Carb running rich

[Scott 'Gunny' Perdue]

Anyone with some AN-1340 carb experience I've got a vexing problem. It seems with the changing of the seasons I have to adjust my idle mixture.... cold to warm it tends to run WAY too rich. Does anyone know what the root cause of the problem is? I've heard that it is common.... but that doesn't really help me figure out the problem. Any help would be appreciated.

Thanks,
gunny

[bdk]

What is your indication of running too rich? Black smoke at idle? Excessive RPM increase at idle cutoff?

Could the primer be leaking? Gravity feed carb installed instead of fuel pump carb?

No real experience with T-6 carbs, just some guesses.

[51fixer]

Is the carb installed with a gasket?
I was told at the last engine change I did by Covington that a gasket changes the amount of heat exchange the carb body absorbs as it acts as an insulator and can cause minor issues such as this.
Most cars that had a carb used some method to heat the manifold where the carb was attached whether it was hot coolant or exhaust gases.

[Scott 'Gunny' Perdue]

Is the carb installed with a gasket?
I was told at the last engine change I did by Covington that a gasket changes the amount of heat exchange the carb body absorbs as it acts as an insulator and can cause minor issues such as this.
Most cars that had a carb used some method to heat the manifold where the carb was attached whether it was hot coolant or exhaust gases.

Rich-

I don't know. I'll look. Someone else suggested that the float was going bad. A vexing issue.

gunny

[Scott 'Gunny' Perdue]

What is your indication of running too rich? Black smoke at idle? Excessive RPM increase at idle cutoff?

Could the primer be leaking? Gravity feed carb installed instead of fuel pump carb?

No real experience with T-6 carbs, just some guesses.

bdk-

Excessive black smoke, one to several jugs shutting down due to the rich mixture (I have a fuel flow and a JPI engine monitor), plus I can feel the miss. No to your last two suggestions.... remember that the AN-1340 has a supercharger.

gunny

[bdk]

If the float was leaking (sunk) I would think it would always run rich. You say it goes from normal to rich with changes in temperature?

Does it just run poorly at idle, or in flight/at higher power settings too?

Could you have a leaky intake pipe? Sucking air at MAP below barometric but sealing up under pressure?

The primer only goes to the top cylinders and they see suction at idle (don't you have to be well above idle for MAP to equal field barometric pressure?).

[The Inspector]

If the float was leaking (sunk) I would think it would always run rich. You say it goes from normal to rich with changes in temperature?

Does it just run poorly at idle, or in flight/at higher power settings too?

Could you have a leaky intake pipe? Sucking air at MAP below barometric but sealing up under pressure?

The primer only goes to the top cylinders and they see suction at idle (don't you have to be well above idle for MAP to equal field barometric pressure?).

Quick way to find out, a 2 lb COLEMAN propane bottle from the hardware store, some vaccum hose with the loose end taped to a yardstick or piece of small dowel. Run the engine at idle and wave the end of the vaccum hose across all the connections, if the engine speeds up, thar be yer leak. One other question, is it the same carb or a rebuild/replacement? if it's new to the engine it could have an internal porosity issue

[Scott 'Gunny' Perdue]

If the float was leaking (sunk) I would think it would always run rich. You say it goes from normal to rich with changes in temperature?

Does it just run poorly at idle, or in flight/at higher power settings too?

Could you have a leaky intake pipe? Sucking air at MAP below barometric but sealing up under pressure?

The primer only goes to the top cylinders and they see suction at idle (don't you have to be well above idle for MAP to equal field barometric pressure?).

I agree with your observation on a sinking float.... as I said it only occurs at idle, not at higher power settings. Adjusting the idle mixture setting has always cured the problem. A leaking intake system... there's a thought worth checking out.... but on the other end, at higher power settings wouldn't it have trouble making full MP? It's supercharged and it makes MP at takeoff power.

Inspector... are you saying that you crack the propane valve slightly so that the engine will suck in the propane and speed up because of that? The carb came with the engine at overhaul 250ish hours ago. It has always run a little rich on the idle side.

gunny

[The Inspector]

If the float was leaking (sunk) I would think it would always run rich. You say it goes from normal to rich with changes in temperature?

Does it just run poorly at idle, or in flight/at higher power settings too?

Could you have a leaky intake pipe? Sucking air at MAP below barometric but sealing up under pressure?

The primer only goes to the top cylinders and they see suction at idle (don't you have to be well above idle for MAP to equal field barometric pressure?).

