Carb Heat

[GlennM]

10 hours ago, Roger Lee said:

Your carbs are controlled by air flow.

I agree, but they are designed to provide the right fuel mixture for a given airflow, so how could they end up lean unless something is wrong inside the carb?

I think of a blocked air filter as a conventional choke, where you have a plate that blocks the airflow to richen the mixture. I can't see it going lean with a blocked air filter. 

[TDJ737]

36 minutes ago, GlennM said:

I agree, but they are designed to provide the right fuel mixture for a given airflow, so how could they end up lean unless something is wrong inside the carb?

I think of a blocked air filter as a conventional choke, where you have a plate that blocks the airflow to richen the mixture. I can't see it going lean with a blocked air filter. 

I would think the same way.. Engine sucking on the filter and generating extra low pressure in the carburator.. 

[TDJ737]

.. although I don't see the extra fuel burn on my FF meter: 16,5L/H at 5000RPM at 1500'MSL

[GlennM]

With less air from a blocked air filter,  you would need less fuel for a given mixture. I am not sure if this would be seen on fuel flow.  Certainly not higher flow.

With a misfire,  could it be ignition related? A drip of water grounding something? 

I have had poor running (misfire, roughness) after a hose change with debris in the fuel lines that were not properly cleaned. On removing the float bowls, the debris was seen and removed. The only other time was from a loose ground to the engine from the ignition module. Rough running and intermittent.

[Roger Lee]

The carbs only correct so much. They aren't like fuel injection. If carbs could correct that much there would be no need for FI. They have limits and when you try to get too much rpm and demand more work out of it the carbs can not correct enough and the balance tube at that point in the rpm plays no part. . This is one reason the carbs can have different jetting and needles.

[GlennM]

I don't agree.  If these carbs could not correct for the whole flight envelope and ran so lean the engine misfired,  I doubt Rotax would have chosen them.  Or Flight Design would have placarded these areas to avoid.  The jets and needles are to adjust mixture ratios and adjust for different engine sizes.

 

There must be something else going on...

 

 

[Roger Lee]

Hi Glenn,

Ther carbs can only correct so much. That is why they run so rich as you climb to higher altitudes. This is also why if you live at airports over 7K and fly at 10K+ all the time you should lean the mixture with the needle. Even Rotax has a chart for this. These carbs have been around for decades and aren't new wonderful technology and that's also why Rotax came out with a computer controlled FI system. It's why the FI engine has better fuel consumption at high altitudes and and adjust for overly rich mixtures that the carbs can't. Don't get me wrong I prefer the carbed engines over the FI ones, but carbs have their limits. 

When I teach one of the questions on the test is what happens when you go from needle position #3 to #2. It leans the mixture. If the carbs were all inclusive for ALL operating parameters then carbs would have needle adjustments or jets that can be changed. Carbs can be further messed up if someone overhauls them and doesn't know some of the adjustments inside. Things like the butterfly valve must be position in a certain way, the needle clip in slot #3 or how to adjust the float height. Carbs have adjustments and limitations.

Plus airflow can be affected by types of filters, size of filters and air intake styles which affects carb / engine performance. I've corrected many with issues.

[Tom Baker]

Roger, you still haven't explained why an air filter restriction will cause the carbs to run lean enough to cause a misfire.