Animal engine vibration, help needed

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yalipito

Member
I have an animal engine that is basically new (about 15 minutes of run time after initial warm up). I noticed a strong vibration at about 5400 rpm, above that it is very smooth. The chain was disconnected to isolate the problem and it is the engine. It is so bad that the handlebars are uncomfortable to hold (engine is on a minibike). Is vibration a normal thing with these engines? Thanks

Yalipito
 
Realizing this is a dirt racing Kart forum.
Need better info.
Stock or modified?
Typically no vibration issues.
 
All parts are new, oil is still very clean (zero visible parts wear in the oil) therefore I don't think wear is the problem. The head was cut .060" and valve/piston clearance was checked and it was plenty. Thanks and keep the thoughts coming.
 
You removed chain to isolate problem, is the clutch still on crank ? If so what style, and did you remove it and inspect -may be out of balance. Be Careful and remove flywheel side cover and run. Check for wobble on that side also. Lastly the animal must be bolted at all 4 corners and flat to its mount. Hope some of this helps. Never ever had a Animal Vibrate. Keep us posted. -Brian-
 
You had the head cut .060"? What thickness head gasket did you use, and how much piston pop-up was there? Sounds like the cylinder head / piston "quench" is VERY small. At .060" you've completely removed the .020" lip from the combustion chamber and the biggest area/diameter of the chamber. With a .050" head gasket and even with .000" piston pop-up, you've got a pretty small chamber left. If you've left the stock jetting, I would think that it would vibrate with the extra compression. The fact that it clears up at a higher rpm is relieving -- remember it's still a single cylinder engine -- balancing them is not easy, and generally to a specific rpm. It could be a balance issue, but I doubt that it is the flywheel or rotating assembly if they are all still stock. The balance of the engine from the factory would be best at 34-3600 rpm.


--
Thanks and God bless,
Brian Carlson
Carlson Racing Engines
Vector Cuts
www.CarlsonMotorsports.com
Celebrating 25 years of service to the karting industry
765-339-4407
bcarlson@CarlsonMotorsports.com
 
I've cut .060" or more from the head and never had it cause a vibration issue.
At zero Pop up and a .047" (average used) head gasket thickness. You have .047" clearance between the piston and the head. Plenty of room.
Cutting .060" from the head gasket surface will give you about 27 cc for the combustion chamber with the head gasket.
If using a stock stroke you still have a pretty low compression ratio.
 
Thanks for the replies. I didn't understand why some thought that higher compression could cause a vibration. Anyway I guess I should have mentioned that the vibration is there with or without the clutch and is static (vertical) and not horizontal. Like I said before at higher rpm it is very smooth. Thanks to all for your help.
 
Here is a follow up. I swapped the flywheel and that made no difference The crankshaft is the only new part in the rotating assembly. Is it possible that it was not balanced properly? Does anyone here balance them and to what rpm are they spun? Thanks
 
Just guessing really, but I would think the factory would balance the crank to the rpm range where the engine would most likely be run in everyday use. I'm thinking about 3600 RPM. At all other RPM ranges, the stock engine would be, out of balance.

When the harmonics, (vibration) of two attached parts synchronize, the perceived vibration can be enhanced. Or, just as easily, they can be nullified.

Have you checked the run out of the crankshaft? What surprises me is; above the point of the worst vibration, the vibration goes away. Not what you would expect from a single cylinder engine.

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.(Al Nunley
 
Myself, I would take it out and bolt it to something very stable and run it to see if it is the environment or circumstance it is currently mounted in.
 
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