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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.

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