever resolved?

I think he sold it.