ASK THE EXPERTS
The exhaust is smoking; to break the ice or not; chimney height; which hull thickness?
Q The BMC 2.5 engine in my 25-year-old boat recently started producing lots of black smoke from the exhaust. The injectors and fuel filter have been changed, and the air filter thoroughly cleaned, but the exhaust is as smoky as ever.
Immediately before the smoking started, I had just changed the fuel lift pump because it had been leaking.
I have had a compression test done: results (cylinder 1) 310; (2) 315; (3) 390; (4) 400. The boatyard is suggesting cylinder head or head gasket problems. Any thoughts? If I am looking at having the engine rebuilt, would it make more sense to replace it with something more recent?”
HELEN, Colchester, via email
A TONY REPLIES… We typically allow for a 10% compression variation between cylinders. You have 25% between the highest and lowest, which could cause smoke but I would expect it to be white vaporised fuel when cold, plus difficult cold starting and rough idling, especially when cold.
Check the valve clearances in case one is tight and leaking, further checks will involve the cylinder head being taken off, which might show an overhaul is required.
Leaking lift pumps and injector pumps can leak fuel into the sump (as can a leaking main shaft seal in the injector pump leaking, which could be misdiagnosed as a leaking lift pump) so it becomes too thin and overfull. This can lead to oil getting up between the cylinder rings and cylinders but that would normally produce a bluey grey smoke.
Black smoke is usually indicative of too much fuel or not enough air in the cylinders. BMCs do not have the best crank case breather oil separators so they can easily clog the air filters up with oil. I would replace the engine breather pipe with a longer one and lead it down into a bottle stood beside the engine, then change the air filter.
Try running it with a deck board wedged up. If the smoke then clears, you know that not enough air is getting into the engine bay.
A common cause is overloading, where something is preventing the engine reaching its full revs for the fuel being injected. Make sure the propshaft is still perfectly in the centre of the gland and that you can turn the shaft/coupling by hand, in case the shaft is running out of line and gradually seizing up.
Changing to a different engine usually involves a lot of work to line up the new engine with the shaft. It would probably be cheaper to get your engine properly overhauled – depending on what the overhaul involves. I suggest you talk to BMC experts Calcutt Boats.