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Mariner 75 misfiring

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bpratt

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"I have a 1995 Mariner 75hp..a

"I have a 1995 Mariner 75hp..always ran great. Had it since new. It sat for about 5 months then I took it out but after a 15 minute idle speed area, it would only run at full speed for a few seconds then drop down to maybe 2000 rpms...then it would pop back up for a few seconds then drop down again. Checked my water serarator and it was filthy...changed it, no help, checked spark - had all 3, plugs looked good, assumed it was clogged carbs, sent it to the shop, rebuilt carbs, still not right, even though they checked the spark also when I first brought it in, they thought it might be coils, no help. Now they are saying that it might be the 'trigger' or the computer. What do you think?"
 
"Bob, first thing I think you

"Bob, first thing I think you better find a different shop if they are "guessing" and willing to just swap out expensive parts.

As far as I can see, your motor doesn't have a computer - just a regular switch box.

It does have a low oil sensor and a temp sensor, but these appear to be wired only to display a "warning" and do not appear to restrict the engine in any way (some temp sensors will cause the rpms to restrict to avoid damage).

That means your most likely problem causing the loss of power to be somewhere in the fuel system.

If the carbs were clogged enough to require a rebuild, it's possible that you have gummed up reed valves - you should enquire if they were checked when they rebuilt your carbs..."
 
"Thanks, Graham. I have had a

"Thanks, Graham. I have had a good 'barter' relationship with the owner of this shop for years..I do his computer work. His mechanic is the one that I doubt, so I'm trying to be diplomatic. I agree with your diag at this point and I will ask about the RVs. I may have used the wrong term...the switch box you mention..does it determine when to limit firing to 1 cyl and when to go with all3...can you briefly explain that to me? Thanks again."
 
"The switch box performs a cou

"The switch box performs a couple of funtions.

The stator (alternator) produces AC voltage when the magnets on the flywheel pass. This voltage is sent to the switch box.

The switchbox "rectifies" it (converts) to DC and stores it for the ignition system to use.

The trigger, which sends the signal for each cylinder to be fired in the correct order is also handled by the switch box, which rectifies then passes the charge along to the respective coil etc.

That's it's primary function - the kill or ignition stop switch and neutral start switch (and rev limiter if applicable) is also usually wired up to the switch box.

From all I have read (and I could be missing something) all 3 plugs fire on every rotation of the flywheel - this does not appear to operate similar to some of the other versions where 1 bank fires and the other only receives a fuel/oil mix to keep the cylinders lubricated when at idle/low speed operation.

However, the voltage to the coils is somewhat less at idle compared to over 1000 rpms..."
 
"I have been told that this en

"I have been told that this engine does drop cylinders when at idle by more than one person and it would be crucial to know...how do I find out for sure...serial # ? model # ?....Also, is there somewhere I can view a manual online, with exploded views, elctrical diags, etc.?
Thanks again"
 
"Bob, I have the Mercury Servi

"Bob, I have the Mercury Service Manual for this model - and it doesn't mention one peep about dropping during idle (the way it does when discussing the V6's in their respective manuals).

Call a Merc dealer and talk to a tech???? Unless someone on here "knows for sure"...."
 
"Ok...I didn't realize you

"Ok...I didn't realize you were so sure. Well...this throws some more doubt on the ability of the tech that's working on it.....he also tells me that the trigger is about $70 and the 'computer' or switch is about $250...he thinks the trigger may just need to be cleaned and lubed since the problem is so intermittent...one minute 1 & 3 fire, the next 1 & 2 fire, then 2 & 3 fire...with only occasional 1, 2 & 3 firing.....etc."
 
"Here's the latest. They h

"Here's the latest. They have removed the flywheel and the stator and trigger look brand new, very clean, however, the ohms readings they get from it are not within the required parameters. The replacement trigger runs $120.00 and there are 2 different triggers for this engine. The mechanic can't tell me why. Any comments...?"
 
Thanks for all your help.....t

Thanks for all your help.....this problem was fixed and the solution was the switchbox (power pack).
 
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