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Discussion Starter #1
Hi all! I’m back, or still here, depending on how you look at it. Just have been computer-less for a while (and the typing skills have suffered for it). Took my Jesse Boyce to NY this past summer. Boy, you Long Island Sound-ers sure don’t like us gunk-hole-rs very much, on both the North and South shores! Anyway, I’ve got a problem. I have a 1994 Mariner 8hp with a 25 inch shaft pushing a 28 foot wooden sailboat with a sailboat prop. The motor is mounted on the transom with a pull-up mount. Up until this year I have averaged 5 - 5.5 knots at cruising throttle (the point where the RPM no longer increases) and have consumed approximately 7/8 gallons per hour. Lately the motor has started to run only slightly perceptibly and inconsistently rough and has started to consume close to 1-3/8 gallons per hour at 3.5 - 4.5 (at worst) knots at cruising throttle. I replaced the needle valve and cleaned the carburetor, changed plugs, performed all the sync and link adjustments, replaced the lower unit lube and completely cleaned the fuel lines and installed an external fuel filter but there has been no change. The motor still starts and runs dependably but I just can’t figure out this problem. Any ideas?
 

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The signs and symptoms that you describe are similar to a overly rich mixture. If you have good combustion chamber compression (using a compression gage, etc.) I'd go after that carburator with a complete rebuild ... especially the float chamber needle valve and valve seat ... and making sure that the float is at the correct height inside the bowl. If that needle valve isnt fully seating you may be 'dribbling' raw fuel directly into the combustion chamber. An over rich mixture can well accelerate cylinder wall wear as it dilutes the oil film there and in the piston ring grooves, etc. The carburator would be my first suspicion.

If you can locate a specialty carburator shop (frequented by hot-rodders, racers, etc.) they can (if they have a 'test-bed' engine small enough) perform miracles in getting a small carburator back to optimum set-up ... and can make modifications to correct it. Ive never found an 'outboard shop' knowledgeable with carburators ... other than a wholesale replacement with a new carb.
At a minimum Id replace the float needle valve and seat ... especially one with a needle tip more chemically compatible with ethanol such as EPDM (ethylenpropylenediamine-monomer). A lot of older 'rubber tipped' needle valves are either BUNA/Neoprene or viton ... and depending on the 'quality' may become softened and swollen in 'gasohol' mixes.
 

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Wandering Aimlessly
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Be sure to check both ends of your fuel hose from tank to engine as well. That was the cause of problems with my 8 hp Mariner.
 

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... if what you've done up to now is within original specs and the engine is burning clean i.e. no sheen on the water or smoke; and, there is no restrictions on the air intake or intermittent firing problem (difficult to assess); then, it occurs to me to ask whether anything has changed on the boat you are pushing. e.i. bottom condition, wine cellar inventory gone up, etc.

...
 

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My neighbor's lawnmower had fuel corrosion at the fittings that were easily cleaned off with carb cleaner, etc. That caused her mower not to start/run when no other discernable problems were detected. It was even at the lawnmower shop and they didn't see it! 'Twasn't till I took it apart "stem to stern" that the issue was discovered.
 

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I don't know how in love with this engine you are, but there is likely a Clymer shop manual for the Mariner (which I have the vague notion are a little prone to fouled/misaligned carbs).

As the only carb I've ever rebuilt has been the primitive Bendix of an Atomic 4, I recently bought the Honda outboard manual for $50 because I couldn't find an exploded diagram of the carb and I didn't want to screw up. Excellent reference and a good primer for all four-strokes, I am thinking.
 

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Discussion Starter #7
Thanks! All good replies. I've visited all of these options so far with no luck. I guess I'll haul that carb apart again. I've got this feeling that it has to be something stupid and simple, (like that lawn-mower) that I'm not seeing. And if that dosn't work I'll look into a different one. I'm kind-a stuck with this motor for now as I don't have the $2500 to toss at a new Merc 9.9 with the 25" shaft and 25HP lower unit.
 

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Simple question? .... how OLD is your fuel????????? Without a stabilizer added, gasoline only 'lasts' about 1-2 months then quickly degrades, forms gums and varnishes. its a BIG problem for engines with small jets and air bleeds, etc. in the carburators.
 

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Telstar 28
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BTW, new ethanol blend gasolines have a shorter shelf-life than the older MTBE-formulated ones. The ethanol will start to come out of solution once enough water has been absorbed from the air. This will happen, even with a good fuel stabilizer.
 
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