Low Static RPM on 1978 C-172

Low Static RPM on 1978 C-172

I recently purchased the plane with about 65 hours on an overhauled H2AD engine.  The engine seems to run strong but will only turn about 2100 RPM on a static runup (2500 max in cruise).  I have had two mechanics look at the plane, and have replaced the tachometer, carburetor, air filter element, and rerigged the engine controls.  The magneto timing, compression, spark plugs, oil pressure and temperature, etc., all check out ok.  One mechanic is suggesting that the engine may not have been broken in completely, and is recommending that I replace the oil with straight mineral oil and run it at full throttle for another 20-25 hours.  The other is suggesting that the prop may need to be overhauled.  Any advice?

Jonathon Plaxton
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Re: Low Static RPM on 1978 C-172

Regarding

"...One mechanic is suggesting that the engine may not have been broken in completely, and is recommending that I replace the oil with straight mineral oil and run it at full throttle for another 20-25 hours. >

I think that this suggestion sounds wrong to me.
Did the previous owner tell you what he did to break it in? Do you have any indication that it isn't broken in? e.g blow by. You said it has good compression, was it check when the engine was hot?

Does the engine run hot?  Any specific signs that the rings haven't set?

Do you have the correct propeller as stated in the TCDA below?

Excerpt from the TYPE CERTIFICATE DATA SHEET NO. 3A12

VIII. Model 172N, Skyhawk, 4 PCL-SM (Normal Category), 2 PCLM (Utility Category) (cont'd)

(S/N 17261578, 17269310 through 17274009)
*Engine limits For all operations, 2700 rpm (160 hp)
Propeller and propeller 1. Propeller limits
(a) McCauley 1C160/DTM 7557
Static rpm at maximum permissible throttle setting:
Not over 2400, not under 2280
No additional tolerance permitted
Diameter: not over 75 in., not under 74 in.
(b) Spinner: Dwg. 0550320

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Re: Low Static RPM on 1978 C-172

How did you check your RPM?  If the answer is with the tachometer then that could be your problem.  I think you are allowed 4% error in the tach.  I though I had a problem as well until I determined my tach read low.  I checked it with a hand held tach.

Ron

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Re: Low Static RPM on 1978 C-172

I would look at the RPM using a digital hand held tach and see what the reading is you are getting compared to your mechanical tach. Verify the correct propeller is installed and that it hadn't been repitched. You may also want to check over the parts list of the items installed during overhaul as to the type of cylinders and the correct matching parts were used. Had you checked for any intake leaks etc. Was the cam replaced or reground. Was this overhaul done by a shop that provided warranty service or who did the overhaul. Had the Magneto been properly overhauled by a shop that is familar with the single drive dual magneto. See if you can find out how it was run for breakin.

Bill

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Re: Low Static RPM on 1978 C-172

Hi Barry;

Thanks for your reply. Unfornunately I can't get much on the history of the engine following the overhaul.  The owner died, and I bought it from his estate.  The logbook only shows a 50 hour annual inspection where Aeroshell W100 was used at the oil and filter change.  The shop that did the overhaul did not install the engine in the plane, and the log entry for the installation does not show what oil was used.  The plane has the original prop as you noted, which was overhauled about 2200 hours ago.  The oil temperature is in the green, but seems a little high during cruise.  Oil pressure is good at start and runup, but is at the lower end of the green arc when the engine is warm, and barely in the green at idle RPM. It still has the Aeroshell W100 from the oil change from the last annual inspection.  TTE is now about 70 hours.  Engine consumes about 1 quart for every 6-7 hours.

Jon

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Re: Low Static RPM on 1978 C-172

To All;

I will have my mechanic check the RPM with a digital tach.  The new unit I installed shows it was calibrated with less than 50 RPM error at that RPM range, but based on your feedback I am guessing I should not trust it.

Thanks

Jon

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Re: Low Static RPM on 1978 C-172

Sounds like the prop is ready for an overhaul based on the 2200 hours; what is the recommended TBO for the prop?  Maybe you get the pitch measured and compare it to its specefications.  If is out of specs then that is likely to be your problem.  (If pitch is greater than the specifications then it would tend to explain it.)

50 rpm doesn't explain the difference from minimum permissible rpm that you are seeing; but confirming the rpm with a digital hand held or strobe is a good idea.

What makes you think the oil temperature is high?  Has the guage been calibrated and graduated to show actual oil temperature?

I guess you don't have CHTs available to see if the cylinder temperatures are high?

If it were me, I'd get the prop checked out.  That doesn't mean a high oil temperautre is acceptable, that could be a separate concern.

my 2 cents

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Re: Low Static RPM on 1978 C-172

Hello Barry;

Thanks for your reply.  I stopped by the airport on my way to the office this morning, and took a good look at my prop looking down along the leading edge from the propeller tip to the hub, and compared it to a few other 172s on the field.  The leading edge on my prop seems very blunt compared to the oter props I examined, so it well could be the prop.  It looks like there were a few good sized dings in the leading edge that were filed out, and that may have affected the airfoil to the point where I'm loosing a few hundred RPM.  I've contacted a shop to arrange for an inspection and overhaul.

