High frequency vibration

[Callahan]

Geoff....no, the pitch of the vibration does not change with engine speed. It's just rough as hell between 4k & 5k.

 

It did occur to me "mid range" trouble....carb jetting!" Nope! Six carbs & cleanings later..... vibration. Plugs....no. leads....no. coils....no. stator.....no. triggers....no. gearbox....no. flywheel.....no. fan.....no. prop.....no. fuel....no. fuel filter.....no. fuel pump.....no. compression.....no. leaks or cracks.....no. internal.....no.

 

I could go on!

 

 

[facthunter]

I recall a DeHavilland motor in a chipmunk from NEW ran OK but oil pressure and vibration problems, eventually #1 crankpin found to have a diagonal crack right through . Only noticed when crank removed completely when it just fell apart.. The alignment of the mainbearings and the #1 conrod bearing fit kept it together. A pressed up crankshaft can have problems that cannot be found without separating the parts and CRACK testing. It could be the engine has rods and pistons that are not the same weight. These vertical twins have a rocking couple that can't be fully eliminated as they are 180 degree cranks. Still better to have matching parts weight wise. Been a bit of a saga wot!. Nev

 

 

[onetrack]

I've seen an engine that had a vibration that couldn't be found, but it was eventually pinned down to a slight twist in one crankshaft crank.

 

 

[Geoff_H]

Without a pitch change my thoughts are on something else in the engine or airframe that is vibrating at it's natural frequency when an engine frequency is near that frequency. When they dug a tunnel under my house I mapped the vibration and amplitude when the machine approached 50 metres down in sandstone. I found that the house had a natural frequency of approximately 50 hz, the tunnel boring machine actually only had very small amplitudes at this frequency. You may have something similar, maybe air ductwork or a panel. To find this you would have to put your hand on various parts and see what moves. Difficult around the Engine I admit. By the way this is only a wild guess, don't spend too much time or money on it.

 

Geoff

 

 

[facthunter]

That can happen with a twin 2 stroke. the coupling in the centre is a design weakness. Nev

 

 

[Blueadventures]

Geoff....no, the pitch of the vibration does not change with engine speed. It's just rough as hell between 4k & 5k.It did occur to me "mid range" trouble....carb jetting!" Nope! Six carbs & cleanings later..... vibration. Plugs....no. leads....no. coils....no. stator.....no. triggers....no. gearbox....no. flywheel.....no. fan.....no. prop.....no. fuel....no. fuel filter.....no. fuel pump.....no. compression.....no. leaks or cracks.....no. internal.....no.

 

I could go on!

Geoff....no, the pitch of the vibration does not change with engine speed. It's just rough as hell between 4k & 5k.It did occur to me "mid range" trouble....carb jetting!" Nope! Six carbs & cleanings later..... vibration. Plugs....no. leads....no. coils....no. stator.....no. triggers....no. gearbox....no. flywheel.....no. fan.....no. prop.....no. fuel....no. fuel filter.....no. fuel pump.....no. compression.....no. leaks or cracks.....no. internal.....no.

 

I could go on!

I once discovered on a rough running 912 UL 80hp 4 stroke (different carb to yours and not two stroke) that after cleaning I discovered in the second carb a build up of fuel muck that had hardened just above the main jet and before it enters the mixing tube. To see this area when the main jet is removed look up into the threaded hole and the muck was set hard around the furthermost inside that area. It looked a little bit like an 'o' ring. Once this was removed and I redid the other carby the engine ran perfect. It took me about six weeks of trouble shooting and parts etc to run the problem down. the muck there must have upset the flow of fuel into the mixing chamber. May be worth a try by yourself or some experienced in maintaining the carbs. This also occurred to mine after period of about 6 months that I did not fly. Also before that the engine had an every so slight miss at full rpm for which I was searching for a air leak on the intake side of things. Cheers Mike