knock on start up

Hey guys as always I've been out on the XJ700. In the past week on start up after sitting all night it knocks. Just quickly for maybe 3 or 4 seconds on start up. I'm using 10W-40 oil which is clean and filled to the proper level. Everything on the bike is tight and secure, no exhaust rattles, leaks...etc...

thoughts, suggestions, ideas.....help?

 

By the way it only has 7,6xx original miles.

 

What kind of a knock? Top end or lower end?

 

Well it only does this for 3 seconds tops and only on cold start up so I don't have all the time in the world to check it out. I'm 99% sure it's the piston/rod assembly. When I'm sitting on the bike it's the far left cylinder.

 

I'm curious how cold it has to be when it knocks.

I could see where a pin bearing that was too loose might knock until the oil pressure comes up, but I'd expect it to do that after 10 minutes of sitting hot. Also, I'd expect the knock to be back when it's good and hot.

Are you sure your fuel valve is set to "ON" and that the float heights are set properly and floats valves not leaking? I could see a situation where you get raw fuel running into a cylinder while it sits, then raising compression so high it knocks until the extra fuel burns off. You want to be sure about this one because enough fuel in there can break a rod and create real big headaches.

Also, that first few seconds tends to be rough running. I'd check to make sure I didn't have anything loose (exhaust collar) that rattles with that roughness.

 

I'm thinking you might be right about extra fuel leaking into that cylinder because it is the one that is lowest when the bike is on it's kickstand. The fuel is to the ON position as normal.

Could the cause of this problem be a leaky fuel petcock?

edit: for how cold it has to be when it knocks, it just has to sit overnight. I don't think it's a temperature thing but more of a time thing as it only does it after a certain number of hours of sitting.

 

Run the bike and get it up to temp.

While the Engine is running ... disconnect one Plug at a time and see if the knock goes away.

If you have Piston Slap, Wrist Pin problems or a Big End Bearing that's the offending item causing the knock ... It will become "Quiet" when the Plug Cap is pulled and there's No Spark for a moment.

 

i can't do that, the knock goes away in a matter of seconds. It only happens when the bike sits overnight. I start it and it goes away in less than 5 actual seconds. Then it won't knock again unless I let the bike sit for hours and hours again. After it knocks once in the morning I can re start it all day and it's fine.