Warbird Information Exchange

TBM decends from 8000 - after initial descent at cruise power engine runs rough, clears, runs rough; level out all OK; further decent - rough again! in circuit rough again; lands OK. ground power check etc etc - normal! check known things like water/plugs/vents etc etc. test fly; takeoff fine; again on decent engine runs rough.; lands OK; ground run fine!

Any ideas?????

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..defeat is never an option!

I'm not an aircraft engine mechanic, but I've done my fair share of work on auto engines... my first thought would be spark advance. Does the 2600 distributor system have any sort of mechanical device to advance the spark under load, like a vacuum advance? If it's vacuum operated, would it be possible that the different air pressures may be affecting the "pull", thereby causing the ignition system to not adjust correctly?

Again, this is just wild speculation, please forgive me if it's already been thought of and checked.

Lynn

I would suspect a carb problem check for fuel stains, it sounds as if it may be flooding on decent. Check your exhast stack for color.

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IF YOU CAN FIND IT WE CAN FIX IT

it sounds like the auto mixture control is not working. When you desend the engine is going lean, you level out, the mixture slowly enriches, engine runs good until you desend again. There maybe a clogged vent hole on the carb, or the mixture control needs to be overhauled.

To Imritget, aircraft operated on fixed timing, there are no timing changes. There is a starter vibrator to cause a shower of sparks for starting, but once the engine is running, the timing is set between 20-25 BTDC.

What's the power profile you're using for descent?

Thanks Matt, I've learned something today.

Cheers,

Lynn (aka "LMRitger"... I need to have Scott change the case sensitivity on that, as it looks like I Am Ritger)

Does the aircraft have the Holley "automatic" carb.? Could be something to do with altitude compesating. Just guessing.
Robbie

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Fly Fast Make Noise!

Or just try cleaning mixture control, remove bellows clean.

Interesting topic. Sorry I'm in it late. I have wondered what the starting vibrators were for... seen them for sale but wondered what they are used for.

Matt, do they have anything to do with Booster coils? Does the "shower of sparks" happen as multi sparks on the same plug, in timing order?

I'm a former racecar mechanic just learning about how round motors work. Cool stuff.

I need to find a basic book on radials.

Dave

When the engine roughs up on descent, have you tried applying full Carb Heat for a minute or so to see if it might just be Carb Ice? Carb ice can form even in warm air masses in a humid atmosphere as a result of intake air cooling while passing through the venturi which can reduce it to freezing even though the carb air temp reads +20* or more.

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It's what you learn after you think you know it all, that counts.

Most favorable condition for carb ice is high humidlty, temp 68* F or less. and low power setting. I do not know TBM, it may have pressure carb and not be prone to carb ice, the pilot note shoul dsay if there is even carb heat to try.

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Bill Greenwood
Spitfire N308WK

Had a problem like that on a R-2600 on a B-25 (Stromberg carbs). Found a loose carb on the engine. Almost same problem, only on decent, never on climb or cruise.

Would also check the intake pipes for security at the same time.

Attributed it to lower power settings in this phase of flight.

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If God had wanted man to fly behind a flat motor, Pratt Whitney would've built one.

The starting vibrator causes the plug to fire at TDC, in firing order to start the engine. It fires the plug with electricty from the AC, when the engine is running, it runs on electricty generated from the magnetos.

I think the booster coils you are talking about are the low tension ignition system like what is on the Connies, and C-1A, S-2. they have a single coil mounted on each cylinder firing both plugs.

Thanks for the tips chaps. We're looking at the carby now. The initial power setting on descent was 2000/28...which is cruise..

We had a similar situation with a R-1830 in a PBY-5A back in the early nineties. One day while in a holding pattern to go into SFO for an open house at the Coast Guard station the right engine started running rough. I applied carb heat and that seemed to clear it for a while. The instrument panel would shake in this rhythmic cycle. We called the tower and they got us in with no problem. When we were leaving we did a good run up. Our fuel tanks were absolutely full (1750 gals.) No problem flying home. Maintenance inspections found nothing. Wasn't until a few flights later one of the induction pipes blew out, taking out one of the cowl flap sections and starting a fire. We got on the ground ok. We eventually pulled a couple of jugs as the center electrodes in a few spark plugs were melted. In addition to one induction pipe being loose we found out later during the engine tear down that 3 cylinders that were chrome also had chrome rings. The rings should have been steel. Too much friction. I think checking the tightness of induction pipes might be in order.
Myles