On another topic, the plane does have an EGT and CHT, but other than using the EGT to determine the peak RPM when leaning, I am ignorant when it comes to the actual information it is giving me (my last plane only had an EGT).  What should I look for with regards to CHT and EGT when at various power settings?  With oil temp and pressure in the green, I assume I am OK on the heat range, but the CHT and EGT probably gives more specific information.

With regards to the oil temp being a little high, I was comparing the gauge reading to my friend's 172.  It is a 1979 172 with the same H2AD but with about 1700 hours on the engine.  His gauge reads about half-way between the low and high end of the arch, where mine reads about two-thirds of the way towards the high end.  The other difference is he has a Powerflow exhaust, and I have the standard Cessna exhaust (with a notable performance difference).

Thanks

Jon

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Re: Low Static RPM on 1978 C-172

There is a lot of information on this website by people that have a lot more experience then me in this regard.  I suggest you do a search on this website for "CHT" and stand back.  Walter Atkinson is the local expert on this subject. 

There is a very good articles at AVWeb (I think www.AVWeb.com) look under Pelican's Brief.  See article #18 "Mixture Magic" and you will find a well written and informative article.

CHTs are cylinder head temperature.  It has been suggested that they be kept under 380 F.  At 420 F, Mr. Atkinson has warned that he has experience "thermal runaway." It has been explained that high temperatures distort the shape of the cylinders, which can create higher friction.  The additional heat produced tends to cause the temperatures to climb rapidly.  So keep it under 400 F using your throttle and mixture control.  See the article Mixture Magic.

Regarding the Prop:  See if they can confirm that the prop is out of spec and by how much.  Ther is a Pelcian Brief article #16, which is all about propellers and it's affects on engine operation. 

Powerflow exhaust is another hot button topic.  Do a search on it if you want to be entertained and educated all at the same time. 

OK now you have some reading assignments and seaches to do...  Seriously though, the articles are very informative and would be very meaningful to you at this time.

good luck and keep us posted.

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Re: Low Static RPM on 1978 C-172

You may want to check out the operation of that EGT/CHT. That will be an important item for good engine performance and maintenance. This is very useful especially if all cylinders can be monitored. Some like the JPI, GEM, and Insight can track data that can be downloaded to a palm pilot or laptop or PC. If this is the case that data may be stored in unit. It could be usedful at this time.

Bill

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Re: Low Static RPM on 1978 C-172

Thanks, Barry;

I went to the web site and read John Deakin's articles, which were very informative.  I'll post my results after the prop overhaul.

Jon

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Re: Low Static RPM on 1978 C-172

You never mentioned the cylinders. Are they chrome or steel? Chrome cylinders have orange paint on the fins near the sparkplug. They are very hard to break in. I am having a similiar problem with a 0-235 in a Cessna 140. The cylinders will not break in, high tempertures 2100 on the static runup, no power. If all other things check out, i  would do another breakin. rc

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Re: Low Static RPM on 1978 C-172

Hi Rich;

Yes, I have chrome cylinders.  I am going to try the prop overhaul, and then if I am still having the problem, I will change to straight mineral oil and run it for another 20-25 hours and see what happens.  At this point I am assuming that a freshly overhauled engine should still turn static RPM even during the break in period.  I have been told that the oil temps will run a little higher until the break in has been accomplished, which is longer with chrome cylinders.  I also understand that I should expect a little more oil consumption with chrome cylinders.  Thanks for your reply.

Jon

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Re: Low Static RPM on 1978 C-172

Jonathon,  do one more search on this Website regarding breaking in an Engine.  Walter Atkinson has some very important things to say about it.  For one thing, the break-in does not occur because of mineral oil.  Mineral oil just means that the base oil is petroleum.  The oil that is currently in your engine is fine.

Review what Walter recommends.  It is was done right it doesn't take that long for the rings to set "Break In".  Take it a step at time.  Have them measure the prop and report to you.  See if the results explain the the reason for the shortfall in the RPM.

Good luck

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Re: Low Static RPM on 1978 C-172

Just got the overhauled prop back on the plane, and it made absolutely no difference in the static RPM.  Still indicating 2100, both before and after flying it for an hour.  Other than that, the plane performed very well in climb and cruise and the oil temp and pressure was normal.  Much better than before I overhauled the prop, so I think I will just fly the airplane for a while and see if it comes up any after 10-20 hours of flying.

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Re: Low Static RPM on 1978 C-172

Just an update on this problem - I went out and bought a digital tach checker, and the mechanical tach checks out.  At full power static runup, I get an indication about half way between 2100 and 2200 on the mechanical tach, and 2160 on the digital tach.  It varies a little each time I check it, probably due to atmospheric conditions at the time, but I never get the 2280 to 2400 recommended in the service manual.

