C150/O200 roughness on startup

[Chilito]

Flew a long xc from MN to MO and back two weeks ago, put 8.6 hours on and flew perfectly both ways. Haven't flown since until today. Started up, and immediately ran rough. Sounded "louder", with vibration that increased with RPMs until the dash was visibly shaking. Brought it back down to 1000 rpm and tach would slowly drop, engine would sputter, catch itself/recover, and repeat. Leaning didn't help. Primer was in and locked. Cycled mags, all normal. Gauges in the green. Shut down and let it sit, tried a few more times, same result. Went home.

It has not flown since it came back from the long xc two weeks ago, and it ran great. What happened in that down time?

Current thoughts are: stuck valve, carb float, fouled plugs (100LL) or vacuum leak. Your thoughts are appreciated.

 

[Der Fliegermeister]

I think you mean induction leak, not vacuum.

I've had this happen on smaller Lycoming engines and Continentals alike. Did you ever notice that there's usually only a single cylinder with a primer line running to it?

 

[RyanB]

I think you mean induction leak, not vacuum.

Same thing.

I can’t imagine it would be fouled plugs, especially if the mag checks were okay. I had a very similar issue happen when the Archer had an air leak, which caused a cylinder to run excessively lean. Start simple, pull the cowl, check the plugs and any intake (or exhaust) gaskets that might be damaged.

 

[Der Fliegermeister]

Same thing.

Wrong.

That aside, mag checks might not always reveal fouled plugs. But @RyanB is right: it's simple to check them. If the induction system has a decent leak another telltale sign would be the the engine trying to stay running for a bit when you pull the mixture to idle cutoff. Definitely inspect those gaskets and consider replacing them with better ones. If you do, be extra careful to remove all remnants of the prior gaskets before installing any new ones. If you put RTV on there I'll personally come to your hangar and slap you.

When you rotate plugs at annual (you are doing that, right?) They should go one forward in the firing order and top to bottom. Check that magneto timing.

Also, are they Slicks or Bendix?

 

[RyanB]

Wrong.

I am most certainly not wrong. A vacuum leak has nothing to do with the vacuum pump. It’s when air can enter the system by abnormal means and not through the induction system like it’s designed to do, which is synonymous to an intake or induction leak. Loose hoses, clamps and bad gaskets are the common causes.

 

[Der Fliegermeister]

I am most certainly not wrong. A vacuum leak has nothing to do with the vacuum pump. It’s when air can enter the system by abnormal means and not through the induction system like it’s designed to do, which is synonymous to an intake or induction leak. Loose hoses, clamps and bad gaskets are the common causes.

Next time educate yourself before calling out someone's error.

Read it again.

You said that a vacuum leak and an induction leak are the, "same thing," so I thought it fit to show you a picture of a vacuum pump to remind you that those are two different systems and that the vacuum system has nothing to do with the engine running rough (unless somehow it catastrophically takes down the accessory case).

Keep automotive terminology in the garage. Even tried to give you a small nod in my post for your troubleshooting recommendations. But you came unglued when I pointed out that your terminology is incorrect (which it is) and you went into attack mode suggesting I'm not educated.

People come here for help. Sometimes that means members will correct each other for the benefit of the OP so that they're not misled or learn something that isn't accurate based on the first things they read. It's called the law of primacy, but that's an educational term...I don't know anything about being educated

 

[Magman]

It sure sounds like a sticky valve to me . ( morning sickness)

Observe safety precautions and pull engine through 2 revolutions.

You should feel similar compression every 180 degrees .

If not ; call the Tech.

If all are similar pull cowling off.

Run about 30 seconds.

See if one exhaust stack is cooler than the others.

Laser Thermometer or wet finger or squirt water.

if one jug not working it should be obvious.

You can pressurize the Induction System by disconnecting Carb Heat Duct at

the Muffler.

Connect to CLEAN vacuum cleaner pressure side or use a Leaf Blower.

Open Throttle and spray soap and water on all hoses and gaskets.

You don’t need a lot of pressure and bubbles will tell tale.

 

[RyanB]

Read it again.

I read it. You’re still incorrect.

You said that a vacuum leak and an induction leak are the, "same thing,"

Correct, they’re synonymous terms.

so I thought it fit to show you a picture of a vacuum pump to remind you that those are two different systems and that the vacuum system has nothing to do with the engine running rough (unless somehow it catastrophically takes down the accessory case).

If you read my post from the beginning, I stated that a vacuum leak does not involve the vacuum pump that moves your gyros. An engine is essentially a big vacuum pump, as the motion of the pistons create a vacuum that draws in air to mix with fuel, which creates power. When air finds its way into the system by abnormal means, it's called a vacuum leak. It can also be called an intake leak or an induction leak and each of these terms are more or less synonymous. If you have a vacuum leak, the engine will absolutely run rough.

People come here for help. Sometimes that means members will correct each other for the benefit of the OP so that they're not misled or learn something that isn't accurate based on the first things they read.

Just make sure that what you’re correcting someone on is actually something that needs to be corrected and not just for the sake of being pedantic. If you go to your A&P and say “I suspect I have a vacuum leak somewhere given XYZ symptoms” he’s not going to look at your vacuum pump - just sayin’.

 

[Dan Thomas]

It sure sounds like a sticky valve to me . ( morning sickness)

Observe safety precautions and pull engine through 2 revolutions.

You should feel similar compression every 180 degrees .

If not ; call the Tech.

If all are similar pull cowling off.

Run about 30 seconds.

See if one exhaust stack is cooler than the others.

Laser Thermometer or wet finger or squirt water.

if one jug not working it should be obvious.

You can pressurize the Induction System by disconnecting Carb Heat Duct at

the Muffler.

Connect to CLEAN vacuum cleaner pressure side or use a Leaf Blower.

Open Throttle and spray soap and water on all hoses and gaskets.

You don’t need a lot of pressure and bubbles will tell tale.

That there. If the engine ran perfectly right up until shutdown on the last flight, it's most likely a stuck valve.

 

[Dan Thomas]

I've had this happen on smaller Lycoming engines and Continentals alike. Did you ever notice that there's usually only a single cylinder with a primer line running to it?

No, There isn't. The small Continentals have a single primer nozzle in the intake spider, and that spider feeds all four cylinders. Larger Continentals have a primer in each of the two intake runners, left and right. Lycomings typically have primer nozzles in the intake ports of three of four cylinders, or four of six. I have never seen, as an aircraft mechanic, any engine with only one cylinder primed. Where did you see one?

 

[Chilito]

Update: #4 cylinder stuck exhaust valve. Thanks to all for the responses.

 

[Magman]

You may want to assure correct orientation of Rocker Arms.

While similar in appearance; the exhaust unit has an oil passage at the tip to

cool and lubricate the valve.

I’ve often found Intake and exhaust switched.

Usually when pulling the jug due to low compression from a worn out Valve Guide.