Originally Posted by
Sore arms
So it idles smoothly, but goes rough when you rev it?
Did the problem appear suddenly on an otherwise healthy engine or has it been sitting in the shed untouched for the last 20 odd years?
If it was running well before, my money would be on a blocked main jet.
The following is what I would be trying first if it was my engine.
Pull the fuel bowl off the carby of the bad cylinder and pull the jets from the carb and clean em out. You wont even need to pull the whole carb if you have room to get the bowl off in situ. I wouldn't bother with further disassembly just yet. If you do need to pull the carb to get the jets then you might as well pull the reed valve out and make sure its not cracked/chipped etc and there's no crap stuck anywhere.
Reassemble and run a short piece of hose from a funnel straight to the carbs, pour some fuel in and run it straight off that.
If that doesn't fix the problem then do a quick spark check.
Sometimes ignition components will bench test fine, but break down under the load of combustion pressure. Hit all three cylinders one at a time with your timing light. If they are all still firing and at the correct time while revving it then it's probably not an ignition issue.
Try that first and let me know how you get on, more often than not these problems are something very basic.
Just my 02c.
Alex.