0-200 RPM Loss on Take Off

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Nick P

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I recently took my 150M to my mechanic as I have noticed the engine runs rough when pulling out the throttle from 2400-2100 rpm. My mechanic said he found a little dirt in the carb and cleaned it up.

I picked up the plane from the shop and on departure had a loss of RPM on take off. At around 600' AGL I noticed the engine stumbled and the rpms were reading 100-150 RPM low. This stumble happened again before I pulled the throttle back and pushed it forward. This time I noticed the engine stumbled as soon as the throttle was advanced. Told the controller I needed to come back and they declared an emergency for me. I landed with several fire trucks waiting.

I should have tried to trouble shoot and definitely should have pulled the carb heat right away. I was focused on getting back to the airport as I didn't have a lot of altitude to spare.
My mechanic did a hot compression check and found cylinder #1 reading 52. He retested the cylinder and got 74. I was told it seems the exhaust valve on cylinder #1 is sticking and that's why there is a big difference between the two compression tests. He is going to ream the exhaust valve on cylinder #1. He noted the RPM rise on mixture idle cut off is varying from 50rpm to 80 rpm to 200 rpm. Mechanic is going to replace the carb and intake pipe couplings (not sure what they are called, but the rubber clamps). The carb is 10 or 11 years old.

Curious if anyone else had dealt with this on the 0-200. Maybe it was just carb ice and the sticking valve was noticed after the fact? It was around 50-55 degrees (10/M1 on TAF) and 60% humidity. It is frustrating as I bought the plane 2 months ago and put 8 hours on it since. It came with an annual and the lowest compression was 74. Now have compressions of 52 (retested to 74), 68, 68, and 74
 
The O-200 has an odd carburetor spider mounting arrangement. If it's not assembled correctly, rough running and big RPM loss is possible. We've dealt with this before. Here's what I have posted many times:

The Continental O-200, as you have in your 150, has a carb "spider" intake manifold. The carb bolts to it and it, in turn, is bolted to the crankcase. The O-200 is different from its older A-and C-series granddaddy engines in that the spider is mounted loosely on the case studs, with special "Lock-O-Seal" washers on the studs between the case and spider and between the spider and castellated retaining nuts. These washers are metal with a rubber insert molded into them, like an O-ring. The overhaul manual says that those castellated nuts are to be turned only finger tight and the cotter pin installed. The reason is that the MSA carbs don't like being shaken, or they'll spill fuel out of the bowl vent and into the carb throat and the engine will run rich and rough. The old Strombergs weren't nearly so prissy, and those old A- and C-series engines had much stiffer rubber engine mounts so the engine couldn't move around so much. With the carb spider a little flexible on the studs, and the rubber hose connections on the intake tubes, the engine can torque-vibrate around the crank while the carb will stay relatively still. I have smoothed out O-200s just by installing the proper washers and doing up the nuts exactly as the manual says.

So your mechanic needs to refer to the O-200 overhaul manual and look for that paragraph. This is critical for that engine. The whole carb and spider need to be able to flex sideways a bit. Go here http://veteranflyg.se/wordpress/wp-...75-C85-C90-O-200-Overhaul-Manual-Aug-2011.pdf and see page 103 and read paragraph 16-10. The Lock-O-Seal washers are used to permit movement, not to seal anything.

The other big possibility is weak ignition. Pulling the throttle back will often lean the mixture, and a weak spark will misfire when lean. How long since those mags were off for internal inspection? Are the sparkplugs the old Champions with the stupid spring-loaded resistor in them? Look down the plug's connector well and see if the contact at the bottom has a screwdriver slot in it. If so, those are horrible plugs. They have caused many people many problems.

Ignition is 90% of engine performance problems, and yet most people will chase the fuel system until they're pulling their hair out in frustration. They think that a spark is a spark. It's not. A weak spark is at the root of a lot of troubles. It's one reason we have two mags and one carburetor: the ignition is far less reliable than the fuel supply.
 
