Loud metal on metal noise under acceleration

[Tools99]

Got my 85 FJ1100 out on the road today for the first time since buying it. Restored all master cylinders and slave cylinder rebuilt, all fluids changed and carbs rebuilt, new plugs etc. idles great, pulls clean with plenty of power, but loud clacking noise upon acceleration like a mini jack hammer is the best way I can describe it. Definitely metal on metal seemingly each rotation of at least one piston. any ideas?
Timing chain..? Valves? - Nipped up the header bolts so I'm confident it's not that.

[Motofun]

This will sound weird but it happened to me.  The '85 FJ has double walled exhaust pipes that are prone to internal rusting.  The internal pipe can separate and it then bangs against the external pipe.  I thought I had a rod knock.

[ribbert]

Got my 85 FJ1100 out on the road today for the first time since buying it. Restored all master cylinders and slave cylinder rebuilt, all fluids changed and carbs rebuilt, new plugs etc. idles great, pulls clean with plenty of power, but loud clacking noise upon acceleration like a mini jack hammer is the best way I can describe it. Definitely metal on metal seemingly each rotation of at least one piston. any ideas?
Timing chain..? Valves? - Nipped up the header bolts so I'm confident it's not that.

Well, from your description I would have said start by checking the header bolts, but you've done that. A leak there would fit your description, clacking sounding, consistent with one cylinder per cycle and noise relative to load.

I know you nipped up the header bolts but seeing how it was there on the first ride, is it possible there's a flange gasket missing from one cylinder or it's pulled in crooked?

Noel

[FJmonkey]

The 84/85 header pipes are double walled. When one separates, it sounds like a major internal engine failure.

[T Legg]

I think the valves would be a good thing to check . You should also check your ignition rotor and make sure it isn't hitting the trigger coils .

[Pat Conlon]

Yep, also happened to me on my '84....the inner wall on my header tube cracked and started flopping around....sounds like hell.  It seems as though all the 84/85 FJ's will eventually suffer this fate.

Good news, all the single wall 1986 to 1995 FJ header tubes are interchangeable (plug and play) and they are stainless steel so they polish up nicely. If you buy a set of 4 make sure they are a set. On the bottom, the header tubes are stamped with numbers 1 thru 4 to correspond to the cylinders they serve. They are not interchangeable between cylinders.

Cheers.   Pat

[Old Rider]

If it is the inner tube that has loosened you can fix it by drilling a hole and insert a pop nail .I did that on the 1100 i had before

[Tools99]

This bike was bought with a 4into1 system.
Would this issue still occur in that case?
I know this issue was with the stock system...

[Pat Conlon]

Nope, the 4 into 1 system is single wall. Check to see if the header tubes are loose.