Rough Running at Full Power

[rotaxflyer]

My BC-12D with an A65 is running rough at full power. This typically develops something like this:

-starts and idles fine.
-run-up at 1700 rpm is fine with about 25 rpm drop on the right mag and 75 on the left.
-take-off develops full power and runs reasonably smooth
-About 20-30 seconds later, it begins to lose power, runs rough and drops to about 2000 rpm from the usual 2300 (kinda scary actually).

Tied down (safely on the ground), the same sequence is repeated. After running at full power, the roughness/power drop returns after 30 seconds or so. After a short period of idling, then I can once again get full power for a little while before the roughness returns.

So far I have checked points and timing and both mags are within a degree or two. Also, I have disconnected the fuel line from the carb and checked fuel flow and there is plenty.

Removing spark plugs after the above running test shows them to be very clean and rather light in color. So I am thinking it is running lean... Am I on the right track?

Also, for the last few flights the engine temperature has been about 180f which is higher than I was accustomed to - I didn't think much about it since it was august, but maybe that fits the lean theory as well.

Should I tear down the carb and look for obstructions in the jets or some such?

Thanks in advance for all the help.

Andy Sinnamon
#952

[Howard Wilson]

Re: Rough Running at Full Power

Have you put in new sparkplugs or changed them recently? I had two bad 'new' plugs that had roughly similar symptoms....

[rotaxflyer]

Re: Rough Running at Full Power

The bottom plugs were cleaned this spring and top plugs new at the same time. That was over 25 hours ago...

Andy

[Howard Wilson]

Re: Rough Running at Full Power

And the gascolater is clean? h

[lfries]

Re: Rough Running at Full Power

I had an intake valve that would stick at full power and then just become unstuck and the engine run fine.

[Acroeric]

Re: Rough Running at Full Power

What kind of mags are you using? Had a similar issue with some older bendix mags- replaced everything in them after ruling all else out. We got 2 bad condensers - both were new in the box. Once we got good ones the problem went away. Is it worse on any particular mag? To find out you are going to have to check the mags when it messes up. Ours did this same thing. I know what you mean about it making you nervous- I had it happen on a test flight after a 20 year restoration that was absolutely beautiful. All I could think about was scratching his plane.

[dennispip]

Re: Rough Running at Full Power

I had the same problem and eventually developed a totally dead mag. Would only do it intermittently and after the engine was hot. Would always run up fine on the ground before taking off. Could be other things, but my bet is on one of the mags, too.

[rotaxflyer]

Re: Rough Running at Full Power

Well at first it seemed to be the left mag since it had a higher drop during run-up. I took it off changed out the points and condenser from a spare mag, set the gap, checked the e-gap, reinstalled and timed the thing to within a degree. I also changed the spark plugs just to be sure. After all this it ran pretty well during run-up so I did the test flight. During the (scary) test flight, I did switch between left and right mags to try to smooth out the engine and it was rough on either or both.

Andy

[magman]

Re: Rough Running at Full Power

Your description seems to indicate LEAN.

If you put on CARB HEAT it will cause the mixture to get "Richer".

I 've seen where this results in RPM Increase!

Another good thing to do is check the harness /distributor system for "leakage".
Use a high voltage lead tester for this.
As a mag turns faster the voltage increases which increases the possibility of
flashover in distributor or leads.


Weak valve springs can do things like this as well.

[Hank Jarrett]

Re: Rough Running at Full Power

I haven't seen it on an aircraft engine, but when I had ignition lead leakage on a sports car you could open the hood on a really dark night and it looked like the 4th of July under there.
Not sure I'm comfortable with someone walking around a spinning prop in pitch black, but if it lit up like my car you could probably see the leads arcing through the bottom rear of the cowl.
Hank

[t-crafter]

Re: Rough Running at Full Power

Andy, check your intake sys. All the intake tubes and clamps,carb mount and spider. My first bet is an intake leak. I would pull all your rocker covers off and look real close at the condition of the valve springs for a cracked spring. Compare the total movement of all of them to each other. These are easy checks but sometime this type of problem can be an adventure!! Good luck and keep us posted. Carlton

[M Jones]

Re: Rough Running at Full Power

Andy, check your intake sys. All the intake tubes and clamps, carb mount and spider. My first bet is an intake leak.

