Ro-Jett76 with a side muffler - venturi diameter

[Matt Piatkowski]

Hello,
I will be breaking this engine next week, using a 12x5 two-blade Master Airscrew propeller and 5%nitro, 22% fully synthetic oil (KLOTZ) fuel.
What venturi diameter is recommended for breaking?

To fly the pattern, I will use 13 x 6.3, 13.4 x 6, and/or 14.2 x 6 wooden CNC propellers from Ukraine.
What venturi diameter is recommended for flying?[/b]

Thank you,
M

[Dave_Trible]

Matt I’ve been flying my (3) RO Jett .76’s for about five years.  I break them in on the original stock venturi which is about .180 I think.  10-6 wood prop so you get a good rpm while still in a cool four cycle.  The last 1/2 hour I go to my flight size venturi.205,  flight prop 12.5x 6 ish three blade carbon, and 10% nitro, 23% oil-50/50 Klotz/castor fuel.  These are NOT high rpm motors so resist trying to run it like that and huge props aren’t nessessary or desirable.  They just kill your turn rate, cause more trim issues and can overheat the engine.  It will still pull any airplane you can attach it to.

Dave

[Matt Piatkowski]

Hi Dave,
My Ro-Jett 76 venturi diameter is 0.202-0.205".
I have received this black, nylon venturi with my engine from Dub Jett and never touched it.

I will use this venturi, unmodified, to break the engine with a 12x5 two-blade Master Airscrew and 5% nitro, 22% fully synthetic KLOTZ oil and will report the results on this forum.

I know that Ro-Jett76 is a low RPM, high torque engine.

The take-off RPM for flying the Pattern I am aiming for is the 7,000 -7,500 RPM range with 13 x 6.3, 13.4 x 6, and/or 14.2 x 6 wooden CNC propellers from Ukraine.

Thank you for your suggestions and opinions,
Regards,
M

[Brett Buck]

The take-off RPM for flying the Pattern I am aiming for is the 7,000 -7,500 RPM range with 13 x 6.3, 13.4 x 6, and/or 14.2 x 6 wooden CNC propellers from Ukraine.

    That sound safe, but 7000 rpm for a launch RPM with relatively light props and a .205 venturi sounds like it will be pretty slow.

     Brett