I am searching for possible reasons for engine power changes that cycle from full power to idle and back again in a constant 10 second cycle. This occurred at 12,000 ft. with an outside temperature of 30 degrees and continued for approximately 20 minutes. During this time I added carb heat and tried different throttle settings with no discernible effect. After this event, the engine ran normally for another hour until I reached my destination. In addition to the disturbing engine performance, I should add that this was at night and in the mountains. Not my most enjoyable flying experience. I can not think of a good reason for this to happen. The first reaction was to assume carburetor ice, but I have never heard of carburetor ice producing a cycling surge like this. It usually just reduces power and then either the carb heat melts the ice and power returns to normal, or if it is bad enough the engine just quits. At this point I need ideas to pursue as to a probable cause. Thanks in advance for any help you can give me. The engine is an O-300 and a 172B.
Charles

My first guess would be a plugged fuel vent. Did you switch tanks during this episode?

Or carb jet or float issue. These 60 year old airplanes are getting a bit long of tooth (like us) and stuff gets plugged up!

Gump

Contaminated fuel ?

My first thought was also venting or lack of. Eng. sucks fuel from an alum. can with a tight ass vent. The can collapses until the tight ass vent surrenders. Ice on vent? Do not recall the 172 plumbing right off.
Do remember a bulletin about vented caps.

I put a vented cap on both sides of my 53-170B with Lyc 0-360. Also had the overhead ram tube.

How long before the problem started? Did it continue on the ground?

Still Curious Chris C

I'll join the vent choir. Check for blockage.

It does sound like the engine is sucking something in that it shouldn't be. Air, fuel contamination, etc.

I almost soiled my shorts reading your post.

I assume there was no place to land on route?

One 'surge' (sounds more like temporary loss of engine power to me) I'd land nearest safe strip, thank God, and start making phone calls.

Mountain Doctor wrote:I almost soiled my shorts reading your post.

My thoughts exactly.

The original post sounds like something from an Earnest Gann novel. "There we were, over the Atlantic in the dead of night, and the engines started to act up..."

Or like an entry in a volume of: "No Shit! There I Was..."
Some old friends are scattered through those volumes.
Best written story in the first one is:
"The Tai Chi Of The Critical Moment"

Sorry for minor drift.

Your right side tank vents through a vent tube located above the windshield to the left tank, which has the only visible tank vent, which comes down behind the wing strut. However, the right side tank is also supposed to have a vented cap. Normally, the left side tank's cap was not vented from the factory. But it's not uncommon for the caps to be reversed (or for an unvented cap to be installed on the right side as a replacement for a lost cap) so that there's no outside venting for the right side tank, and the crossover vent tube either gets plugged, or sometimes gets a blockage of fuel in there, or the anti-siphon check valve needs replacing.

See this thread for that last issue: https://www.backcountrypilot.org/commun ... uawk-20004

Sometimes the vented caps plug up, too--they're pretty simple vents.

The end result of any vent blockage is that fuel stops flowing for long enough that air can get into the tank(s). Your tanks are aluminum, and while they will "oil can" enough that after many years, many of them will crack (usually on top) from the oil canning, they don't flex enough to allow a blocked tank vent to draw air and flow fuel very quickly. Incidentally, when they crack and need to be repaired, it's 'spensive, Lucy! The actual repair is cheap, but draining and pulling the tanks is a very labor-intensive job.

Me too, on landing right away--that's what I would have done.

Cary