rpm increase during climb out

[KG]

This morning as I was taking off I had an odd thing happen and I'm trying to figure it out.

C145 with fixed pitch prop.

First take off of the day. I took off normally and climbed out indicating 70 mph. It seemed to be climbing a little slower than usual but wasn't so bad as to get my attention. At about 800 ft. the rpm suddenly, and without any input from me, increased by about 75-100 rpm. (I'm guessing as it happened unexpectedly) It was then indicating about 2350 rpm and I think that prior to this it was indicating about 2275, more or less. It was enough of an rpm increase that I noticed the different sound and the climb rate improved.

I continued and made four take offs and landings, paying closer attention to the power setting. Climbing at 70 mph indicated, it was consistently at approximately 2350 rpm and seemed to be running fine.

I had warmed up prior to take off, run up, checked carb heat and even left it on a little longer than normal as it was quite humid. All seemed normal. I had no indications of carb ice during the run up or on any of the subsequent take offs and landings.

I recently read on another website where someone said something similar to this is an indication of a valve sticking (on a Lycoming)
link to what I read: http://www.supercub.org/phpbb2/viewtopi ... ing+valves

Anybody here agree with that assessment?

Any other ideas?

Thanks, Keith

[GAHorn]

A stuck valve would be accompanied by considerable vibration, so I doubt that was the situation.

A restriction to the intake (which let-go and cleared up) might explain it. Or a magneto which was oil-contaminated due to a leaking oil seal and cleared up due to centrifugal-force of the distributor rotor-gear/etc. would also do that. Just two possibilities out of several.

[KG]

A stuck valve would be accompanied by considerable vibration, so I doubt that was the situation.

Thanks George. That is what I thought too. It wasn't running rough.

I keep thinking carb ice that cleared itself but I had just moments earlier completed the run up with extra use of carb heat. Temp / dewpoint was 18/17.

I'll keep an eye on it and try to get more detailed data next time.

Keith

[Bruce Fenstermacher]

You would not take off with any of the stuck valves I've had. The engine shakes horribly.

I'm thinking carb heat was on and then closed. What you describe fits that perfectly. Check your carb heat cable and carb heat flap in the box and make sure it's all working as it should.

[KG]

I'll do that Bruce. That would explain it. Thanks.

[mod cessna]

All ideas so far are possible. I did have 300rpm increase when i pulled carb heat on the ground today. That was around noon! Good day for carb ice.

[jrenwick]

When you check the mags, do you always look for a return to the starting RPM after returning the switch to both? (I'm asking because it's a mistake I've made in the past, resulting in taking off on one mag.) If the switch were intermittent on one of the mags in the "both" position, or it wasn't quite returned to "both" but fell into it after becoming airborne, the symptom might look like what you saw.

[hilltop170]

Keith-
I agree with John, a dead mag could cause an RPM drop. The mag might be ok but the mag switch might be going out. I had the right mag quit working suddenly while flying thru Canada going to the Galveston Convention. The engine had a noticable drop in RPM but I didn't note how much at the time. We were just south of Grande Prairie, Alberta so we diverted there and found a really nice mechanic. He checked out everything and determined the mag switch was bad on the right mag. He said it would take about three days to get a new one. I asked if there were any other options. He said he could not legally do it but if I wanted to take the switch apart, it was probably just dirty contacts. The switch was original from the factory as far as I know and he was right again. The inside of the switch was full of carbon. With the switch cleaned out, reassembled, and reinstalled, the engine started right up and ran fine. That might be your problem, or it could be carb ice.

[KG]

When you check the mags, do you always look for a return to the starting RPM after returning the switch to both?

I do, but, of course, yesterday I must have been daydreaming during the run up because I can't remember what I saw. Like driving home and not remembering passing the stop sign.

I agree with John, a dead mag could cause an RPM drop.

This might be my answer. I usually get a 75 rpm drop checking one mag and 100 on the other so my 75-100 rpm increase makes sense.

Thanks for the suggestions. My buddy who helps me with maintenance is out of town so it will probably be a few days before I resolve this.

Keith

[Bruce Fenstermacher]

I concur with John and Richard. (bet they feel better now ) A temporarily inoperative mag would produce the same effect of rpm drop or gain as it came back on.

Besides the switch the P lead might be chaffed somewhere in route to the mag grounding it. Also it is not unheard of for a capacitor lead inside the mag to break killing the mag.

