Mercury 2005 75HP 2-Stroke Carbon Buildup

airkidd

Recruit
Joined
Mar 22, 2015
Messages
5
I would appreciate some input from those who deal with outboards on a regular basis. Thanks for any help in advance.

Last spring I bought a used V177 Warrior with a 2004 or 2005 Mercury tiller outboard on it. The engine is an electric start, electric choke 3 carb, 3 cylinder two stroke. This is the second engine for this boat and I believe it was installed by a dealer.
Model: 1075412DD
Serial: 0T983271

RPMs have been sort of a guessing game since the tachometer does not display true. Getting to know the boat through last season it seemed as though the outboard was a little off. So, one of the first things I did when I got the boat was to drain the fuel tank and replace the fuel gauge sending unit. When I did that I noticed there was a lot of sediment in the plastic built in fuel tank. I tried to remove as much of it as possible but don't think I got it all. I ran Shell V-power with no ethanol the rest of the season.

The motor is hard to start. It requires quite a bit of cranking (yes I prime the line) and some generous throttle to start and keep running when it is cold/first started for the day. Warm restarts are generally ok but can require some extra cranking. The motor will idle once warmed (20+ seconds) and will perform ok through all RPM ranges. It idles a bit rough and it does have a couple RPM ranges that cause some moderate vibration though-out the boat. Additionally, running at WOT across the lake the engine will occasionally drop a few hundred RPM for about a half second and then recover.

Last week I investigated a bit more. Running with the eamuffs on and out of an auxiliary tank of V-power fuel I got the same kind of rough idle, random moderate puffs of smoke (possibly fogging oil this time) and vibration at certain RPMs. Fuel condition would seem to be ruled out. Compression seems good: 109,111, & 112 psi. I am not sure on the ignition. It was hard to see the spark across the gap of my adjustable gap tool because I was in direct sunlight. I replaced the spark plugs with the correct surface gap plug: BUZHW-2.

The only smoking gun I could find was considerable carbon buildup on top of the piston. It is almost identical on all three pistons. The carbon covers 2/3's of the piston with the heaviest deposits on the exhaust side. I will be running some Yamaha "Ring Free" to reduce the carbon but I think I need to look at the carbs or maybe the fuel pump. Any thoughts?
 

bilge rat jim

Petty Officer 1st Class
Joined
Jun 28, 2012
Messages
330
Yamaha ring-free is a good choice as a de-carbonizing agent- it contains a fairly high percentage of PEA. I tend to use Chevron techron, only because it's more readily available than the Yamaha product where i am. I think the formulation of both products is about the same.
 

airkidd

Recruit
Joined
Mar 22, 2015
Messages
5
Sorry, but I lost interest as soon as I read "seafoam":sleeping:

So is that 1 vote against Seafoam?

Ok, so now I have a couple ideas how to quickly remove the carbon buildup, but don't think removing the carbon will change my running issue. I think that the carbon is the result of the running issue. Do the carbs need adjustment? Should I pull and clean the carbs? Should I rebuild the fuel pump?
 

airkidd

Recruit
Joined
Mar 22, 2015
Messages
5

So I was able to get out on the water yesterday to try the de-carbonize procedure described. I was skeptical of the "miracle" properties of Sea Foam. That said, I am modestly impressed by the results. I ran about a half gallon of the 6:1 mixture through the motor as described. After that I pulled the plugs (which looked great) and inspected the cylinders. About 75% of the carbon had been burned off. I could now see the cast top surface of the pistons and the markings in them. The exhaust ports appear to be completely free of carbon. I am very satisfied with the results.

I got the carburetor and fuel pump rebuild kits today. I am a little nervous about rebuilding the carbs but they look pretty simple. If anyone has some advice, please chime in. I will follow up with how that goes soon.
 
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