I have such a specific issue that I hope I can adequately describe. I have been reviewing the service manual and I was wondering if someone could help me close the gap. I have a 350 mercruiser with a Alpha one drive. Its on a 1986 23ft Wellcraft Nova.
The issue is while at cruising speed the tach is oscillating by as much as 1000 RPM, usually in the increasing direction. In addition to this the engine missing. There is a loss of power while this is occuring. I noted that when the actual engine RPM is falling due to the missing indicated RPM on the tach is rising by a large amount 1000-1500 RPM so I know the it is not actual. The condition is intermittant and not all the small tach oscillations result in a noticable loss of power. The tach was dead steady prior to this and now it dances continously. My concern is primarily with the engine missing. I can tolerate the tach oscillations if it is not real (which I can hear that actual RPM is falling vice rising) but the engine missing is intolerable. The two started occuring at the very same time so they must be related considering they are moving opposite of each other. The missing doesn't seems to happen at low RPM or if it is it is not noticable.
What is the relationship between the coil and the tach? If this is not enough info what further trouble shooting would reveal the source? As it is July Fourth I am going to try to get it out today and explore the issue, hopefully it won't leave me stranded.
One extra piece of information. I very recently replaced throttle and shift cables and my control box. The cables were adjusted per the manual. I can't make the leap between that and this issue. I considered that the throttle cable was dancing around causing this but I ruled that out by observing the thottle linkage at cruising RPM and it is not moving. Additionally this would not adequately explain the tach increasing while actual RPM was falling. I was thinking that this information may be helpful though, just because I can't make the leap doesn't mean that someone else may not be able to.
Thanks for the help.
The issue is while at cruising speed the tach is oscillating by as much as 1000 RPM, usually in the increasing direction. In addition to this the engine missing. There is a loss of power while this is occuring. I noted that when the actual engine RPM is falling due to the missing indicated RPM on the tach is rising by a large amount 1000-1500 RPM so I know the it is not actual. The condition is intermittant and not all the small tach oscillations result in a noticable loss of power. The tach was dead steady prior to this and now it dances continously. My concern is primarily with the engine missing. I can tolerate the tach oscillations if it is not real (which I can hear that actual RPM is falling vice rising) but the engine missing is intolerable. The two started occuring at the very same time so they must be related considering they are moving opposite of each other. The missing doesn't seems to happen at low RPM or if it is it is not noticable.
What is the relationship between the coil and the tach? If this is not enough info what further trouble shooting would reveal the source? As it is July Fourth I am going to try to get it out today and explore the issue, hopefully it won't leave me stranded.
One extra piece of information. I very recently replaced throttle and shift cables and my control box. The cables were adjusted per the manual. I can't make the leap between that and this issue. I considered that the throttle cable was dancing around causing this but I ruled that out by observing the thottle linkage at cruising RPM and it is not moving. Additionally this would not adequately explain the tach increasing while actual RPM was falling. I was thinking that this information may be helpful though, just because I can't make the leap doesn't mean that someone else may not be able to.
Thanks for the help.
