I have a 1997 Yamaha TRX850.
When I open the throttle I'm hearing a thud knock sound near the left crankshaft cover. That sound does not always appears and I can't find any correlation between appearances.
Clutch and speed of opening the throttle didn't affect that sound.
In the meantime, the engine works perfectly well and has never been opened since 1997. I have changed the oil and antifreeze recently and seems like the motorcycle in a very good condition.
Is that OK for a TRX?
If not, what should I check to get rid of that sound?
Maybe it would be better to not open the engine if it works well?
The sound appears only once in the beginning of a process of opening the throttle. So the flow is as follows:
engine idle working -> beginning of opening the throttle -> BANG -> continue the process of opening the throttle (no thud knocks further)
The thud knock sound happens simultaneously with the roar of the engine only once.
The sound is very similar to the sound of smashing two heavy steel hammers together except it more thud. I discovered it only after the 4 months of owning. It's not so easy to hear because of the engine's roar. Only if trying to hear it specifically.
Here is the link to video https://www.dropbox.com/s/55omtvm9bpccy5e/20160419_175448.mp4?dl=0
That sound appears on
Hope you will recognize it