Engine run-up issues

[jackh]

So the Cessna 170A that I purchased five months ago (and I have been working on since) had its first real run-up today. We ran into an issue that I'm hopeful someone has seen before.

So during the static run-up, the engine would only make 1900 RPM. At idle, the right exhaust seems to be at a lower pressure (I would even say "much lower pressure") than the left exhaust.
- The engine has not been started in over 5 years. But always in a dry hangar.
- Compression checks (twice now) are all above 70.
- McCauley 1A170 7653 prop. installed, C-145 engine.
- Borescope inspection of the cylinders and as much of the exhaust as we can get to [without removing it], look good.
- Aircraft has original exhaust system (not saying it's "the original system" - but that it is the original factory design - and seems to be "ok").
- Engine starts with no issues (very easy to start) and runs smooth at all RPMs
- All cylinder heads are hot and all cylinders seem to warm-up at the same-rates
- Mag check was good.
- Fuel system has been completely removed, cleaned and flushed many times. Aircraft has fresh, clean fuel.
- Engine has 1100 hours SMOH. But done over 20 years ago. (I ok with doing an overhaul, but wanted to have a "fun-summer with the aircraft"...and then decide this fall what to do with the engine).

The key issue seems to be that the right-side exhaust pressure is weak, compared to the left side exhaust pressure (at all engine speeds - but again...engine runs very smooth and starts with no issues) - to the point I could put my hand on the ride exhaust tips within mins after engine shutdown.

So what's the issue? Or maybe a better question: What should I/we do next?

Note: I'm not an A&P, but I am working with a local and well-trusted A&P I/A. ...and we are both 'scratching our heads' right now. I'm sure he's coming up with a good plan - but I wanted to work the issue with this group also (maybe you have seen this before and could give good direction on what we should do next).

[jackh]

Updated picture of the aircraft...we are SO ready to go flying. Just fix the engine issue and we're flying again!

[johneeb]

Jack,
Nice clean looking bird.

You say the engine, while not able to get over 1900 rpm, is smooth so you need to look for something that affects the whole engine and not just one cylinder or one bank (low pressure on the right bank should be rough running). You did not mention checking the magneto timing, if retarded the engine would not be able to make rated horsepower. Another thought is the diffuser (my word) located in the intake just above the carburetor, they have a history of breaking and blocking the fuel air flow. Have you tried running the engine without an air-cleaner (maybe it is plugged up). Is the carburetor heat valve closing properly?

[GAHorn]

Uhhh... "low" exhaust pressure on one side...??? Have you removed that muffler and washed it out for mud daubers and checked it for collapsed baffles/blockage?

[jackh]

John,
Thanks for the input. I just talked to the A&P. He had checked the magneto timing (I missed him doing that part). I talked to him about the "diffuser" issue and we're going to check this as a possible issue. The air-cleaner is new (I missed saying that) and he said he checked the carb heat valve (again...I missed him checking that).

We also plan to remove the right exhaust and intake manifold, checking for mud daubers or collapsed baffles (...THANKS for the pointer).

What other issues could it be? Why would an engine start so easily, run so smooth, but have low RPM and have low exhaust pressure on one-side (right side)? (more info: The aircraft has CHT and EGT on all cylinders...all had normal temps during the run-up (nothing to high....nothing to low).

I'm off to the airport again...to troubleshoot the issue.

[lowNslow]

Have you checked the accuracy of your tach?

[jackh]

Karl,
I just purchased a hand-held laser tachometer from Amazon (next-day delivery and tachometer was $18.98). A&P is offsite and can't have his "real" digital tachometer onsite until Monday. $19 total for overnight delivery of a laser tachometer. ...hey...why not.

[GAHorn]

There is one other think which comes to mind.... just above where the carb is mounted... inside the sump, as part of the intake/induction... on some older sumps was a riveted distribution-baffle which can come loose and block the intakes on one entire side. Later versions were cast/moulded into the sump castings.

