Low rpm

[cverdoljak]

Just bought a 170 with 145 and will only run about 1900 -2000 rpm, any suggestions

[c170b53]

There's a bare canvas....
What do think you have or better yet what have you checked so far?

[Bruce Fenstermacher]

How about a bit more description what goes on and when.

We're pretty good but when you stump Jim right out to the box like that the rest of us don't have much of a chance to make a call.

[cverdoljak]

It was a stuck valve on #4, just bought it and had an annual done, picked it up from the mechanic went to fly it home and 1900 rpm was all it would do.

[c170b53]

Good try Bruce (Oh-Oh its never good when the Pres is buttering up the staff prior to a board meeting ) and I wish I was as smart as you suggest but the regulars know whom amongst us really knows their stuff.
Cverdoljak; not trying to annoy but often its hard to give advice (and not offend at the same time) when there's a lack of info on your diagnosis of the problem and your first attempts to resolve the issue. Give us some info and we will begin building the darts.

[cverdoljak]

I know, we were in a hanger with 3 mechanics standing around scratching there heads and I didn't have good enough cell service, I just paid a premium for an inspection 700 miles from home and I show up and the plane didn't run properly. I guess we'll try again tomorrow. Should I have all the valves cleaned on that side? 1,3 and 5 were done about 70 hrs ago by looking at the logs, 2,4 and 6 have not been done. #4 was stuck. Engine has 326smoh

[blueldr]

What in hell kind of an inspection did you get that had your airplane come out in that kind of condition? Are you at all sure that they were AIRPLANE mechanics? The conditions you describe could be caused by any number of things, but I would have expected them to be detected in any sort of competently performed inspection other than an alleged "Pre-Buy Inspection".

[GAHorn]

I presume you're speaking of static RPM. (Although one can never "assume" or "presume" too much, especially if 3 mechanics fail to notice the engine vibration of a stuck valve.)
Have your "mechanics" confirm the accuracy of your tach using a digital tach-checker.
Confirm the pitch and length of your prop, (compared to type certificate.)
Determine the density alititude. (Less than 3K above S.L. shouldn't matter much....but if you're in the high desert it might be off by 100 or so... Be certain to lean during the static check.)
At idle... is there any occasional "popping" going on in the exhaust? (weak valve springs, ...not uncommonly overlooked when "field overhauls" of cylinders are performed.)

[Bruce Fenstermacher]

You know I almost said stuck valve on a whim because that will cause the problem you described. Had you included "it shock like the devil" that would have cinched the diagnoses for me.

In my opinion if one valve is sticky, the rest are likely to also be. Or said a different way, why would one valve and valve guide build up with carbon and not the others.

I would likely do only what is necessary to get the plane to friendly territory (home) and then consider a preemptive checking and cleaning of the other 5 exhaust valves.

As far as it not running when you arrived, that is just the luck of the draw or otherwise in this case. The engine probably ran fine right up to the last time it was shut down. Yes I've experienced this and it only took 3 days to diagnose because I suspected a mag problem.

[KS170A]

If they did an "annual," did they check and/or adjust mag timing? If adjusted, did they adjust the correct direction and degrees? One of my first lessons out of A&P school and working in a GA shop was just because someone was certificated, doesn't mean they knew squat about correctly working on/fixing airplanes. Conversely, just because someone is not certificated, doesn't mean they don't know a thing or two.

Did you fly it at all, or abort the flight when you found the RPM low? If you flew, what sort of performance did you get (a la, is the tach correct, which can also be verified using George's aforementioned tip of using a digital laser tach).

[cverdoljak]

The plane wouldn't fly, it was my first time in a 170, and my first plane, we checked the tach, it was good, we were at 5000' in Colorado, it ran great after the valve was un stuck. The prop is a 7456. We flew the plane to Wisconsin today, everything worked great. I can't believe nobody even read the engine log to see that one side already needed to be cleaned because of stuck valves, if I didn't mention stuck valves who knows what they would have done. The IA there all but insisted it wasn't a valve problem. I learned a lot about what not to do when getting an annual done, I was just in a bad place with the plane so far away and no time. I'm just glad I got the plane home and never have to deal with that shop again.

[Bruce Fenstermacher]

The prop is a 7456

What make prop. If McCauley 1A170 or 1A172 if should be between 74.5" and 76". If a Sensenich M74DR if can be between 72" and 74".

I just saw that that half the valves were cleaned and now I assume #4. You only have #2 and #6 left to do.