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Old 07-11-2006, 09:53 PM   #11
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Thanks for the tip Terry, I'll look into it.
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Old 07-12-2006, 06:54 AM   #12
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Ok! it's DA so don't be worry..

like you said ,try checking the fuel tubes and the tank

and please let as know what going on
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Old 07-12-2006, 09:32 AM   #13
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Originally Posted by SJrcflyer
Thanks for the tip Terry, I'll look into it.
I'll bet you will find your solution in the following text...



This is typically caused by a positive pressure building-up inside the cowl, or the carb diaphragm vent hole(s) being exposed to direct prop blast or ram air.

The vent hole(s) is in the plate on the metering side of the carb (the side opposite from where the fuel tubing attaches). Sometimes the hole(s) is centered in the plate and sometimes it is towards one edge.

When extra air pressure (anything greater than normal atmospheric pressure) is applied to the vent hole(s) it can depress the diaphragm on the other side of the plate. As the diaphragm depresses, it opens a metering valve causing more fuel to flow through the carb, which makes the engine run rich.

More often than not, the additional pressure is not realized or noticed until the aircraft starts moving and picks up speed. It is therefore not unusual for the engine to "go rich" during the take-off roll and remain rich while it is flying if the diaphragm is subjected to extra air pressure. When the aircraft lands and is held stationary the problem goes away and the engine runs fine.

Possible Solutions:

If the vent hole(s) is facing forward and appears to be in a position that is subjected to ram air or prop blast, then installing a dam or small plate in front of the carb to deflect the air away from the vent may cure the problem. If this does not cure the problem, then try solution-2.

The idea is to let the diaphragm sense normal air pressure. This can be done by forcing the vent hole to breathe through a piece of fuel tubing that is routed to the inside of the fuselage.

Remove the carb plate with the vent hole(s) in it and solder a piece of brass tubing (3/8" - 1/2" long) into or over the vent hole so that the vent must breath through the tube. Reinstall the carb plate. Now take a piece of fuel tubing and attach one end to the brass tube and run the other end through the firewall into the fuse. The vent will now sense the air pressure inside of the fuse, instead of the pressure in the cowl.

If there are vent holes located towards the edge of the plate, then seal them with glue, solder, or some suitable material that is not affected by the fuel. Drill a new hole in the center of the plate and solder the brass tube into this hole. The diaphragm must breath through the brass tube!

Make sure the brass tube does not protrude past the back of the plate where it may interfere with the diaphragm.

Use a low temperature silver solder such as "Stay Bright" to solder the brass tube into the vent hole.

It may be necessary to seal-off the fuse at the firewall or hatch to keep the entire fuse from being pressurized.


If the engine runs steady on the ground but fluctuates while in the air, especially when changing aircraft attitude, it is a sign of erratic airflow over the carburetor intake. This airflow can change the pressure that is sensed by the carburetor venturi and can cause richening and leaning of the mixture during flight.

This condition is often present when the carburetor is sticking out of the cowl and is subject to the slipstream. The cowl will block airflow, or will cause air to be rammed into the carb, depending on the attitude of the aircraft.

Possible Solutions

An aluminum plate may be bolted to the front of the carburetor such that it hangs down approximately 1 below the carb body and blocks airflow over the venturi. If this does not work, the plate may be made longer and bent rearward so that it deflects air away from the carb venturi. However, do not get the plate too close to the venturi. Two of the screws that hold the diaphragm cover may be used to attach the plate.

Obtain a carburetor stack or velocity stack and bolt it onto the carburetor. It lengthens the air column over the venturi and will block/smooth much of the erratic airflow. These stacks typically have a bevel and can be rotated to provide a little ram-air effect. Experiment with the position of the bevel in order to achieve the best air flow/performance.
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