One thing I didn't mention earlier, when the prop was overhauled at Santa Monica Propeller, the shop owner told me that it was pitched at the "high end" of 57", and that he could re-pitch the propeller to give me a better static RPM.  I didn't consider it at the time.  It seemed to me that since it is the original McCauley 7557, overhauled twice since new (as far as I can tell from the logbook), it should make static RPM with this prop set the way it is.  Is there a range within a single pitch setting, and would it have that much effect on static RPM?  How much range is there before it would be a 58" pitch or whatever?

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Re: Low Static RPM on 1978 C-172

Jonothan,
One possibility that I haven't seen mentioned here is carb heat. Make sure your carb heat mechanism is working and completely closing. Also an exhaust blockage. It would be unlikely on your system, but possible. What RPM are your getting on rollout, climb and cruise?
Del

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Re: Low Static RPM on 1978 C-172

Hi Del;

Thanks for your reply.  My son and I flew to Camarillo from Chino this afternoon.  Takeoff roll was 2100-2200 RPM, with climbout at 70 KTS and 700 FPM climb.  Leaned mixture and got 2300 RPM in climb.  Cruise at 6500 FT was 2450-2500 RPM.  All RPM numbers are at full throttle and just rich of peak mixture (lean of peak produces a slightly rough running engine, and the power drops of quickly).  Digital tach readings agree with the mechanical tach reading.  My mechanic installed a new carb, and rerigged the carb heat and throttle linkages.  I do notice some times that the carb heat control creeps out about 1/8", but pressing it back in doesn't seem to make any difference.  Also, in the normal runup, the carb heat check does not produce a drop in RPM like my C-150 did.  However, my mechanic didn't seem to be concerned with that.  The carb heat control does open and close the plate in the heat box, and it seems to open and close to the full position.  He also inspected the exhaust system, which was repaired before the engine was overhauled.  It does seem like the throttle is not opening fully, but inspecting it shows that the throttle is opening to the stop on the carb.  I appreciate any insights you might have.  I am thinking of getting a third opinion from another mechanic, as I think this is a problem that could be fixed with the proper disgnosis and repair.

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Re: Low Static RPM on 1978 C-172

Jonathon

Strange that there is no RPM drop with carb heat applied. This may indicate that heated air is already being put to engine.

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Re: Low Static RPM on 1978 C-172

Jonathon
I agree with Bill. You should have a noticeable RPM drop with the application of carb heat. You say your mechanic rigged the carb heat, but is it possible that the seal material has deteriorated?
You definately need another mechanic to look at this. If your mechanic wasn't concerned about carb heat being inop per the run-up procedure, I'd find another.
Del

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Re: Low Static RPM on 1978 C-172

I was getting about a 100-150 RPM drop on my 150.  Should I expect the same from the 172?  The carb heat on the 150 came from one of the mufffler shrouds, whereas the 172 is pulling it from around an open exhaust header.

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Re: Low Static RPM on 1978 C-172

To all that joined in on this discussion, I thought I would post a follow-up.  It seems that when the engine was overhauled, the camshaft to crankshaft alignment was off by one tooth on the gears (apparently a common problem on H2AD engines when they are assembled).  This will mean a very expensive repair, and lengthy downtime since I don't have the money to fix it.  If the owner at the time had addressed it, the shop would have covered it under their warranty.  As it was, I bought the plane after the 12 month warranty was up.

There is a simple check for this, which you can find on the Textron Lycoming web site, and Sacramento Sky Ranch web site.  Anyone experiencing this, especially on the H engine (since it requires spliting the case in order to fix it), should do this quick check before accepting an overhauled engine that does not turn static RPM.

Has anyone else had this problem?  I did not learn about this from my mechanic, but from another owner that had exactly the same problem.  Lucky for him his engine was still under warranty.

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Re: Low Static RPM on 1978 C-172

Jonathan:

Thanks much for the update. I'm going to keep this tidbit in my back pocket, since this is one of those "needle is a haystack" discoveries.

Hope you can get this resolved soon.

 

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Re: Low Static RPM on 1978 C-172

As a further update, the shop that overhauled the engine agreed to repair the problem at their cost, for which I am grateful.  However, they said they can fix the timing problem on the H2AD engine without splitting the case, saying "It's not easy, but it can be done."  Both my mechanic and the Lycoming tech support person I spoke with said that you can only set the timing as the case halves are put together, so you must split the case to reset it.  Is anyone familiar with a different procedure for doing this?  Are there other concerns I should have with them fixing it this way?  I would be interested in hearing from anyone that has had this problem, or has fixed this problem.

Thanks

Jon

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Re: Low Static RPM on 1978 C-172

This is (hopefully) the final chapter regarding this problem to anyone interested.  I picked up the plane from One Stop this last Friday, and correcting the crankshaft to camshaft timing gave me 200 more RPM on static runup, and I get within 50 RPM of redline in cruise at full power.  I am grateful to One Stop for taking care of the problem for me and giving the story a happy ending.

It does make me wonder if there are a lot of 172s with the H2AD engine out there flying with the same problem.  I flew it quite a bit before getting the problem fixed.  The engine seemed smooth and reliable, I just couldn't get full power out of it.  Thanks to all who joined the discussion.  I learned a lot through the exchange of opinions and ideas.

Jon

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