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Too rich, or something else?

L

Lotro

PATRON
Hey Guys,

(TLDR: Have you every had a marvel carb run too rich?)

It finally got hot up here in Canada the other week (30C/86F + humidity) and my ‘51 Super Cub with an O-320 A2B Wide Deck (160 hp) started giving me grief.

In the morning takeoff run everything seemed fine, but in the hot afternoon I was only getting 2250 RPM on the takeoff run with full throttle. I nursed it into the air (it’s good to be lucky) and managed 2500 RPM in cruise with full throttle, but it also sounded wrong. Gauges all looked good except EGT was cold (VERY cold) I’m inexperienced, but it sounded like a misfire maybe or something like that. No violent shaking or anything, I could have got it home, but my passenger was spooked so we did a precautionary on a big lake on route (I’m on floats).

Checked mixture cable, throttle cable, carb heat cable and air box operation, air filter, mag drop normal - everything obvious, found nothing. Float struts and exhaust pipe were black with soot though. I got a flight home with a friend and came back with a mechanic a few days later.

One mechanic speculated “stuck valves” except no morning sickness that I noticed, and cylinders are only 300 hours since new and I run marvel with oil changes.

Mechanic pulled valve cover in the field and tried a wobble test, but nothing seemed off. We did a taxi, run up and mag check all good, went to go full power and again 2250RPM. Mechanic said “try again, but pull the mix” and low and behold it turned up full power (and then some) leaned out. Flew great leaned out all the way home.

The diagnosis right now is “Running too rich, get a new carb” which seems logical, but I’ve not really heard of carburetors (Marvel MA-4SPA) misbehaving. It was rebuilt at overhaul (22 year, 700 hrs ago) and logs say it’s been taken off and inspected a few times since then, but nothing conclusive noted.

Some guys have suggested broken exhaust baffle (air starvation?) but I’m running a Sutton exhaust (no baffles I think) and the problem only presented on hot afternoons at full power.

Everything to me says carb except I’ve never heard of it happening to anyone else. So… anyone have a similar failure due to carburetor?

Thanks for reading.
 
Lotro said:
... exhaust pipe were black with soot though.
Click to expand...

exhaust pipe were black with dry soot = excess fuel

First, pull the carb bowl drain plug and let some gas flush through. This will clean out the bowl of any contaminants which could cause an issue.

Rig a clear plastic hose to the drain so that it extends above the carb parting surface.

Pull the tails of the floats up on the shore enough so that the carb parting surface is level.

Measure the distance from the bottom of the parting surface gasket to the top of the fuel.

Look up the float level measurement, I don't know it off the top of my head. This will tell you if the float level is high = rich mixture.
 
The fact that the engine made normal power when leaned shows the problem is excess fuel not restricted air flow.

I would suspect a stuck float valve or a leaking float. I would check both before replacing the carb. Not an A&P so that advice has no FAA approval.
 
I would take the carb apart before replacing it. A legitimate function for an A&P. Shame on him if he doesn't.
 
I can’t imagine slow taxiing full rich on a warm day. My engines would have loaded up and quit before I was positioned to take off. At 55% power or less you can’t do any damage by over-leaning. Lean too much and the engine will run rough, so enrich just enough to smooth it out. Taking off with clean plugs works better than with carbon-crusted plugs, as you’ve found out. Just remember to go full rich for take off power.
 
stewartb said:
I can’t imagine slow taxiing full rich on a warm day. My engines would have loaded up and quit before I was positioned to take off. At 55% power or less you can’t do any damage by over-leaning. Lean too much and the engine will run rough, so enrich just enough to smooth it out. Taking off with clean plugs works better than with carbon-crusted plugs, as you’ve found out. Just remember to go full rich for take off power.
Click to expand...
Aren't your engines fuel injected? Some fuel injected engines develop vapor in the injector lines at idle on hot days. This makes them run rough or sometimes quit. Turning on the fuel boost pump pressurizes the lines eliminating the air bubbles and roughness.
 
If you have a round filter grab the dome and see if you can get any side to side movement. Otherwise find a way to see if there is any play between the carb halves. Another check is for excess rpm drop with carb heat. This problem creeps up on you at first showing symptoms only on a warm engine. Eventually the play between the halves gets to the point where the the addition of carb heat on landing can almost kill the engine.

Jerry
 
Lotro said:
My AME has a strict "do not lean" policy. I realize that's controversial...
Click to expand...

well I was taught different, at sea level that might be acceptable, dont know what your field elevation is but add couple 1000 ft of density altitude and you could easly be at 5000 ft that requires leaning, in my opinion your AME is wrong.
 
Cubflier beat me to it. Loose float bowl would be my number 1 guess, look for blue stains on the side of the float bowl. Could also be a leaking float, I had that happen also. Both are simple fix. As far as leaning, I do lean when I taxi and in cruse but full rich for run up and take off. Most of the flying I do is out of runways lower than 100 ft above sea level. Find a fixer not a parts changer. DENNY
 
Read the article StewartB posted for proper leaning. Everybody is right, every body is wrong, just depends on altitude.:lol: DENNY
 
A simple check is to access the carb (or injector body) throat (venturi) and take a look visually. I have had multiple Lycomings run intermittently rich over the years simply from a piece if baffle seal from the induction heat butterfly coming off and draping over the venturi or sense probes on injector servos.
Will cause from minor to extreme symptoms and is probably the easiest to check. Problem is not limited to Lycoming either.
Just a shot, but one I take as a first step every time I have an issue come in for rich running. I found one a few years ago after a cylinder had been removed by a mobile mechanic frustrated that all the deep troubleshooting had not produced an answer. One of those things that is hard learned the first time then becomes another tool for the future.
Ken
 
skywagon8a said:
Aren't your engines fuel injected? Some fuel injected engines develop vapor in the injector lines at idle on hot days. This makes them run rough or sometimes quit. Turning on the fuel boost pump pressurizes the lines eliminating the air bubbles and roughness.
Click to expand...
My Cub is injected, the 180 has a carb. I’ve leaned for taxi power in every plane I’ve flown and it works the same for injection or carb. Any plane with adequate fuel flow for cold weather ops will need to be leaned for warm weather ops. I still use full rich at takeoff regardless of OAT, but my DA is rarely higher than 1500’. It’s those winter -5000’ DAs that worry me more.
 
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