Whats causing misfire on volvo penta 250 4cyl engine?

Question

it was running fine and just started missing and losing power,i noticed the rev counter started jumping from one side of the scale to the other so thought it must be an electrical misfire,i have changed the plugs,leads,distributer cap,points and rotor arm,to no change! Even when sat out of gear if you rev it past 2000rpm the rev counter jumps all over the place and the engine starts missing,it will tick over all day long.I havnt changed the condenser as it is located in a c**p place and i would need to remove the whole distributer to change it,could it be this at fault?? I have also cleaned the carb jets and fitted new fuel filter although it was new anyway,just to satisfy myself that it isnt a fuel problem.just the other week i had the plugs out and they were a lovely light brown colour,now they are very sooty and black with unburned fuel.In fact there is quite a whiff of fuel from the exhaust when you rev it as the engine isnt burning it correctly.Any ideas will be welcome.

Answer 1

i have found that when the revcounter starts to break down it can cause a misfire and even stop it runing try disconecting the revcounter in see if the misfire stops

Answer 2

Does it have a vacuum operated advance/r****d?-if so it could be the pipe off or broken somewhere between the inlet manifold and the distributor-drawing air. That dosn`t account for the rev.counter problem though-that sounds electrical. I assume the rev.counter is driven from the distributor and not a magnet in the flywheel and a reader  in the bellhousing? It`s a process of elimination really- you might have to change that condenser yet!

Answer 3

I would look for a loose ground strap, chafed wires, shorts.

Run it at night in the dark and look for arcing around the spark plug wires.

You might also check your separator filter and/or dip the tank to check for water.

Answer 4

The previous answer is a good one, also check to see that the coil wires and tach wires have not chaffed through and shorting out.

I was going to say a weak points spring, but I see you changed those.

Have you checked the timing and ignition advance? A worn distributor can also cause this.

Answer 5

my nephews boat was running like c**p and theres a pickup in the fuel tank and it was clogged (food for thought)

he cleaned that out and it runs like a champ