I agree with your observation on a sinking float.... as I said it only occurs at idle, not at higher power settings. Adjusting the idle mixture setting has always cured the problem. A leaking intake system... there's a thought worth checking out.... but on the other end, at higher power settings wouldn't it have trouble making full MP? It's supercharged and it makes MP at takeoff power.

Inspector... are you saying that you crack the propane valve slightly so that the engine will suck in the propane and speed up because of that? The carb came with the engine at overhaul 250ish hours ago. It has always run a little rich on the idle side.

gunny

Yep! same technique as finding manifold leaks on any other engine, just be REALLY careful! I mentioned possible porosity issues because after 800 years of vibration, stripping, rebuilding, occasionally a small crack or porous spot turns up and causes a leak of either air or fuel caused by a speck of casting sand finally becoming disloged. See if you can borrow a known good carb and see if the problem goes away, if it does you're out the cost of another carb but also have a cool door stop since yours would now be unairworthy, if it doesn't then the troubleshooting continues. Troubleshoot in small, simple steps. You'd be amazed at how many flights have been delayed or cancelled over a burned out light bulb and the mechanics wanting to go after the main pivot washer only to discover much later, it WAS the lightbulb. Just because the carb came with the engine don't make it guaranteed as 'good to go'.

[bdk]

I agree with your observation on a sinking float.... as I said it only occurs at idle, not at higher power settings. Adjusting the idle mixture setting has always cured the problem. A leaking intake system... there's a thought worth checking out.... but on the other end, at higher power settings wouldn't it have trouble making full MP? It's supercharged and it makes MP at takeoff power.

Inspector... are you saying that you crack the propane valve slightly so that the engine will suck in the propane and speed up because of that? The carb came with the engine at overhaul 250ish hours ago. It has always run a little rich on the idle side.

gunny

After a little more thought...

If you were to drill an 1/8th inch hole in an intake pipe, I would expect a lean mixture in that pipe at idle since you would be sucking in air. At MAP above atmospheric, you would blow a small amount of your intake charge out, but it would not create a lean condition, just a slight loss in MAP. I'm not convinced that would even be noticeable as an MAP drop since there is some tolerance in the field barometric pressure/MAP RPM check.

Now if the idle mixture had been set to compensate for the lean condition in some cylinders to get smooth running, you might actually be masking your lean problem and calling it a too rich condition. The "good" cylinders could actually be the too rich ones while your bad ones could be the lean ones.

What you may be fighting is too great a variation of mixture in one cylinder vs. another.

If you could idle the engine for 10 or 15 minutes in this too rich condition, do you think you could tell which cylinders are actually running rich? If they all get evenly sooty, variation isn't your problem, that would point to an induction leak at the carburetor or an internal carb problem.

The intake pipes do have rubber seals AFAIK, so they may have perished due to calendar time.

That's my theoretical thoughts, maybe Rich can tell us if his experience could discount any of that.

And I still haven't given up on the leaky primer thought. If the sooty plugs were the ones with the primer lines, that would be quite a coincidence.

[Scott 'Gunny' Perdue]

bdk

You missed the part in my post that mentioned I have a fuel flow and an engine monitor.... I know exactly how rich the engine is running and which cylinders are not firing.... I appreciate your inputs.

At the moment the leading theory is one of the floats is cracked. That is the one I'm going with.

gunny

[bdk]

So then the rich condition and the change in weather is merely a coincidence?

[Scott 'Gunny' Perdue]

So then the rich condition and the change in weather is merely a coincidence?

I don't know. It happens the way I described then I adjust the mixtures and it goes away until the next change. But I started this thread to throw out the net a little farther for a mechanical explanation of the problem. I just don't buy the idea that a change in air temperature will cause this problem. Somehow it is an indicator.... so I would say it is not a coincidence at all.

[bdk]

Please let us know when you figure it out!

[Matt Gunsch]

bdk

You missed the part in my post that mentioned I have a fuel flow and an engine monitor.... I know exactly how rich the engine is running and which cylinders are not firing.... I appreciate your inputs.

At the moment the leading theory is one of the floats is cracked. That is the one I'm going with.

gunny

If you know what cylinders are not firing, why don't you let us in on which ones they are, it might help figure out what is going on.

Since the engine is supercharged, the fuel air charge should be evenly mixed before it gets to the cylinders, so there should be no lean or rich cylinders unless there is something going on with a cylinder or cylinders. You said the engine had 250 hrs, have you run the valves ? Checked the packing gland nuts on the intakes ? a leaking primer ?