MicroMac,

I have made some very similar experiences as you have with the MVVS 91. I use this engine in a CMPRO Leo with a Hatori 700 tuned pipe and a APC 14x8 prop. For fuel I use 16% synth oil and 5% nitro. After 4 flights and some 2 litres fuel on the break-in test bed performance is still not at its max.

First of all it seems to me the MVVS engine needs quite some time to break in before it a) develops good power and b) runs reliably. My previously used MVVS 61 engine in the very beginning quit many times in flight and developed it's power only after about 5 litres of fuel had gone through it. I admit that I also had some difficulties to set up the carb.

What considers the perry pump: I had exactly the same problem. I needed to rev out the screw of the pump as far as possible and still had to close the main needle of the engine to about 1/2 turn open position. Otherwise the engine would completely drown in fuel. Then again it was very difficult or lets say impossible to adjust the idle needle. A fellow at my club said, that the MVVS carb is probably not suitable for use with a pump. I therefore removed the pump for the time being and changed the tank's position to behind the firewall which was quite difficult in the Leo.

I do not know about your KS pipe but I have my Hatori pipe set up at about 490-500mm. Maybe it's worth to try to reduce the distance carefully by 1 or 2 centimetres on the test bed and look at the revs.

What regards the OS 91 FX: I also used this engine for quite some time in a Swallow with very good results. It did not need any particular break-in time to develop fair power and was 100% reliable. I just changed to the MVVS because of the rear exhaust mode. Maybe we have to be patient with the MVVS engine.

Jan