The International Cessna 170 Association

Attention engine gurus....
I'm trying to determine what may be causing the O300A of my newly acquired '52 to run rough between 2000-2300 RPM! in flight....
Figuring it to be induction related, I replaced the carburetor, (O/H'ed MA3-SPA by MS), since it was obviously old and hadn't been updated with recent mods, installed new intake gaskets (and exhaust since the mufflers were overhauled) and new intake hoses.
The the roughness came with airplane when I acquired it earlier this year. At that time, it was in the 2000-2250 RPM range and obvious and annoying. Since completing prior mentioned maintenance, it shakes/runs rough, at 1500-1650 RPM on ground during static runs, smooth above to 2200 RPM max. Now this roughness I honestly can't confirm existed or not prior to carb replacement, maybe because I was always transiting that 1500 RPM and after engine work, I was now looking out at an uncowled engine!?
Anyway, now in flight, the engine runs even rougher in the 2000-2350 RPM range! Nice and smooth smooth above 2400 and below 2000 RPM!
All compression's are in the high 60's-mid 70's. Roughness cannot be eliminated with carb heat, leaning or eliminating a mag.
All engine mounts look to be in good shape. Have not actually checked tightness of mount bolts, since engine seems quite secure when I've attempting to move it. The propeller track checks out almost perfect at about 4-5" in from tip. I have not had it statically balanced, but going for a dynamic balance later this week. Re-indexed propeller as recommended in clubs 170 book. (11:00)
The ignition harness is new and the plugs have been recently cleaned and tested satisfactory. The mags have not been off and tested, mainly because one of the guys helping me diagnose this issue, is a very sharp engine guy and runs a mag shop. I'm sure he would've suggested mag inspection if he thought it may be the culprit...
So any helpful suggestions appreciated! Don't think this is normal for what I'm told is supposed to be a very smooth running mill.....

Gary S

A very quick check on the ignition system can be doe at night with cowls off. I know the ignition leads are new but it's easy check. Had a similar problem a few years ago on a another aircraft and found the coils in the magnetos were shot . They were good until hot then broke down at 2200 rpm . The mags were good at red line also until you put it into steep climb and then they would break down .

Gary, when you run out of ideas, you might look to see if you have any sticking or tight exhaust valves. I'd also consider week valve springs.

If the roughness is repeatable every time at the same rpm, it might be a balance problem. My engine ran rough after one annual where the prop had been removed and installed 180°out. When reversed back correctly, the vibration went away.

I assume you've looked at other posts on engine roughness such as this one and this one. There is some discussion in these about the intake baffle which might be worth looking at. Eliminate the easy/cheap possibilities before doing such things as examining the counterweights. If you don't have 6 point CHT/EGT an infrared thermometer might help in detecting a cold cylinder.

To add to what Richard said, make sure your prop is clocked on the crankshaft correctly. With cylinder #1 at TDC, the blades should be at 10:30/4:30 as seen standing in front of the airplane looking back. 45 or 90 degrees out will definitely cause roughness. I hadn't heard that reversing the prop 180 degrees would be a problem, but I guess it could. The blades are stamped 1 and 2, so once you find the sweet spot, write it down for future reference.

I certainly would check the prop location first. And all other ideas.

The sticking valve can be mysterious. Usually is diagnosed when one sticks open and your engine won't develop more that 1900 rpm and shakes worse than a Lycoming. Weak springs would allow a tight guide to stick quicker. If the guides were good but the springs weak the valves would float at upper RPM. Tight guides usually show themselves when the engine is cold, disappearing a the cylinders warm.

I've had a valve stick on low throttle decent showing up when I advanced the throttle in the pattern. Most other times the valve sticks after shut down and you don't know it has till your next start.

Hey thanks for all of y'alls responses....
Found propeller indexed at approximately 90* from TDC and reinstalled at more like the 11-11:30 position. Didn't look at intake baffle when carb was off. What am I looking for there?
And as far as sticking valves, could that appear as a consistent symptom at a consistent RPM range, and regardless of engine temperature?

Sounds like your prop might have been 45 degrees out of position.That definitely could have caused your problem.

The last post in this thread shows what the flow divider looks like. Later sumps had these cast in.

If you still have a problem, you might also check the intake hoses at each cylinder for tightness. An intake leak will cause roughness.

An intake leak should make the engine run roughest at idle where the throttle is closed creating the lowest manifold pressure/highest vacuum in the intake. One or more cylinders would drop offline due to the lean mixture. Then, as the throttle is increased the manifold pressure increases as the manifold tries to equalize with atmospheric pressure. At some point the manifold pressure gets close enough to atmospheric pressure that the leak becomes insignificant and the cylinders come back alive and the engine should run smooth again until the throttle is closed again.

This situation does not sound like an intake leak to me.

I saw that propeller reindex diagram in the 170 book and did so. Unfortunately no help. Though I'll look to see if my prop is marked 1/2 blade....

cessna170bdriver wrote:... I hadn't heard that reversing the prop 180 degrees would be a problem, but I guess it could. The blades are stamped 1 and 2, so once you find the sweet spot, write it down for future reference.

I too thought that it wouldn't make any difference if the blades were 180 degrees off. Interestingly though Cessna specifies the position for the number one blade in SNL 3-23-54. Any idea why being 180 degrees off would make any difference?

Bruce, that was so unkind ,a shaky Lycoming ?? .Never, they only more around to stop the dust settling.

Aryana wrote:

ghostflyer wrote:Bruce, that was so unkind ,a shaky Lycoming ?? .Never, they only more around to stop the dust settling.

I got a kick out of that one too!

It's a feature. It helps to shake the rust off the cam lobes.

Thanks for suggestions, and the humor...
Still haven't eliminated issue. Getting ready to run a dynamic balance test, but in the mean time does anyone know the engine mount bolt torque value for my application? O300A on '52 B model?

Gary