Good idea! I've seen a loose carb mount on other small engines give those same symptoms. I had a carb loosen up on one of my chain saws to the point that I was using full choke to keep it running at power. Tightened it up and it ran like a champ (uhh,Tcraft). There've been lots of good suggestions, hope you get it resolved soon.

[rotaxflyer]

Re: Rough Running at Full Power

I have now removed the carburator and dismantled it looking for obstructions. I checked the float level and it was a little low at 1/2" below the parting line. I removed one washer from the float valve seat and it came up to a bit higher than spec. I inspected the intake system looking for any holes, checked each of the intake elbows for tightness. Reassembled everything. Tied it down, started up - same as before. Runs great for 30 seconds or so and then starts to miss at any throttle setting above 3/4 power.

How likely is it that both mags are bad? Could I have had one bad mag for quite a while and not known it until the second mag acted up? They are eisemann's. Also, does anyone know if the gears from the eisemann's can be fitted on to new mags if I choose to try that

There is pretty decent resistance for the usual four flips before starting, so does that basically rule out valves? Could a lifter work great for while and then deflate? But only at full throttle.

I guess I'll remove the rocker covers next and compare the valve movement.

[Guest]

Re: Rough Running at Full Power

My BC-12D with an A65 is running rough at full power. This typically develops something like this:

-starts and idles fine.
-run-up at 1700 rpm is fine with about 25 rpm drop on the right mag and 75 on the left.
-take-off develops full power and runs reasonably smooth
-About 20-30 seconds later, it begins to lose power, runs rough and drops to about 2000 rpm from the usual 2300 (kinda scary actually).

Tied down (safely on the ground), the same sequence is repeated. After running at full power, the roughness/power drop returns after 30 seconds or so. After a short period of idling, then I can once again get full power for a little while before the roughness returns.

So far I have checked points and timing and both mags are within a degree or two. Also, I have disconnected the fuel line from the carb and checked fuel flow and there is plenty.
Removing spark plugs after the above running test shows them to be very clean and rather light in color. So I am thinking it is running lean... Am I on the right track?

Also, for the last few flights the engine temperature has been about 180f which is higher than I was accustomed to - I didn't think much about it since it was august, but maybe that fits the lean theory as well.

Should I tear down the carb and look for obstructions in the jets or some such?

Thanks in advance for all the help.

Andy Sinnamon
#952

Andy,
Check the fuel flow thru the float bowl.

Do this; put a pan or bucket under the carb and then remove the float bowl drain plug.

Let at least one bowl full drain out, I am wondering if after the bowl is drained you see a reduced flow rate out of the empty bowl. That would suggest a restriction somewhere between the drain plug and the tank. In your case probably in the carb. But certainly do this to verify that you are maintaining a good flow thru the carb after the bowl drains. Since you loose power after 30 sec I suggest watching it drain for 30 seconds.

I have gotten a similar power reduction due to the nose too high on a less than full nose tank so it sounds like a loss of fuel flow to me.

I assume that you are running off the nose tank and that the plumbing is simple?

Hope this helps and doesn't end up being a goose chase.

Dave

[flyguy]

Re: Rough Running at Full Power

Just as Dave says, it sounds exactly like you are sucking the fuel level down in the carb bowl because the flow to the carb is poor.

When the engine is idling or off the bowl fills up and then immediately the fuel level in the bowl starts down when it goes to full throttle and it takes 30 seconds to get low enough to go lean. Just like pulling the cut off valve.

Bet it is a stopped up filter screen into the carb or debris at the bottom outlet of the gas tank. Anyway just walk thru the fuel delivery path, fuel line from tank, gascolator (it isn't installed backwards is it?), second fuel line, screen, ect.

Dave is on the right track.
Darryl