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Catalina/Capri 25/250 Sailor's Forums

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Yamaha 9.9 4-stroke anomaly?

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OJ
Master Marine Consultant

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USA
4382 Posts

Initially Posted - 07/03/2002 : 16:22:05

When I took my motor in for a new impeller (BTW, it didn't need a new impeller, the tubing between the water pump and the power head was blocked. You think that spider was smart enough (4 years seems kind of short for gene mutation) to build his/her nest further in where where my custom made spider nest extractor wouldn't reach?) I explained that I seem to foul a plug or two every season. The service tech acted kind of surprised - then asked what kind of boat (a C-25.) He explained that the high thrust motors with the 3:1 gearing were actually designed for heavier boats and that my motor may not be getting warmed-up enough because of my lighter duty application!
<img src=icon_smile_question.gif border=0 align=middle>
I thought maybe it was because of my leaving the gas line connected and/or not running the motor out of gas at the end of the day. I seem to foul a plug after the motor has sat unused for a couple of weeks.
Any thoughts or similar occurrences on either topic?

Steve Madsen
#2428
OJ (Ode to Joy)

Edited by - OJ on 07/08/2002 18:49:31

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Douglas
Master Marine Consultant

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1595 Posts

Response Posted - 07/03/2002 : 17:14:19

One though Water ski ???

Doug&Ruth
Wind Lass
Tacoma Wa.

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Steve Milby
Past Commodore

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USA
5913 Posts

Response Posted - 07/03/2002 : 17:28:09

For what it's worth, I don't buy your mechanic's explanation of why your plug is fouling. How much heavier a boat could a relatively small (8-10 hp?) outboard motor be designed for than a heavily ballasted 25 foot sailboat with a cast iron keel?

My old 2 cycle outboard was smoking and loading plugs a couple years ago, and I completely drained the fuel tank, and then washed the tank out thoroughly with kerosene, to get any old varnish, undissolved oil and crud out of the tank. Next, I rinsed and drained the fuel line. I carefully measured the correct amount of oil and gas, and added it to the tank. Then I put some fuel additive into the tank that I obtained at a marina, that is made to clean accumulated carbon from the inside of 2 cycle motors. (Sorry, but I don't remember the name of the product.) I put two new plugs in the motor. Since then, I am very careful to measure and add the correct amount of oil to the gas. The motor immediately stopped smoking.

Sometimes, when we mix oil with gas for our 2 cycles, we don't measure very carefully. To be on the safe side, we add more oil than is called for, so that we don't burn up the motor. But, if we don't completely drain the tank before we add more gas, and if we add too much oil in 2-3 consecutive tanksful, excess oil accumulates in the tank, and it can load up the plugs, and not be fully burned in the combustion chamber.

I never disconnect my fuel line, or run the motor out of gas at the end of the day, and have never known anyone who did. The only time we do that is at the end of the season. That probably is not the cause of your problem.

My suggestions might not solve your problem, but it won't cost much, and it's worth a try before you turn it over to your mechanic. Good luck!

Steve Milby "Captiva Wind" C-25 T/FK #2554

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Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 07/03/2002 : 22:09:46

<BLOCKQUOTE id=quote>quote:<hr height=1 noshade id=quote>
He explained that the high thrust motors with the 3:1 gearing were actually designed for heavier boats and that my motor may not be getting warmed-up enough because of my lighter duty application!
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote>
Baloney! It's pusing 6000 lbs of displacement hull (with some people and stuff). Do you run it for long periods without putting any load on it? That'll foul any engine. Oil is not the issue with a 4-stroke unless you have some blow-by in from the sump or a bad valve. If you always foul the same plug, I'd say oil is getting to that chamber. If both, it sounds like a fuel-air mixture problem--maybe as simple as the air filter.

You may also have something regarding running the fuel out... I like to do that when the boat will sit for several weeks. Usually it's the carb that suffers from evaporation, but I suppose that could mess up your mixture, too.

Dave Bristle - 1985 C-25 #5032 SR-FK-Dinette "Passage" in SW CT

Edited by - Dave Bristle on 07/03/2002 22:18:39

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Dave Seely
1st Mate

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46 Posts

Response Posted - 07/03/2002 : 22:17:43

You might want to take a look at the fouled plugs to determine if it is a gas fouling or an oil fouling. Oil generaly leaves the plug wet with lumpy deposits on the plug. Gas fouling generaly leaves the plug dry with a powdery black film evenly distributed on the plug.
Since it is a 4 stroke and your service technician didn't try to sell you on a new motor, I'm going to guess that it is gas fouling. The first things I would check are 1 Is the choke opening completely. 2 Is the needle and seat leaking. 3 Is the float level correct.
A service manual would be quite usefull in explaining how the float level is checked. The needle and seat can be checked by removing the carburetor but leaving the fuel line connected. Hold it at the same angle as it would be in if the motor was running. Pump the ball until its hard. If fuel dribbles out of the carburetor and down you arm, replace the needle and seat.

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Gary B.
Admiral

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USA
969 Posts

Response Posted - 07/05/2002 : 03:24:39

I have a Yamaha 9.9 and it does NOT have to run very hard to push a C25, but I doubt that's it. I disconnect the line and drain the carb if I won't be back to the boat for 2 weeks or so; it probably isn't necessary, but it's a good habit, I think.

While I have never fouled a plug with my 2000 model, I want to suggest that if there is any chance that your carb is allowing excess fuel into the chamber, it would be a good idea to check the crankcase oil frequently. I am sure they fixed this problem, but the earliest Yamahas had a design flaw they didn't want to ever admit. My 1985 model burned up because it was "making oil". The auto choke stuck and raw fuel would go down through the rings and end up in the crankcase where the oil turned to mostly gas! After the first rebuild, it DID it AGAIN, at which point I SCREAMED at Yamaha and threatened to write every sailing mag. in the country. They paid for that rebuild under duress. At the same time, a friend's Yamaha burnt up as well with the same problem. We were in Neah Bay, Wa, looking at a 130 mile trip down a relatively unprotected coastline and over the Columbia River Bar. With some clever "monkey wrenching", we got it running (we were at the end of the world, with few resources) and sailed down and he got home 200 miles up the river. Then came the major rebuild. I was sure Yamaha had corrected this flaw or I would have never bought a new one in 2000. Please do yourself a favor and check your oil level (smell the dipstick for gas smell, too) frequently until you get your problem diagnosed and fixed. I don't want to sound alarmist, but for me, years ago, this was a major PAIN in the transom...

Gary B. Encore! #685 SK/SR

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frich
Captain

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USA
418 Posts

Response Posted - 07/05/2002 : 07:57:38

Just had a similar instance with my 88 Yamaha 10hp 4 stroke. If your has a carb set-up similar to mine there are tiny air passages that adjust the air to gas mixture. All choking and fuel supply is done pneumatically.

My carb had dirt in these passages causuing the mixture to be very rich, also it was hard starting. If this is the case simply take off the carb and clean it out throughly. If you have an electronic enrichment valve ( I didn't) check that also.

Good Luck
Frank Rich
84 C25 SK

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