Engine roughness

[tcraftpilot]

Hello, my name is Ryan. My Dad and I recently purchased a 1953 Taylorcraft 15A Tourist this past summer. I am posting this question here because it has a Continental C-145 engine, and all Cessna parts firewall forward. Here is the situation.

Our engine is running rough especially at higher RPMs 2350+ range. Application of carb heat immediately smooths it out. pushing carb heat back in brings back the roughness no matter how long it is applied. RPMs are also reduced when running rough. Atleast 150 -200 RPMs. This problem comes and goes. When purchased airplane had flown about 10 or so hours in last 2+ years, very little in last year. After getting it home we put about 20 hours on it. It stuck a few valves. It began to develop roughness in the 2350+ RPM range. We figured it was more valves sticking/burning. We removed all cylinders and cleaned the heads and valve guides/valves of carbon. #2 cylinder assembly was totally replaced. one cylinder 72/80, all others 75 or 76/80. After cleaning out cylinders/valves we replaced all intake/exhaust gaskets, and intake elbow rubbers. After cylinder reassembly it would barely run up. Sparkplugs were replaced and problem seemed to be fixed. Flew it about three hours after all the work and it ran just fine. no roughness, and good RPMs. Yesterday, the engine roughness returned. 2350+ RPM range (that is at least where it is noticeable), application of carb heat smooths it out, but never clears the problem so don't think it is ice. Mag drop is even on both mags and well within limits. We are suspecting the Carb (Marvel MA3-SPA), but was looking for opinions before taking it apart. Both my dad and I have alot of experience with 4 cylinder Lycomings and small Continentals, but this is our first six cylinder. Any help is greatly appreciated!

Thanks
Ryan

[Joe Moilanen]

Carb heat makes the mixture richer so my guess would be that it is running too lean. Maybe cleaning out the carb would help or you may have an intake leak somewhere.

Joe

[Bruce Fenstermacher]

Ryan, the C-145 is a 4 cyl Continental with 2 more cylinders. There should be no mystery for you if you know the 4 cyl.

It would seem that application of carb heat improves the engine roughness. As stated carb heat richens the mixture so one could conclude the engine was/is running to lean. Why? Leaking intake. Wrong carb. Bad carb.

Other things that could cause roughness are sticking valves, which weak valve springs can attribute to. Your symptoms don't and my gut does not lead me to believe sticking valves from carbon buildup is the problem here.

You are sure you didn't have carb ice which can be a mystery sometimes (you can get it when you least expect it). After that make sure there is no leaking intake manifolds. And after that I'd be going after the carb.

[hilltop170]

Roughness caused by intake manifold leaks will most likely be more pronounced at low rpm when the throttle is closed. The lower pressure in the intake at low rpm draws in air thru the leak leaning the mixture excessively and causing misfire. At more open throttle settings/higher rpm, the intake manifold pressure is closer to atmospheric pressure and intake leaks have less effect.

To test for intake leaks, use scat tubing and duct tape to connect a leaf blower to the carb heat at the carb. Pull out the carb heat control on the instrument panel and turn on the leaf blower. Use a spray bottle with soap water and spray all of the possible places that can leak. The smallest leaks will produce soap bubbles that are easily seen.

[blueldr]

It might be a good idea to "Bomb" test the spark plugs: that is, test them on a pressure machine. If they're old spark plugs,they could be breaking down under high pressure.

[tcraftpilot]

Thanks for the suggestions guys. The spark plugs are brand new so ruling them out (the old plugs were a problem). We are going to pull the carb on Fiday and go through it this weekend. Will also check for induction leaks before pulling it off.

[canav8]

Thanks for the suggestions guys. The spark plugs are brand new so ruling them out (the old plugs were a problem). We are going to pull the carb on Fiday and go through it this weekend. Will also check for induction leaks before pulling it off.

Ryan, you cannot rule out the plug unless they have been tested. I have seen brand new bad before. Also if you drop them, there is a high chance for insulator fracture.

[n2582d]

What part number is your carb? I agree with Bruce; the O-300 is an O-200 with two more cylinders. Differences include the carb and the intake diffuser located downstream of the carb.

[blueldr]

I would bomb test those spark plugs anyway. There is no other way of knowing if they're breaking down under pressure and it someimes only takes one.

[hilltop170]

And also test the resistance of the plug between the center electrode and where the wire lead connects. Anything over 5000ohms is a failure and the plug should be replaced.

[johneeb]

I have forgotten what are the symptoms of a bad venture?

[c170b53]

You're sitting around with your buddies having a few beers and someone says "Hey watch this"

[johneeb]

Ok! Jim you got me, I meant venturi, the old one piece two piece discussion and the effect on how our engines run!

[cessna170bdriver]

As I recall, the problem with the early one-piece venturis was over-rich mixture. I would think most of those should be gone by now.

I was going through some old photos recently and a couple of them caught my eye as a possible problem. I believe the original poster said he had an O-300D which should have a later model sump with the cast-in flow divider, but in case it was replaced with an older one for whatever reason, you might check for a loose or missing riveted steel divider. You might have to pull the airbox, but you shouldn't have to pull the carb just to look, just open the throttle.

This is what a riveted divider should look like.

A friend of mine once found one of these pieces (I presume the one on the left) floating around in one of the intake manifolds on his ragwing 170. He found the other piece still riveted in the sump after I told him what the first piece was.

[blueldr]

I recommended bomb testing even new spark plugs under the above conditions simply because newness does not necessarily guarantee perfection. The plugs may well nave been handled by unknowing persons and dropped onto a hard sufface causing an invisable crack in the ceramic insulator. Who's to know?
I was taught, early on, that a dropped spark plug was never to be installed unless it was first bomb tested for proper serviceability.

A sick spark plug in a TCM O-300 is no big problem to deal with. Just think about when the ignition analyzer on a P&W R-4360 engine on a Boeing 377
tells you that out of 56 spark plugs, a rear plug in a center row, bottom cylinder, on a four row 28 cylinder engine needs changing. It's almost enough to make an A&P tear up his gertificate. It's a three man job just to get the cowling off.