Mercruiser runs rough SOMETIMES- but not always

KevinKnuth

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Jun 20, 2011
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10
I have a 4.3L Merc in a 2003 Bayliner. My engine is NOT fuel injected.

Here is my issue: Last year the tach quit working (i only mention this because I have read in other threads that the tach could point to an issue). The boat, which ran fine the year before, was running REALLY rough. You could smell gas. I took it to a marina and they replaced the distributor and cap. It seemed to run okay after that (though the tach still does not work).

This year the boat is running rough, but I have not smelled gas. It also seems to rev higher on its own sometimes- so I can be cruising along at 10mph and it will all of sudden rev up to maybe 15mph for 30 seconds or so, then back down. I cannot get anywhere near my usual top speed of 42Mph- limited to about 30mph.

Any suggestions?
 

searay3

Senior Chief Petty Officer
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Apr 7, 2005
Messages
655
Re: Mercruiser runs rough SOMETIMES- but not always

Is the distributor cap and rotor the only maintenance done on that boat in the last couple of years? If the engine and boat are of the same year, it's been seven years on the plugs/wires/fuel filters etc. Try a full tuneup with plugs and filters, fresh gas and see if it does any better. If it still runs erratic, you need to rebuild the carb. Also, get the tach working. Even if it means replacing....you need it for ballpark performance info like idle speed and wot rpms.
 

KevinKnuth

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Jun 20, 2011
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Re: Mercruiser runs rough SOMETIMES- but not always

Great suggestions. I did replace the fuel filter last summer. Was planning on plugs and wires this weekend. My mechanic friend thinks plugs or wires may be the issue.

Thanks for those ideas- anyone else???
 

81 Checkmate

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1,360
Re: Mercruiser runs rough SOMETIMES- but not always

Sounds like a fuel issue not am elect issue.

Did you change the water/fuel seperator filter also?

A elect issue would lead me to it happening all the time.
A surge issue would be fuel related.

Just my thoughts!
 

KevinKnuth

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Re: Mercruiser runs rough SOMETIMES- but not always

I replaced the water/fuel seperator last year- I can do that again- they are cheap! It did run like it had a moisture problem at the beginning of the season, so I put in some fresh gas and some sea foam.
 

KevinKnuth

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Re: Mercruiser runs rough SOMETIMES- but not always

To clarify- I have done more reading on the forum- seems my faulty Tachometer COULD be causing a problem? yes/ no?????
 

dubs283

Vice Admiral
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Jul 27, 2005
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5,058
Re: Mercruiser runs rough SOMETIMES- but not always

seems my faulty Tachometer COULD be causing a problem?

its possible, yes - to eliminate it, disconnect the grey wire from the coil (-) side

definitely still do a FULL tune-up, spray down carb and choke and do a carb adjust
 

Pete104

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Apr 30, 2011
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1,439
Re: Mercruiser runs rough SOMETIMES- but not always

Disconnect it! Probably not. Finish your tune-up.

Oh, clean the flame arrestor, also!
 

KevinKnuth

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Jun 20, 2011
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Updated problem- Is my tach causing this??

Updated problem- Is my tach causing this??

Here is an update to an earlier thread. Initially I wrote this: I have a 4.3L Merc in a 2003 Bayliner. My engine is NOT fuel injected.

Here is my issue: Last year the tach quit working (i only mention this because I have read in other threads that the tach could point to an issue). The boat, which ran fine the year before, was running REALLY rough. You could smell gas. I took it to a marina and they replaced the distributor and cap. It seemed to run okay after that (though the tach still does not work).

This year the boat is running rough, but I have not smelled gas. It also seems to rev higher on its own sometimes- so I can be cruising along at 10mph and it will all of sudden rev up to maybe 15mph for 30 seconds or so, then back down. I cannot get anywhere near my usual top speed of 42Mph- limited to about 30mph.

Any suggestions?

I did get some suggestions, including that it was worth unhooking the tach (grey wire at the coil). But when I do this, the boat will not start. Hook it back up, and it starts fine....unhook it, it will not start.

The tach has not worked since last year, when I started having some issues..

Question- shouldn't I be able to unhook the grey wire with no issues? Why can't I???
 

Don S

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Aug 31, 2004
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62,321
Re: Updated problem- Is my tach causing this??

Re: Updated problem- Is my tach causing this??

I merged both of your posts together, please don't start two different threads on the same subject. This is just a continuation of your original problem.
 

Don S

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Re: Updated problem- Is my tach causing this??

Re: Updated problem- Is my tach causing this??

Question- shouldn't I be able to unhook the grey wire with no issues? Why can't I???

I have no idea why you can't, why can't you? wrench to big, can't reach it, what?
 

KevinKnuth

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Joined
Jun 20, 2011
Messages
10
Re: Updated problem- Is my tach causing this??

Re: Updated problem- Is my tach causing this??

No, I can reach it. When I unhook it the boat will NOT start. Hook it back up, and the boat starts...but stills runs rough.
 

nikogto

Seaman
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Apr 25, 2010
Messages
56
Re: Mercruiser runs rough SOMETIMES- but not always

Did you replace the plugs yet? I had a similar issue with my 260 Merc. Would not run rough, but would take a lone time to plane out and it would also speed up for a few seconds then slow back down, I found #7 plug damaged, replaced plugs and it's back to the way it should run. It sounds like an ignition problem to me the way your rpm's speed up and back down again.
 

Don S

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Messages
62,321
Re: Mercruiser runs rough SOMETIMES- but not always

If you only have a single gray wire on the coil hook it up, and disconnect the gray wire from the tachometer itself, not the coil.
 

KevinKnuth

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Joined
Jun 20, 2011
Messages
10
Re: Mercruiser runs rough SOMETIMES- but not always

Problem solved! I replaced my spark plugs- turns out 2 of them were cracked!

Thanks for all the help guys!
 
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