[McPilot]

Hello,

Does anyone notice that the fuel flow indication has a bit of fluctuation?

I set the fuel flow in cruise and it seems to vary +/- 0.8 GPH....

 

[McPilot]

Here is an example...

 

[lr172]

Hello,

Does anyone notice that the fuel flow indication has a bit of fluctuation?

I set the fuel flow in cruise and it seems to vary +/- 0.8 GPH....

My red cube on my D120 is quite steady. I will get occasional drift right after setting the mixture. I think this is slop in the mixture cable / linkage and vibrations force it to one side of the slop or the other. But once set it's very stable. I am FP and even small deviations from straight and level cause changes in the mixture, due to changing MAP / RPM. It needs to be a significant departure from level though to get a .8 change.

I would check the connections to the red cube. That is their most common cause of errors and problems.

Larry

 

[McPilot]

Thank you Larry. I spoke with my A&P. He tells me that the FF indicates fuel in to the bowl of the carburetor will vary with the carburetor float position. The mixture cable s controlling what actually goes from they bowl to be mixed wth intake air. I guess this makes the actual value less significant than in a fuel injected engine. Hence leaning by egt is more relevant than fuel flow from what I understand

 

[lr172]

Thank you Larry. I spoke with my A&P. He tells me that the FF indicates fuel in to the bowl of the carburetor will vary with the carburetor float position. The mixture cable s controlling what actually goes from they bowl to be mixed wth intake air. I guess this makes the actual value less significant than in a fuel injected engine. Hence leaning by egt is more relevant than fuel flow from what I understand

I do not agree with your A&P. While the fuel does go into the bowl, the fuel level in the bowl is fixed (float position is just a regulator to keep it at that pre-determined level and it is quick reacting) so what goes in is directly related to what went out through the jets. My engine started life as an O-320 and I also had very stable FF indications with the carb, especially in cruise conditions where the flow is stable and consistent.

Larry

 

[McPilot]

I do not agree with your A&P. While the fuel does go into the bowl, the fuel level in the bowl is fixed (float position is just a regulator to keep it at that pre-determined level and it is quick reacting) so what goes in is directly related to what went out through the jets. My engine started life as an O-320 and I also had very stable FF indications with the carb, especially in cruise conditions where the flow is stable and consistent.

Larry

The level might be fixed within certain limits but the flow in to the bowl and out via the needle valve are two different items. A float wouldn't be needed otherwise Kind like like a toilet tank.

 

[lr172]

The level might be fixed within certain limits but the flow in to the bowl and out via the needle valve are two different items. A float wouldn't be needed otherwise Kind like like a toilet tank.

If the level of fuel in the bowl is always at a fixed level, you'll have to explain to me how the in and out flow amounts are different over any given period of time longer than 1 second. I'll agree that if you used a time span of 5 or 10 milliseconds, the flow rates could be slightly different, but when measured over a reasonable time span, like a second or two, they must be the same. If you take 3 ounces of soda out of a full can and fill it back to the top, the inflow will be 3 ounces.

Don't mean to argue, but you really don't need to accept a .8 GPH variability. Something is wrong. Maybe others will chime in with their experience, but I have not seen this type of complaint. Lot's of guys with carb'ed engines that get steady flow ratings, regardless of what your A&P says.

Larry

 

[lr172]

The level might be fixed within certain limits but the flow in to the bowl and out via the needle valve are two different items. A float wouldn't be needed otherwise Kind like like a toilet tank.

You fuel pump delivers about four times the flow rate your engine requires. It operates just like your toilet and insure the tank always fills to the same level. The difference with the toilet is that the outflow capacity is MUCH higher than the inflow capacity. A carb is the exact opposite.

By the way, a flow meter would also work perfectly on a toilet. Given the fixed water level, the inflow must also exactly match the outflow. The difference is the time it takes to reach equilibrium.

Larry

 

[McPilot]

I just got off of the phone with none other than Mike Busch..

He agrees that the fuel flow fluctuations are as a result of the float in the carburetor bowl opening and closing the valve which does vary the fuel flow.

He says that many fuel flow instruments have a carburetor/fuel injection option to "smooth" this fluctuation out...

He doesn't see anything out of the ordinary with the above graph..