Rough Running 150

tonycondon

tonycondon

Gastons CRO (Chief Dinner Reservation Officer)
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Tony
I could've sworn that this particular problem has been discussed. I flew a 150 yesterday that liked to run a little rough on climb out. The owner said it was going lean and he couldnt figure out the problem, and neither could any A&P he had talked to.

It didnt feel like carb ice. Plus the guy said this has been going on for a while and its highly unlikely that they would get carb ice on every takeoff over a course of many flights on different days and conditions. Second, turning carb heat on only smoothed out the engine, there was not an inital drop in RPM and increase in roughness as ice melted and was ingested.

So using carb heat smoothed it out, which led him to believe that the engine was for some reason running lean in the climb. I played around with it a little, and seemed like slightly reducing RPM helped smooth it out too. This didn't jive with his explanation. The 150 has an accerator pump that shoots extra fuel into the intake when at full throttle, to assist engine cooling. reducing throttle would actually lean the mixture.

Im wondering if the accelerator pump isnt putting too much fuel into the intake, and it can't atomize properly. With carb heat on the warmer air assists atomizing this extra fuel and it smooths out. thoughts?
 
tonycondon said:
I could've sworn that this particular problem has been discussed. I flew a 150 yesterday that liked to run a little rough on climb out. The owner said it was going lean and he couldnt figure out the problem, and neither could any A&P he had talked to.

It didnt feel like carb ice. Plus the guy said this has been going on for a while and its highly unlikely that they would get carb ice on every takeoff over a course of many flights on different days and conditions. Second, turning carb heat on only smoothed out the engine, there was not an inital drop in RPM and increase in roughness as ice melted and was ingested.

So using carb heat smoothed it out, which led him to believe that the engine was for some reason running lean in the climb. I played around with it a little, and seemed like slightly reducing RPM helped smooth it out too. This didn't jive with his explanation. The 150 has an accerator pump that shoots extra fuel into the intake when at full throttle, to assist engine cooling. reducing throttle would actually lean the mixture.

Im wondering if the accelerator pump isnt putting too much fuel into the intake, and it can't atomize properly. With carb heat on the warmer air assists atomizing this extra fuel and it smooths out. thoughts?
Click to expand...

dunno 150 carbs. But in general, an accelerator pump only puts in the extra fuel when the throttle is moving. Think of a piston or diaphram pump - moving the throttle to wide open is one stroke of the pump.
If the pump is bad, the engine will likely stumble as you open the throttle, but then recover.

There are other things found in some carburetors such as pull-over circuits that can provide additional enrichment at WOT but I don't know if those typically are found in aircraft carburetors.

Carb heat warms the air making it less dense which makes the mixture richer. But it will also help the fuel in the manifold evaporate. So it could be that it is running generally too lean (seems most likely) or there is a fuel distribution problem.
 
right geoff, maybe 'accelerator pump' is not the correct term.
 
tonycondon said:
right geoff, maybe 'accelerator pump' is not the correct term.
Click to expand...


Are you sure the mixture is full rich during the climb?

this sounds to me like a dirty carb.

or

a mixture enrichment valve is not fully open at full throttle.

or

throttle shaft bushings are worn.

or

the venturi is loose.

or

he has an induction leak.
 
oh crud, i forgot to mention, it seemed to also be somewhat associated with pitch attitude, when we would level off (still full throttle) it would smooth out a bit.
 
tonycondon said:
oh crud, i forgot to mention, it seemed to also be somewhat associated with pitch attitude, when we would level off (still full throttle) it would smooth out a bit.
Click to expand...


OMG remove the carb, and send it to me, it's one of them damn, thingies that holds the what's it in place.
 
Tom i always knew I could count on you for a technical opinion :)
 
Rotor&Wing said:
Time for a new carb $$$$$$.
Click to expand...

Do not do that........ the new kelly carbs are a POS, just fix it. keep the 2 piece venturi, and metal float, throttle shaft bushings are 30 cents, gaskets and accelerator pump plunger might be 200 bucks, the new carb is 1000+ and runs like crap.
 
tonycondon said:
oh crud, i forgot to mention, it seemed to also be somewhat associated with pitch attitude, when we would level off (still full throttle) it would smooth out a bit.
Click to expand...

IIRC the 150 is gravity fed? Could be a restriction in the fuel supply from the tank that has slowly been getting worse and you are just on the edge of a real problem?
 
Capt. Geoffrey Thorpe said:
IIRC the 150 is gravity fed?

Yes.

Could be a restriction in the fuel supply from the tank that has slowly been getting worse and you are just on the edge of a real problem?
Click to expand...

I don't know how that could happen, the system is 3/8" hard line, with a gascolater. and a on/off valve.
 
Tom-D said:
Do not do that........ the new kelly carbs are a POS, just fix it. keep the 2 piece venturi, and metal float, throttle shaft bushings are 30 cents, gaskets and accelerator pump plunger might be 200 bucks, the new carb is 1000+ and runs like crap.
Click to expand...

You could. However as an A&P there is no way I would repair a carb without approved data or make a logbook entry that I repaired said carb. The liability is just too excessive.
 
tonycondon said:
oh crud, i forgot to mention, it seemed to also be somewhat associated with pitch attitude, when we would level off (still full throttle) it would smooth out a bit.
Click to expand...

