Rojett76 vibration

[Steve Bakac]

Hi guys
  Quick question I have a bad vibration in my new rojett 76 . It's had  over 2 litres of fuel through it with a 11/5 apc ,with the 11/5 had slight vibration but with the eather 14/5.5 two blade near shakes the model apart . I have rechecked balance of everything props spinners all good.plus it is four two  at only 6600rpm and it is only tacking out at 7300 rpm this doesn't sound right. Set up Venturi .187 fuel 15 syn 5 castor 10 nitro. Thunderbolt idle bar. Motor was very tight first up could it be still to tight. Any ideas ?
Regards Steve

[Tom Niebuhr]

I suggest that you talk to Richard Oliver.

[Dave_Trible]

Pretty unusual for this engine.  I'd suspect a bent or defective crank or rod.  It's a big torque machine but not a vibrator.  I'd email or call Dub Jett for his thoughts.  Also maybe a bad bearing.  I do have one that needed a bearing replaced that corroded.  Dub is adamant about running them out dry,  leaving no fuel in them to cause corrosion.  As a matter of opinion I feel your fuel is light on lube.  My Jett's run best for me on 23-24% oil,  50-50 mix of castor/technoplate and 10-15% nitro.  They start losing good running characteristics for me with fuel heavy on synthetic.  I also lean toward smaller props though I know the .76 will turn a rather large tree.

Dave

[Keith Miller]

Just wondering if the vibration is present on a test stand or different airframe?  Maybe the airframe you're mounted to amplifies the modes at certain frequencies.  I've had well-balanced electric motors hit a resonance on a particular airframe which was cured by stiffening the structure.

[Steve Bakac]

Thanks for the replies
 Dave I will try your mix and a smaller prop I have a 13/5 three blade
Keith if fuel n prop doesn't work I'll stiffen nose up
Tom and then I'll email Dub if still no good
Thanks again and I'll post what I have found
    Regards Steve

[frank williams]

Steve,
Take the back plate off and see if there might be a small crack in the connecting rod at the crank pin end.
Frank

[Randy Cuberly]

Thanks for the replies
 Dave I will try your mix and a smaller prop I have a 13/5 three blade
Keith if fuel n prop doesn't work I'll stiffen nose up
Tom and then I'll email Dub if still no good
Thanks again and I'll post what I have found
    Regards Steve

If this is happening as you say then you definitely should not run it.  Call Dub and tell him what is happening.  I have one of these and it is not a vibrator.  In fact it is a very smooth runner on the bench.

Do as Frank Williams suggested and pull the back cover and look for something bent or cracking...if you have the resources check the runout on the shaft at the front.  Also these engines use a large set screw in the front of the shaft to hold the prop...make sure that set screw is not bent!

My RO Jetts do not like much castor oil in the fuel and run best on about 18 % total oil with only 3% castor and the rest Klotz.

In fact my 65 runs very poorly on 20% half and half fuel.  

I believe Brett runs a similar mix of synthetic in his RO Jet 61's.  In fact I think that's where I got the info for my fuel problem from!

Randy Cuberly

[John Leidle]

  I opened my venturi up to .205" & for sure mine is the smoothest engine I've ever had . Like an electric motor.
  Good Luck , John

[Dave_Trible]

  I opened my venturi up to .205" & for sure mine is the smoothest engine I've ever had . Like an electric motor.
  Good Luck , John

John I also use .205.  I had to go with that for the .61s in Muncie where there seems to be no oxygen sometimes.  It works great in the .76. 

Dave

[John Leidle]

   Dave , I felt by the way mine ran a  .189" was a little small & since you had told me you ran a .205"  I tried it & I like it better.  Thanks, John

[Steve Bakac]

G/day guys
   Just a update on my motor is running like a clock now ,problem was when I tighten the prop nut up it tighten the prop stud with it which pulled off line on thread .so thanks for the replies n to mark
 Regards Steve