Here's a picture of one (courtesy of Miles Bowen), ...carb removed...looking UP into the intake plenum:

100_1692lo_res.jpg (38.17 KiB) Viewed 17153 times

[Bruce Fenstermacher]

I believe what George pictured is what John is talking about and calling a diffuser.

[jackh]

All,

So we checked both the intake and exhaust systems and found no issues (borescope on both systems). We also installed an overhauled carburetor (there was some concern about the carb in other areas, so an overhauled carb was installed and used to address the carb concern and maybe also fix the low RPM (2000 RPM) static engine run up test).

Folks...I'm at a loss as to what-to-do-next. One issue I have is that I'm a Cirrus pilot - and that I purchased this aircraft to learn to fly a tail dragger - so I'm not really qualified to do a good run-up test. The aircraft is also at an airport that no one wants to come onsite and work on it (I can get phone support - but that's about it - A&P I'm using will come onsite - but only when he has a min - which I FULLY understand as I know this time of the year can be VERY busy).

What other items should I look at. What are we overlooking?

[Bruce Fenstermacher]

How are you doing the static run?

There should be noting blocking prop blast behind you and you should be facing into any wind. Hold the brakes and push full throttle. If you don't think your qualified enough to do that, tie the plane to something.

If it still only makes 2000 and you have confirmed that with a reliable digital tack then, since you seemed to have covered all other ideas, I'd look close at the prop hub for a clear stamp what pitch the prop is.

[c170b53]

And make sure you're hitting the full throttle stop, don' ask me why ,I'm thinking of that

[jackh]

Bruce,
Thanks for the input. I'm ok doing the run-up - I'm just not sure what "normal" is on a Cessna 170. ...and so your input and input from others is 100% welcomed and appreciated. Thursday we plan to do a run-up up/down the runway (maybe a flight if all looks good -- I found a "test pilot" for Thursdays run-up).

As you suggested: I will check the pitch of the prop this evening (the prop is a McCauley 1A170 7653, but as you suggested; maybe it was re-pitched).
I am doing run-ups in two locations; between two rows of hangars and in our airports two "normal" run-up areas (pointed into the wind).

[Bruce Fenstermacher]

Jack,

From the TCDS:

• McCauley 1A170
  Static r.p.m. at max. permissible throttle setting:
  Landplane: Not over 2330, not under 2230
  Seaplane (Models 170A and 170B): Not
  over 2525, not under 2300.
  No additional tolerance permitted
  Diameter: Not over 76 in., not under 74.5 in.

So what this means is this. NORMAL for a 170 is you go to the end of the runway (with nothing blocking the air blast behind) and point into the wind. With brakes applied and the aircraft static (not moving), full throttle is applied. With an accurate tach the RPM must be within 2230 to 2330. Depending on your DA you might lean the engine but I'm at basically sea level and the DA doesn't go hight enough on the ground to make a difference so I don't.

If you can't make between 2230 and 2330 it indicates one of two things. Your engine is not making rated horsepower or your prop is out of tolerance.

You've pretty much covered the engine as far as I can see from my computer. Your prop can be no longer than 76" and I doubt it is cause that was the length they came. If it was shorter than 74.5" the RPM would be higher than normal. That leaves pitch. A '53 pitch should split the difference at about 2280. To much pitch and RPM will be to low. Hard to see the difference of 50 RPM even with a digital tach.

If your engine truly is only making 2000 RPM following the procedure I just outlined, your engine isn't running as well as you think because your prop would have to be twisted to nearly a 60 pitch to drag the engine done that much.

Is your carb heat leaking into the carb reducing RPM?

[wingnut]

Sounds like you've covered the bases, I'd be curious to know if RPM increases if you pulled back on mixture. What type induction filter? Throttle and mixture making stop to stop? Correct carb? Correct mag timing? Don't overlook condition of harness and plugs, and I'd disconnect the carb heat for another test run. Did I miss field elevation in above post?