Another possibility is air leakage at the Carb Split Line caused by a loose float bowl. Easy to check. Lycoming SB 366c ( I know you have TCM) applies to your Carb.
 
Magman said:
Another possibility is air leakage at the Carb Split Line caused by a loose float bowl. Easy to check. Lycoming SB 366c ( I know you have TCM) applies to your Carb.
Click to expand...

Not sure about the carb split line but, my mechanic mentioned the float was an older brass type and that they have a tendency to fill up with fuel.

He reamed the valve and replaced the carb. Engine is smooth on run up. Ill keep this in mind
 
I am NOT familiar with the O-200.
Sounds like the mechanic likes the shotgun approach to troubleshooting.
Ka-ching!
Listen to the advice you get here, and perhaps find a mechanic more amenable to troubleshooting BEFORE replacement.
 
Dan Thomas said:
The O-200 has an odd carburetor spider mounting arrangement. If it's not assembled correctly, rough running and big RPM loss is possible. We've dealt with this before. Here's what I have posted many times:

The Continental O-200, as you have in your 150, has a carb "spider" intake manifold. The carb bolts to it and it, in turn, is bolted to the crankcase. The O-200 is different from its older A-and C-series granddaddy engines in that the spider is mounted loosely on the case studs, with special "Lock-O-Seal" washers on the studs between the case and spider and between the spider and castellated retaining nuts. These washers are metal with a rubber insert molded into them, like an O-ring. The overhaul manual says that those castellated nuts are to be turned only finger tight and the cotter pin installed. The reason is that the MSA carbs don't like being shaken, or they'll spill fuel out of the bowl vent and into the carb throat and the engine will run rich and rough. The old Strombergs weren't nearly so prissy, and those old A- and C-series engines had much stiffer rubber engine mounts so the engine couldn't move around so much. With the carb spider a little flexible on the studs, and the rubber hose connections on the intake tubes, the engine can torque-vibrate around the crank while the carb will stay relatively still. I have smoothed out O-200s just by installing the proper washers and doing up the nuts exactly as the manual says.

So your mechanic needs to refer to the O-200 overhaul manual and look for that paragraph. This is critical for that engine. The whole carb and spider need to be able to flex sideways a bit. Go here http://veteranflyg.se/wordpress/wp-...75-C85-C90-O-200-Overhaul-Manual-Aug-2011.pdf and see page 103 and read paragraph 16-10. The Lock-O-Seal washers are used to permit movement, not to seal anything.

The other big possibility is weak ignition. Pulling the throttle back will often lean the mixture, and a weak spark will misfire when lean. How long since those mags were off for internal inspection? Are the sparkplugs the old Champions with the stupid spring-loaded resistor in them? Look down the plug's connector well and see if the contact at the bottom has a screwdriver slot in it. If so, those are horrible plugs. They have caused many people many problems.

Ignition is 90% of engine performance problems, and yet most people will chase the fuel system until they're pulling their hair out in frustration. They think that a spark is a spark. It's not. A weak spark is at the root of a lot of troubles. It's one reason we have two mags and one carburetor: the ignition is far less reliable than the fuel supply.
Click to expand...

Great info! He replaced the carb and reamed the valve. Engine was smooth on run up according to the mechanic. Ill take a look at the mounting of the carb as you mentioned
 
Nick P said:
Great info! He replaced the carb and reamed the valve. Engine was smooth on run up according to the mechanic. Ill take a look at the mounting of the carb as you mentioned
Click to expand...

I it's the mounting it will often be smooth on runup. In the takeoff roll the nosewheel vibration is often enough to get the carb shaking and sloshing and you lose RPM.

Grab the carb airbox and see if you can move it sideways a bit. Maybe a half-inch or so. If it's stiff the mounting isn't right.
 
Had a stuck valve on an O-200 once, it will definitely get your attention. Way more than a 100-150 rpm loss, more like complete loss of that cylinder. I suppose there are different degrees of stuckness but if it's stuck open even just a bit that cylinder isn't going to fire.
 
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