In cruise flight one time I had a capacitor lead fail. The resulting rpm drop seemed like carb ice and though unlikely I applied carb heat further reducing rpm. When it didn't clear I requested a direct path through a restricted area to landing that I'd previously been denied. After landing everything seemed fine but we didn't do a mag check till the next morning when we found the dead mag.

[KG]

I'm going to the airport this afternoon and I'll do a thorough mag check to see if I can duplicate this. The more thought I put into this, I think you guys are right. I was a little fixated on carb ice because of the humidity and the wispy patches of fog lying around, but I had no other indications of carb ice. I have had carb ice on the ground on a similar weather day recently.

I'll keep you posted.

Keith

[1SeventyZ]

I had a similar issue the other day that perhaps you guys can shed some light on since we're talking about it.

Departed on a 400+ mi XC, had a great climbout rpm from homebase: 53" prop, 88mph, 75F, 1120 MSL, 1900 lbs, and was seeing 2500 rpm. I was surprised it was running that well, as I normally see only about 2475 rpm at Vy.

Coming home, I noticed my takeoff from sea level was only giving me 2425 rpm at same gross and air temp. If anything I should make a little more rpm at sea level, right? It was running smooth at full rich, so I dismissed it. After my fuel stop and thorough runup with scrutinous mag check, carb heat test with about 80 rpm standard loss, I took off and got 2425 rpm again, this time with a very slight shudder. I could feel the barely perceptible shudder occurring rhythmically. Having scrutinized my engine during a dynamic balance 2 months ago, I know how smooth this engine should be. It wasn't missing, but it wasn't as smooth as normal.

I thought maybe I had lead-fouled a plug so after I reach my cruise altitude I ran it wide open for a bit and leaned to roughness, then backed off only a little, trying to "cook" the offending plug clean with a hot-running mixture. After a minute I enriched the mixture for cruise without seeing much improvement. It held 2450 rpm for cruise fine, so I continued home.

So, possibilities mentioned above for KG's case included bad mag, bad mag switch, and sticking valve. Having never stuck a valve before that I know of, and reading about the symptoms, it sounds like my barely perceptible shudder and 75 rpm loss were not severe enough to signal a sticking valve.

Does a fouled plug give a little shudder like that? Or does it just present as a little less rpm? I run 100LL only.

When i get a free minute I'm going to pull and inspect all the plugs (I only pulled one; #2 top; during post-flight and it looked fine.)
Thanks.

[Bruce Fenstermacher]

I've got the little shutter as well. Usually gets a bit better when I lean. I've just checked all my valves and they are clean and free. I chalk it up to an inefficient unbalanced carburetion system and an overly rich cylinder.

if you have a stuck valve, meaning it is stuck and not moving, the valve will be in the open position and the engine will shake and run ruff at idle and higher than an idle it will shake worse and you'll pull throttle because your thinking it is coming apart.

If a valve is getting sticky but not stuck the engine may run ruffer than usual at idle when cold and you won't feel a difference above idle. The warmer the engine gets and cylinders expand the ruffness will disappear and you won't feel it at idle either. This is sometimes called morning sickness because it only shows up at first start till warm up.

Because the engine is warm and operating at a much higher rpm I would not expect to ever stick a valve at any rpm over idle. But I have stuck a valve in the air after pulling the throttle back to idle from full throttle and descending (cooling the cylinders) into an airfield. All but this one stuck valve have shown themselves at start up after the engine had been running fine. I've operated an A65, a C-85 and a C-145 for a combined 1500 hours I'd guess and have had about 4 stuck valves that were stuck hard open. After I've had a stuck valve I've always checked all the other valves cause if one can stick it is likely others are ready to. I've had 3 or 4 others that I found were about to cause trouble and cleaned them.

[Brad Brady]

Just to through more confusion into the Frey....I just replaced a champ spark plug (RHM-40E )in a PA-12 with an intermittent spark (35 Hrs on the plug).....The owner said that run ups weren't great but smooth and exceptable....after I replaced the plug that was bad, he said that he had more power than before.....I saw several anomalies out of this box of plugs I have....Mostly that the machining seamed to be poor......barrels seamed to lob about a quarter of an inch when installing into the cylinder....also I had one pull the helical.....not enough to replace, but enough to get your attention.....I don't have a lot number from Champ. But the Spruce lot number is 30-14771 If any one else has had a problem I would like to Know.....I am thinking about writing Champ and seeing if they have had a problem....Brad

[hilltop170]

They are probably being made in China or Mexico.