50 hp merc problem

wvgale · Aug 12, 2008

we have a 1 1/2 year old 3 cyl 50 hp merc.

it will run fine for about 1/3 of a mile and then drops out the bottom cylinder. we have changed spark plugs and swapped the cdm with the top cyl, but the problem stays with the bottom cyl.

it will kick the bottom cyl in briefly but then drops back to 2 cyl help !!!!!!!!

JB · Aug 12, 2008

Re: 50 hp merc problem

What happens if you squirt some fuel mix into the bottom carb?

wvgale · Aug 12, 2008

Re: 50 hp merc problem

Haven't tried that, but it sounds like a good idea. This motor is on a 15 foot boston whaler with a fiberglass 18 gallon tank using gas with ethanol in it which I am told can create problems ?????

lots of boats · Aug 12, 2008

Re: 50 hp merc problem

did you check to see whther the bottom plug was brown/white in otherwords hot and dry, or black and sticky?

wvgale · Aug 12, 2008

Re: 50 hp merc problem

not yet, what would one or the other mean. I assume hot and dry would mean spark and no fuel and the other would mean fuel and no spark ??????

we're pretty sure it has spark by using a spark tester in line with the plug and at least at idle the tester glows - hard to tell at speed

Silvertip · Aug 12, 2008

Re: 50 hp merc problem

Yup -- another uninformed tech blames it on ethanol. If you have spark, it is a fuel problem. A stuck float can prevent fuel from getting to the bottom carb. Not an ethanol issue. Varnished jets or debris in the bottom carb can prevent proper fuel flow. Poor fuel system maintenance (lack of use of a fuel system treatment like SeaFoam or Stabil) can cause this. Again -- not likely an ethanol issue. Do the tests described above and read what the plugs are telling you.

wvgale · Aug 12, 2008

Re: 50 hp merc problem

thanks, we'll do that. keep in mind that it always runs fine for about 1/3 of a mile before it acts up.

also its not just the ethanol, but the combination of ethanol and a fiberglass tank that has been pretty well documented to cause problems. we are told the ethanol will cause the fiberglass to soften and create a gummy mess. Not at all sure this is related but it seems like it could be.

wvgale · Aug 13, 2008

Re: 50 hp merc problem

wELL THE MYSTERY CONTINUES - WE TRIED SQUIRTING FUEL INTO THE BOTTOM CARB AND THE ENGINE SPEEDED UP NICELY, WE ALSO SQUIRTED INTO THE TOP CARB AND THE ENGINE SPEEDED UP NICELY ALSO WHEN WE PUSHED THE CHOKE THE ENGINE SPEEDED UP NICELY. FUEL PROBLEM, RIGHT???
HOWEVER WE DISASSEMBLED EACH CARB AND THEY ARE PERFECTLY CLEAN.
(HAVEN'T PUT IT BACK TOGETHER YET TO TRY IT)
ALSO PER ONE SUGGESTION, WE CHECKED THE PLUGS, TOP TWO CLEAN AND DRY, BOTTOM ONE DARK AND OILY ?????
NOT SURE WHERE TO GO FROM HERE ????

This_lil_fishy · Aug 13, 2008

Re: 50 hp merc problem

Caps lock please...

Are you sure this is a 1.5 year old motor? Two stroke?

Sounds like a carb issue, possibly the floats or needles and seats need replacement. That would be my first guess as well. FYI the old fuel lines do degrade with ethanol fuel, it turns them to goo. I experienced this first hand, the root of the problem isn't the ethanol though..it's a lack of maintenance, and old fuel lines should be changed anyhow. I used automotive fuel injection hose for replacement, I figure if it will last 20 years in my truck, should be good to go on my marine engine.

Ian

wvgale · Aug 13, 2008

Re: 50 hp merc problem

This motor was installed new (50 hp 2stroke 3cyl) in spring of 2007 and has been seldom used - has maybe 50 hours on it. its my understanding that hoses etc. in motors of this vintage are made to take alcohol.

we just put the carbs back on and will be trying the motor again in a day or so. I'll post the results.

This_lil_fishy · Aug 13, 2008

Re: 50 hp merc problem

Huh, didn't know they still made that motor. Anyhoo...my guess is still a fueling issue. Possibly from a lack of use, the carbs may just be gummed up a bit.

Ian

wvgale · Aug 13, 2008

Re: 50 hp merc problem

We just took the carbs off and disassemled them. they were spotlessly clean. They are back on now and we will try it in a day or so.

it definately acts like a fuel issue, but why is the bottom plug darker and oilier than the other two ???

It will run at 5500 for about 1/3 mile and then drop back to about 4000. if we take the bottom plug wire off it still runs 4000. if we take either of the other plug wires off (with the bottom back on) it won't get on a plane. If we hit the choke while its only running 4000 it will jump back up to 5500 for a short time (until the extra gas is burned)

we thought sure we'd find a problem in the carbs, but they looked perfect ????

JB · Aug 13, 2008

Re: 50 hp merc problem

The carbs can "look perfect" when the orifices into the throat are badly gummed up. They are almost never cleaned thoroughly until and unless they are specifically targeted. I don't think that is your trouble, though. Problems like that don't wait a few minutes to manifest themselves.

The fact that the bottom cylinder runs for a while, then quits and comes back on when enrichment is introduced suggests a problem keeping that bowl full. I would take a close look at the float setting and the needle valve on that carb, and at the fuel supply ability to supply enough fuel for all carbs at WOT.

wvgale · Aug 13, 2008

Re: 50 hp merc problem

Well I have a question. does each carb specifically feed each cylinder or do they go into a common manifold and then feed all cylinders. I'm pretty sure they are cylinder specific, but thought I'd ask an expert ?????

wvgale · Aug 13, 2008

Re: 50 hp merc problem

Well, as of now, it seems to be fixed. I'm writing all this from florida while my brother is doing all the work in new jersey.

the last thing he did was take the bottom carb off again and hand twist a small drill thru the high speed jet and he said he thought there might have been some debris in there. In any event he put everything back together and tested the boat and it seemed to run fine. we'll run it some more tomorrow to be sure

lots of things don't make sense, such as why it would run about 1//3 mile before acting up if a jet was clogged, but why argue with success (or at least seeming success) I'll post a final chapter after more extensive testing.

thanks to all who helped

merc_man · Aug 13, 2008

Re: 50 hp merc problem

I read through the answers and was wondering if you tried checking the spark in the bottom cylinder to see if you have spark after you run it the 1/3 mile or so. possibly your coil might be quiting once it gets hot.

50 hp merc problem

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