Engine Roughness

[tonycondon]

Heyo All,
Had an interesting flight today, Climbing out from airport everything was dandy. about 2000 AGL, start to get some engine roughness, pull back the throttle a tad, engine is smooth. Go back to full throttle, engine rough. Turned back towards airport, called approach and stayed high until over top, spiraled down to beautiful crosswind landing. This was on a 150. Any ideas of the problemo?

 

[markb5900]

I would think MAYBE carb ice.
The engine would the samething to me in when I would get carb ice in the 150's.

 

[tonycondon]

i wondered about that too, but it was only at full throttle, other power settings it ran fine, and was developing normal RPMs for throttle position.

 

[tonycondon]

anyone else? john thought maybe a vacuum seal going bad? ideas?

 

[Let'sgoflying!]

Tom will know this; is the enrichment valve a fixed orifice?

 

[gismo]

anyone else? john thought maybe a vacuum seal going bad? ideas?

Sounds more like a fuel delivery problem (fuel filter clogged?), or possibly a mag problem (high MP requires the most spark voltage).

 

[tonycondon]

lance, im interested in the magneto thought. do the mags lose voltage with altitude? im no electrician, personally hate the stuff, but ive heard issues of high altitude flying without sealed mags. would that explain why our first couple thousand of feet of climb were good, but then it got rough?

 

[Tom-D]

You didn't tell us what happened when you pulled carb heat, it could be a couple things, loose carb venturi, carb heat box dampener free to add heat.

 

[Tom-D]

Tom will know this; is the enrichment valve a fixed orifice?

Its an MA3-SPA, with a fixed oriface. with a choice of 2 different venturis. one has several ADs. which discribes these symptoms.

 

[Tom-D]

i wondered about that too, but it was only at full throttle, other power settings it ran fine, and was developing normal RPMs for throttle position.

If your carb air control is broken, wide open throttle can suck the dampener to the hot air side as soon as you close the throttle a little bit it flops open and you get cold air again.

did you check it going out? coming back?

did the RPM do as it should?

 

[tonycondon]

carb heat worked normally during runup. following the engine roughness, i did not immediately put carb heat on, because carb ice didn't seem likely to me, given that it was smooth at lower power settings. during the approach to land, i turned carb heat on and nothing funny happened, normal rpm drop.

 

[Michael]

this may be a dumb question, but what about your mixture?

 

[tonycondon]

it was full rich. i thought about that too, that maybe at full throttle it was going over rich.

 

[gismo]

lance, im interested in the magneto thought. do the mags lose voltage with altitude? im no electrician, personally hate the stuff, but ive heard issues of high altitude flying without sealed mags. would that explain why our first couple thousand of feet of climb were good, but then it got rough?

Magneto voltage output is pretty much a function of RPM, but the voltage required to fire a plug goes up with cylinder pressure which rises with increased manifold pressure. So if the magneto output was marginal, it could stop firing a plug when you opened the throttle and raised the MP. The low spark voltage could also be the result of the current leaking elsewhere due to a bad wire or distributor cap.

 

[tonycondon]

thanks lance, other CFI at the club flew it today and reported no problems, he thought it was carb ice, but i kinda doubt that. i am scheduled in the plane friday and im thinking a climb above the airport is in order, once i get to similar altitude if same problem shows up i will be checking the mags/carb heat, adjusting mixture to try to get some info on it. if not, maybe it was carb ice, i dunno

 

[Let'sgoflying!]

it was full rich. i thought about that too, that maybe at full throttle it was going over rich.

That describes the enrichment valve's function. Its probably working normally, you just need to adjust more with the red knob.

 

[gismo]

That describes the enrichment valve's function. Its probably working normally, you just need to adjust more with the red knob.

At Cedar Rapids IA, there should be no need to "adjust with the red knob" at full throttle in the pattern. If the base mixture at full throttle is so rich the engine runs rough at full throttle/mixture below 3000 MSL something needs adjusting.

 

[tonycondon]

yea im kinda leaning towards that, it makes sense, problem didn't occur until at about 2300 MSL, field elevation is 750. who knows how long its been since that plane got much higher than that, plus it was only when the throttle was all the way forward, bring it back a tad and it went away. it just all adds up. thanks guys.

 

[Henning]

lance, im interested in the magneto thought. do the mags lose voltage with altitude? im no electrician, personally hate the stuff, but ive heard issues of high altitude flying without sealed mags. would that explain why our first couple thousand of feet of climb were good, but then it got rough?

I would be looking at the induction side first, the mag is potential, but it isn't where one starts. First thing that you do is uncowl, look at the airfilter, the scat tube to make sure the inner layer isn't flapping into the airflow, the mixing box for the carb heat and the flap for it. Check the fuel filters and make sure the carb and induction system are securely fastened. One we have established that we aren't going to starve the engine of fuel or air completely and the system isn't obstructed damaged or falling apart, we go up and try to reproduce the problem. If it doesn't reproduce, it was either something you found before, or absent finding anything, it was probably carb ice.

If it does reproduce, then I would reset the points and check the condensors as well as reset the timing. One thing to do here is to check the dwell angle against the points gap. If you set the correct gap and the dwell angle is coming out wrong, the points cam in the mag may be getting worn out. Go fly, see what happens, report back.

 

[Henning]

Its an MA3-SPA, with a fixed oriface. with a choice of 2 different venturis. one has several ADs. which discribes these symptoms.

Very true, but I haven't seen one in 15 years. Be scary if it was missed for all these years...