Tony,

I had an 0-300D that did this for some time. New carb didn't fix it. Not the o-ring in the primer, not any of the various things that different mechanics did. However, a new set of valve springs did. It started off as a nibble on climb-out that would smooth out when I leveled the nose off. It progressively got worse until it would drop considerable rpms and run really rough. Pushing the nose down would smooth it out. An old-timer suggested that we check the springs after we spent several thousand dollars looking for the problem. It worked. So if the other suggestions don't work, check your valve spring compression.

Deb
 
interesting Deb, i would've never thought of that as a culprit for this particular problem.
 
Tom-D said:
I don't know how that could happen, the system is 3/8" hard line, with a gascolater. and a on/off valve.
Click to expand...

Crud building up on the strainer in the gascolater.

Note: Better running level vs. climbing could also be related to the increase in engine speed. Which would suggest that one would want to look at the magneto timing.
 
Tom-D said:
I don't know how that could happen, the system is 3/8" hard line, with a gascolater. and a on/off valve.
Click to expand...

Probably the same way the Fuel Guage on the Super Cub I have been flying sticks on full fuel even when the tank is near empty. And it is even simpler than the 150 fuel system. The mechanic supposably fixed it once and it still remained full of fuel with ball at the top. Haven't flown it since his 2nd try at it.

Does the 150 have a screen in the tank?

Brian
 
Tom-D said:
that too.
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Bad primer O-rings just let fuel dribble out on your fingers. The primer has a spring-loaded needle on the end of its plunger that plugs the outlet hole so that fuel can't be sucked past the primer's check valves by low manifold pressure. You can feel this needle spring when you push the primer in and lock it. You are locking it, right? The needle or seat can get damaged and let the engine suck extra fuel past it, or the spring can break. Easy to check. The escutcheon nut comes off and the plunger comes out. Turn the fuel off first.

If there was an induction system leak the problem should get worse when the throttle is closed some. On some carbs, the nozzle's atomization is so poor that closing the throttle a bit lets the fuel strike the throttle plate and get broken up into smaller droplets and the engine will run better. We had a 172 that did that; closing the throttle about a quarter-inch on climbout actually increased RPM.

The carb's accelerator pump check valve springs can break or even be left out during manufacture. We had a factory O/H Lycoming (with the same stupid MSA-type carb as you have) on it that ran rough in the climb. Freshly overhauled or new, and the check valve springs were missing and let the engine suck more fuel. The accelerator pump nozzle is in the secondary venturi and it will merrily suck lots of fuel. Badly atomized, too, so that carb heat might help vaporize it.

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.

O-200s don't like the intake valve guide clearances getting too small. The valves will stick and the engine can run rough. 100LL aggravates the problem.

Rough running is often due to ignition problems. At full throttle the cylinder pressures are high enough that a bad plug can't fire or a weak mag can't develop enough voltage. Air is a dielectric (insulator) and the higher the pressure, the harder it is for the spark to jump. If the mags aren't timed reasonably close together one plug can fire before the other and raise the cylinder pressure beyond the point where the later plug can spark and the engine will run rough because of the slower burn on that cylinder. And it won't show up on a runup. Weak mags will aggravate it.

I would check that carb spider mount and the plugs and mags. They're the most likely issues here. The plugs MUST be tested under at least 125 psi. Their internal resistors can fail. Champions are famous for that, and the newer Unisons do it, too. The old Auburns were my favorite. Never had one fail.

Dan
 
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This may sound stupied but Have we checked for water contamenation in the fuel? This is a very simple fuel system gentlemen.I dont mean just with a fuel stainer that we all carry . We had a simular issue with a 150 commuter rough running. We found water in the tanks and fuel system. Whats funny! The plane just seem to make water!:yikes:no matter how much we drained This just took a little patience. We even retrieved a fuel sample from our fuel farm to ensure fuel was good.lol we drained and inspected tanks all fuel lines,Vent (which gets over looked ) ,fuel strainer had some minor sludge buildup. We replaced fuel caps. All is working well now havent had any issues. Just food for thought.

Dan ,The inside man.
 
We had some rough running intermittently.. could not figure it out.. regapped and retimed the mags, checked all the plug wires, cleaned and regapped all the plugs.. checked the throttle lingages.. only thing left was to rebuild the carb thinking some jets were plugged. In the process of pulling the old carb, we found a section was loose.. a gasket was gone.. it was sucking air where it should not have been.

Well worth the rebuild carb, shipped it out and back in a week.
 
Check the tiny spring in the check valve on the carb for the primer, you might be
happily surprised to find it broken. It took us months to figure this one out. It was letting fuel get by when under load in a climb, running too rich and stumbling during climbs only.
 
150F since 1992 said:
Check the tiny spring in the check valve on the carb for the primer, you might be
happily surprised to find it broken. It took us months to figure this one out. It was letting fuel get by when under load in a climb, running too rich and stumbling during climbs only.
Click to expand...
This thread was posted over a decade ago, I’m sure he’s reconciled the issue by now. :)
 
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