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Why is my fuel flo fluctuating?

pa38112

Well Known Member
Why is my fuel flow fluctuating?

My fuel flow is always fluctuating at a given setting. If I am flying straight and level with no throttle or fuel flow adjustments I will see my Fuel Flow vary plus or minus 1gph around my set point. I believe the numbers because there is a corresponding change in RPM, engine smoothness, and EGT. Any idea what would cause this? O-360-A1A carbureted, FP.
 
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If there is no damping in the readings, could you be seeing the float chamber needle valve coming off and on?

If my math is correct 10 gph = 21 oz per minute, or a cupful of 100LL every 25 seconds or so.

Perhaps the system is too good? :)

PS above for a carb. engine only.
 
It sounds like the flow is being metered by the float valve instead of the mixture valve. Time for carb cleaning or OH?
 
It sounds like the flow is being metered by the float valve instead of the mixture valve. Time for carb cleaning or OH?

But isn't it the job of the float valve to cut in and out to keep the carb bowl level even?

The fuel pump can provide much more flow than needed at cruise so the needle valve must perform some sort of metering.
 
Things to consider are air leak and carb throttle shaft wear causing internal carb air leak which effects the enrichment valve
 
Which fuel flow meter do you have and where is it installed. Both the Red cube and Floscan have some guidelines about the installation. One example that is common with both is no 90 degree bend within x inches of the input/output port of the device.
 
I have fluctuating fuel flow readings on my 0-320 and have determined that it fluctuates with g force. In smooth air the FF is always steady.
I have tried every conceivable fix but nothing helped. If your FF meter has an electronic dampening adjustment try setting it to max dampening.

Fin
9A
 
If there is no damping in the readings, could you be seeing the float chamber needle valve coming off and on?

That is what I have always thought; however, the more I am running very LOP, the more I find myself chasing engine roughness with these swings, and I see the EGT changing with them as well. - I need to study how the mixture control is actually working to better understand this.
 
My fuel flow (red cube flo-scan) bounces around when I'm LOP around 7ish GPH, steady at higher fuel flows 8 GPH and up. I always figured it was the float opening/closing momentarily, engine runs smooth.
 
My fuel flow (red cube flo-scan) bounces around when I'm LOP around 7ish GPH, steady at higher fuel flows 8 GPH and up. I always figured it was the float opening/closing momentarily, engine runs smooth.

Unless there is a problem, the needle/seat will smoothly adjust flow without this reported behavior. I never once had anything but smooth readings in the 100 hours that I had a carb. This excludes oddities, such as a failing needle/seat, leaks, etc.

I would first speculate that you have air getting into the system, causing variations in measurement at the FF sensor. The FF sensor will not report accurately when air or fuel/air passes through it, eventhough this issue gets sorted out in the carb with no noticeable performance symptoms. Vapor lock in the FP or FP performance issues can cause the same symptoms for different reasons. In this case, flow is increasing and decreasing, but not due to the needle/seat. Again, moderate fluctation can be tolerated, as the carb bowl is something of an equalizer/stabilizer.

If you are confident that above is not an issue, it could be a failing FF sensor. The orientation of the FF will not cause intermittent issues. It only affects the overall reading accuracy, which can be adjusted out in calibration.

Larry
 
My fuel flow (red cube flo-scan) bounces around when I'm LOP around 7ish GPH, steady at higher fuel flows 8 GPH and up. I always figured it was the float opening/closing momentarily, engine runs smooth.

The original post states that RPMS and EGT fluctuate along with fuel flow, this would indicate to me there is an engine running problem.
 
The original post claims engine not running smooth, RPM and EGT fluctuation which leads me to believe engine issue not instrument problem.
 
The original post claims engine not running smooth, RPM and EGT fluctuation which leads me to believe engine issue not instrument problem.

Correct, I believe the fuel flow numbers. The question is: Exactly what is the lean setting doing in the carburetor? I have searched on line and have not found a good explanation, and I have never disassembled an aviation carburetor. I assume it is just a needle valve in the fuel path to the main jet?
I am trying to understand the mode of failure for the mixture control. - Could it be mechanical slop, worn component, Vibration, ect. There are 700hr TTSN on the carburetor.
 
Filter housing

Check your air filter housing to make sure there are no issues there such as partial blockage, broken alternate air device, etc.
 
In an earlier post I suggested checking throttle shaft because when they wear it affects enrichment valve which affects mixture , this is assuming carb was working properly in the past and was jetted properly and no induction leaks .
Worn throttle shafts are common on high time carbs.
 
In an earlier post I suggested checking throttle shaft because when they wear it affects enrichment valve which affects mixture , this is assuming carb was working properly in the past and was jetted properly and no induction leaks .
Worn throttle shafts are common on high time carbs.

How are you checking this - play in the butterfly? What would be ok VS worn? I'll check it this weekend. - is 700hrs High time on a carb?
 
To check throttle shaft is just a matter of wiggling it , 700 hundred hours is not particularly high time but things like linkage geometry can affect it. When the shaft wears it causes an internal leak in carb . All this is assuming engine was running good in your installation . The main reason for checking is it's easy to check and eliminate so you can move on with your diagnosis.
Good luck, Jerry
 
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