 

[tonycondon]

well i flew the plane today, and others have flown it since with no problems, maybe it was carb ice, i dunno...

 

[spiderweb]

Heyo All,
Had an interesting flight today, Climbing out from airport everything was dandy. about 2000 AGL, start to get some engine roughness, pull back the throttle a tad, engine is smooth. Go back to full throttle, engine rough. Turned back towards airport, called approach and stayed high until over top, spiraled down to beautiful crosswind landing. This was on a 150. Any ideas of the problemo?

I have no idea what the problem was, but I do know that you handled that situation with aplomb.

 

[Tom-D]

Magneto voltage output is pretty much a function of RPM,.

Mag voltage is a product of the primary magnetic field collapsing thru the secondary field. RPM has nothing to do with that, other than how many times it occures per second.

but the voltage required to fire a plug goes up with cylinder pressure which rises with increased manifold pressure. So if the magneto output was marginal, it could stop firing a plug when you opened the throttle and raised the MP. The low spark voltage could also be the result of the current leaking elsewhere due to a bad wire or distributor cap.

The only thing that could cause what you are hinting to, is a bad condenser or points, both should show in a mag drop.

 

[gismo]

Mag voltage is a product of the primary magnetic field collapsing thru the secondary field. RPM has nothing to do with that, other than how many times it occures per second.

Sorry Tom, but that's not entirely true. The energy that produces the charge in the windings comes from a coil moving relative to a magnetic field, and simple laws of physics say that the amount of energy is proportional to the velocity of the field relative to the coil. If what you said were true, a magneto without an impulse coupler would produce full spark when turned slowly and we both know it wouldn't. There is an upper limit on how much energy can be stored in the coil, but normally the coil is sized so that this (saturation) doesn't occur at any operational RPM.

Perhaps what you are thinking of is the fact that the voltage is limited by the spark plugs to what it takes to jump the gap and that is indeed independent of RPM (but not MP) and the result is that the spark voltage is indeed constant across the full range of RPM as long as the plugs are firing (and there's no gaps in the wiring). But if you open the gap of a plug (which increases the voltage required to spark) far enough, it will only spark at high RPM where the magneto can generate the extra voltage.

BTW, what you said is true of most electronic (capacitor discharge) ignition systems, but not magnetos. Also FWIW, a conventional automotive coil/battery/points (Kettering) ignition system exhibits the opposite effect as a magneto in that the available voltage decreases with increasing RPM because the coil charging time (dwell) gets shorter the faster the engine turns.

 

[Tom-D]

Sorry Tom, but that's not entirely true. The energy that produces the charge in the windings comes from a coil moving relative to a magnetic field, and simple laws of physics say that the amount of energy is proportional to the velocity of the field relative to the coil. If what you said were true, a magneto without an impulse coupler would produce full spark when turned slowly and we both know it wouldn't. There is an upper limit on how much energy can be stored in the coil, but normally the coil is sized so that this (saturation) doesn't occur at any operational RPM. .

I think your picking nits in theory on mags. above 350 RPM the mag is at operating range, and the primary and secondary will be at max saturation when the points open.

My Bendix DF7s off my Warner will spark with out rotating, All I need to do is obtain "E" gap and open and close the points by hand.

I have a retard on my mags to allow the engine to be sparked at 5-10 degrees BTDC, with no impulse coupling. and it starts at well below 350 RPM.

All an impulse coupling does in theory is to retard the spark, and give a spark that is equal to operating speeds, for easy starting. All thru the operating range of an aircraft engine the mag will deliver pretty much the same voltage, as it does at idle, or full power.

If the mag in question was SO bad that it gave the symptoms of skipping at full power, it would give those same symptoms during a mag check. These symptoms would have been caused by the points or condenser, not RPM.

Spark plugs on the other hand will brake down and short to ground when under full load, But then again it should have shown up during a mag check.

OBTW while we are picking nits, the term is "impulse coupling"

 

[tonycondon]

well the other instructor at club flew it again today and experienced the same problem. he tried mags/mixture/carb heat and w/the mixture leaned it cleared up after a few minutes. threw some "super lube" or something in the oil and will run 100LL through for a couple tanks. they think it might be a valve now. time will tell...

 

[gismo]

I think your picking nits in theory on mags. above 350 RPM the mag is at operating range, and the primary and secondary will be at max saturation when the points open.

My Bendix DF7s off my Warner will spark with out rotating, All I need to do is obtain "E" gap and open and close the points by hand.

I have a retard on my mags to allow the engine to be sparked at 5-10 degrees BTDC, with no impulse coupling. and it starts at well below 350 RPM.

All an impulse coupling does in theory is to retard the spark, and give a spark that is equal to operating speeds, for easy starting. All thru the operating range of an aircraft engine the mag will deliver pretty much the same voltage, as it does at idle, or full power.

If the mag in question was SO bad that it gave the symptoms of skipping at full power, it would give those same symptoms during a mag check. These symptoms would have been caused by the points or condenser, not RPM.

Spark plugs on the other hand will brake down and short to ground when under full load, But then again it should have shown up during a mag check.

OBTW while we are picking nits, the term is "impulse coupling"

OK, the "impulse coupling" not only retards the spark, it also spins the magneto faster than the engine is turning it and IME without that the voltage is insufficient to jump the plug gap. And I don't see how you can get a spark on any magneto without rotation unless there's a connection to the aircraft electrical supply ala Shower of Sparks or SlickStart.