Carburetor Icing

[Firefighter2143]

To the wiser and more experienced pilots. I just finished my check out on my c180, and have fairly low hours on a 172. I was flying one day. And totally lost manifold pressure and rpm, it was going down and up and down and up again manifold pressure went from like 24 to like 10 and back, don't remember the rpm. I instantly went through all my checks. It did that about for four cycles then was good for two minutes and did it again for like 2 cycles. Then was good for the rest of the very very quick flight back to the airstrip. My question is is this carb icing? Carb heat mixture rich, full throttle, fuel both, etc so fast I am unaware what was the issue. What is the reaction to mainifold pressure in carb icing, up? Down? I never thought the carb icing would feel like my engine is going to fail? Scared the sh!t out of me.

[cgzro]

Carb ice is usually slow to cause power loss but could be I guess quicker.
Could it be prop governor? How was your oil pressure?
Prop going coarse could do that. Hows the oil level?

Fuel quantity? Fuel sector not properly positioned?
Kinked fuel line hose?
Fuel filter/ screen gummed up?

[Firefighter2143]

It not the fuel filter screen cause just got it cleaned less then a week before, even checked the one in the fuel tank I was using fuel out of clean guess I will have to follow the lines and see if kinked. As the govenor I am sending out to a prop shop to get tested cause on takeoff. It goes to redline then drops down like hundred rpm. The prop shop says it may be the govenor there doesn't seem to be and oild leak from the prop even got an ame to take the prop off and have a look down in there. I am nonot sure if these two issues are related or not . Hence why I am asking about carb icing. One would think carb icing would be a slow onset of rough running?

[cgzro]

If you have problems with the prop RPM its possible its causing your other more serious problem by causing coarse pitch. Its fairly delicate mechanism in there.
When was it and prop last overhauled?

My experience with carb ice is its gradual until you apply heat then its kinda suddenly rough till it melts.

[Firefighter2143]

The prop has about 75 hours on since new. That's what I have been told about carb icing. Since the govenor is still on warranty I guess I am going to send it away and get it checked.

[TeePeeCreeper]

To the wiser and more experienced pilots. I just finished my check out on my c180, and have fairly low hours on a 172. I was flying one day. And totally lost manifold pressure and rpm, it was going down and up and down and up again manifold pressure went from like 24 to like 10 and back, don't remember the rpm. I instantly went through all my checks. It did that about for four cycles then was good for two minutes and did it again for like 2 cycles. Then was good for the rest of the very very quick flight back to the airstrip. My question is is this carb icing? Carb heat mixture rich, full throttle, fuel both, etc so fast I am unaware what was the issue. What is the reaction to mainifold pressure in carb icing, up? Down? I never thought the carb icing would feel like my engine is going to fail? Scared the sh!t out of me.

I don't think you had much if any carb ice... You'd have noticed a gradual lowering of MP... Not a sudden "wtf is she doing" moment... Atleast that's always been the case with my O-470K...
Sorta sounds like you may have had a run away prop?
Had that happen a few years back... Caused by some metal which blocked the prop governor...
Are you checking for metal in the filter during oil changes?

[cgzro]

Did the plane lunge forward a little? How quickly did the rpm decay?
A coarse prop will kill the rpm much faster and give you a little shove feeling forward (like when u test on the ground) while a fuel problem would take longer for the Rpm to decay. The oil pressure would also drop if the prop was working hard to move.

[Firefighter2143]

Honestly just bought this plane did an oil Change about a week before didn't look for metal in oil just passed my mind, will definitely next time. It was a pretty quick drop in mp,the rpm drop but dont remember how fast etc, was fast enough to almost make me crap, but seemed to come back to life. And never had any issues till like three flights later. Then did the same thing little longer the second time it seemed like someone was turning one off switch on the mags without letting it shut off completely.

[Colonel Sanders]

If either magneto was accidentally grounded out
in flight, all you would have noticed is a small
fluctuation in RPM.

I suppose if one magneto failed internally (or
came loose) and intermittently advanced the
spark much too far, that could cause pre-ignition
or detonation.

It's a remote possibility that both mags got
grounded out by the switch. Or, that both
p-leads shorted out on the firewall at the same
time, but I think that's a stretch.

PS Could theoretically be sticking valves
but generally you have other symptoms
eg morning sickness.

I would look at fuel delivery. An interruption
in fuel makes for a terrible running engine. This
can be caused by all sorts of things, including
poor venting of the gas tanks, plugged filters,
collapsing fuel hoses, sunk float valve, etc.

Also make sure air can get in and out of the
engine. Is the air filter loose? Or the carb
heat flapper not secure? Or baffles corroded
inside the muffler, occasionally plugging up
the exhaust?

Assuming that there is nothing horribly wrong
with the innards - the valves are opening and
closing at the right time - it's probably related
to fuel, or possibly air, or remotely spark.

[Firefighter2143]

Initial thought I figured fuel too, but looked at fuel strainer in tank, in fuel bowl seemed both are good and clear. Venting on the tanks are something I may have to check, I will be pulling the carb tomorrow morning. With the cowl off may have a quick look at the air intake too. Oh the gremlins that come with aircraft. I figure that if it was mags or something I should be able to get it to act up on ground run up... But to this day knock on wood it has only happened in cruise flight. It seems both times it has happened been the same tank on fuel selector. Maybe just luck? Maybe not.