Originally Posted by charliehows
my yanmar 2qm20
runs about hull speed
(about 7knots for 29 ft waterline) at 1600rpm according to the tach. but flat out i can get 1900 with quite a bit of black smoke. lowest idle i can get on the tach is about 550 before the eng. conks. I'm wondering if the tach is accurate given the highest revs i get without a load is 2300. Also, i have a new set of injector nozzles, black smoke makes me think i should fit them, any ideas/help?
Classically overpitched prop...
You can use a strobe to check the tach but I doubt that is the problem or if in fact therre is any problem at all.
An overpitched prop will be unable to reach max rpm
, it will drive the boat faster for a given rpm
and accelerate more slowly. When the prop and bottom are fouled it will be more susceptible to lugging and producing black smoke as the engine
can't get up on the torque curve. It also makes slow speed handling difficult as thenboat might go too fast even at idle or just off idle.
You may be able to have your prop repitched. This will give better acceleration, be less sensitive to bottom and prop fouling and may improve slow speed boat handling.
Your question about the accuracy of the tach is interesting as the engine
should reve quite a bit higher with no load. Have you physically checked that the throttle pushes to the stop on the injector pump
? Also I presume the intake is clear and the exhaust
is in good shape.