Raflex

Dear all,
I have a Carf models Yak 55 2,6m and I have done several flights with it. My problem is that on the ground my engine is working perfect but during take off I notice that the engine is going rich and stay rich. I decide to weld a brass pipe on the diaphragm hole on the carb and extend with a hose to a balsa box for breathing.
My question is, do I have to open holes to the balsa box?
The dimensions of the box are 7,5x5,5x2,5cm.The thickness of the balsa is 2mm and the box is located in the fuselage to be far from cowl airflow.
Please advice. Dead stick is not an option for any of us.

Thanks in advance.
Alexandros

Lifer

My Feedback: (1)

It will pulse as the engine develops positive and negative pressure pulses. As long as the volume of the box is equal to, or greater than, the volume of the diaphragm's chamber, you will be okay. I have this setup on all my pump equipped engines but I simply run the tube into the fuselage cavity.

All Day Dan

My Feedback: (5)

Make sure the carburetor intake is not too close to the cowl. Also, try flying without the cowl to see if that is the problem. Dan.

speedracerntrixie

My Feedback: (29)

This issue really has nothing to do with cowl pressures. What is happening is that you are adjusting the engine to what you see on the ground and not what you are seeing in the air. Adjust to what the engine is doing in the AIR. With forward motion the prop imposes less load on the engine and the engine gets better cooling so of course it is going to run rich as opposed to being on the ground. This is one of the reasons most guys running these larger gassers don't have their engines running on the ground for extended periods. Lean the H and L needles just a little bit at a time. It sounds like this is a new engine and this will also get better with some run time. Too much oil can contribute too. With a new DA you should be running 40:1 petroleum oil for 3 gallons and then you can switch to 50:1. I personally stay away from the synthetics usually however I have been running Lucas semi synthetic on a new 150cc and have been very happy with it. Just make sure you are running a quality oil and in the correct ratio.

Truckracer

My Feedback: (19)

I've had similar problems with twins when the bottom mounted carb has the regulator diaphragm facing forward into the airstream. As airspeed increases the increasing pressure on the diaphragm causes the engine to go rich. You will also see the same problem anytime the airspeed increases such as on down lines, etc. I use the balsa box trick and use one small pinhole in the box. Has always worked for me.

speedracerntrixie

My Feedback: (29)

I can't say for sure on this particular install but on all Carf Yaks I have seen the kit includes a block off plate that installs into the cowl that only has openings for air to enter in front of the cylinders. If using this plate he should not be getting direct airflow on the diaphragm.

Truckracer

My Feedback: (19)

That would eliminate the airflow problem! He could still be having a cowl pressure issue but I certainly agree with your earlier post, could just be needle settings. Then it would just be an issue of whether the needle settings stay constant at different airspeeds.

Raflex

Dear friends hi,

First of all i would like to thank you for your replies,
What i will do in combination to your experiance are the followings.
First of all , i will open a Pinhole to the balsa box that is located in the fuselage area to be on the safe side.I know that also in the fuselage will be pressure coming from the openings of the throttle and chock servos but for sure it will be a little.
If this is not enought ,i will adjust the needles from the beggining based on the flight perfomance of the engine.

You see here in Greece the temperature is 30-35 C (85-95F) and i beleive that is better to wait for the final tuning during Autumn.
The engine were broke in with BellRay petrolium based oil Plus Red line additive 32:1 for about 4 gallons and now is working with Red line synthetic 40:1 for the last 4 gallons.

I never messured temperature more than 108C ( 225F) and that is because during take off i feel the engine going rich...

I will fly tomorrow and i will give you the results.

Thanks again/

Alexandros

Raflex

Dear friends hi,

I went flying yesterday to check the engine and the results were great.
No *going rich* symptoms during take off any more, the engine needed some more trim on the needles because it behaved like a different engine in the beggining but after that everything was fine.
I realy recommend this idea to all giant Yaks with open cowl.

Thanks again.

Lifer

My Feedback: (1)

Glad it worked out for you!