Yamaha 8A

[raymillard]

I've got an old 8A and it runs on one cylinder only. I've put new plugs in and I've soaked the bad cylinder with model diesel engine fuel to free up the rings, although the compression seems to be the same and good on both cylinders.
I've got new, properly gapped points and if I rev the engine hard it comes in on both. There are no nasty mechanical noises. Any ideas, anyone? Thanks.

[IBWET]

I know you said you checked them but;
1)If the gap in the points is to small, it'll miss at low rpm to wide and you get the reverse (I don't mean going backwards).
2)I don't know your engine ,but if it's a 2, then possibly a sticking reed.
3)Set the gap on the plug again.
4)swop the leads over, to eliminate one breaking down.

If all that fails cup of petrol and a match.....

Just kiddin.

[m chappelow]

i had same problem with a yamaha ,could be that a coil is a bit faulty ,or the plug lead has a bit of corrosion where it pushes into the coil on to the spike ,which was also corroded,i also changed the plug caps to normal motorcycle ones and that seemed to improve it ,also had same problem with a johnson and that was a condenser that was at fault even though they are supposed to either work or not ,you could try closing the gap a little on the spark plug ,i would also check the points again if they are new ,the shoe part will have worn down until it beds in on the cam so they may have closed a bit,when you took the flywheel off you dident happen to put it down on a work bench as the magnet could have picked up old filings that can cause odd running ,

[raymillard]

Thanks for the advice, folks. You were both right: closed up points, and a dodgy lt lead to one coil. Engine runs beautifully now.