yamaha c115 loosing power

[cyrillefrenchy]

i own a 19'hurricane deck boat powered by a 1995 yamaha c115 tlrt since 2 years.i start to loose a little power a few weeks ago.i first replace plugs,still the same, it seems like the cylinder number 4 is working less than the others, when the engine is idling,i disconnect the spark plug wire on the cylindre 4, the rpm drop just a little bit, and a lot on the others cylinders.it seems like this cylinder is working but less than the others, right?
before i was able to spin the engine 5300 5400 rpm, now no more than 4900 5000 and the engine sound a little different.
i tried to switch coils, still the same cylinder missing(there is good spark light).
i tried another used cdi unit, still the same cylinder.i checked the compressions 2 times, 120# on all the cylinders.i pulled the heads and exhaust plate for carbon build up and cylinder scuffing : everything like new inside.
light head shaving and everything back together with new gaskets, still the same cylinder missing half
pulled the carburators, cleaning very well all the canals specialy the concerned cylinder, pulled the reeds valves off for checking (like new) ,switching the reeds blocks starboard to port , everything back together with all new gaskets , still the same cylinder missing half.
when i check for lean or rich combustion, the color of the plugs after a 1 minute full blast ride is brown light on all the cylinders.
when i try to "play" with the mixture screws, the rpm at idle is changing on the 3 cylinders working but not on the #4.
the only thing i did not try is to switch the carburators
do you have any idea to fix my probleme?

excuse my french-english

[boats.net]

You are trouble shooting a high speed problem at idle speed, you need check it high speed, use a timming light to see if losing fire to cylinders and you can spray small amounts of gas in carbs to see if running lean on fuel. Pump the primer bulb on fuel hose while running engine if it makes engine run worst then it's running rich, probably fuel pump diaphrams.

[cyrillefrenchy]

thank's for your reply,i forgot to tell i also try to spray a small amont of gas in the carb while running idle or fast idle, it's worth, y loose the cylindre #4 completely.i am not running lean, right?
when i squizz the bulb while running fast or slow, does not change anything, bulb is hard .(complete fuel line new with no quick connection anymore).the fuel pump kit is new too, no difference
i tried also with a timing light at idle or fast, i never loose spark.
again, when i disconnect the plugs wires at idle or 2500 3000 rpm, the rpms drop a lot on #1,2,3
and a litte bit only on #4.at high rpm i can't feel anything but the engine sound is not like before and is shaking a little up to about 2500 rpm
i used the boat again today , start right up, check many things again, always the same.
i am missing something but what?
thank's again for your help, any idea welcome.
this engine drive me crazy.
i will switch carbs soon and will tell you.

[1Chavez]

Before switching the carbs, be sure the wires to the ignition coil are in good shape (nothing grounded out, no wire chaffing, clean and tight connections). If all checks out ok, try switching the ignition coil. I am basing this on the fact that adding gas to the cylinder is having no change. An engine needs fuel (spraying fuel through the carb, there is no change), compression (you say compression is good), and spark (seems like the only thing missing).
Good luck,
-Rich

[cyrillefrenchy]

thank you rich for your answer, i wrote in my first post that i already switch the coils with high tension wires, and the cdi unit too.
i did that first after new spark plugs because only one cylinder is missing half, the #4.
i already tried all what is in my mind , all the solutions possibles i know before writing this post and i assume i miss something,i will check again all what you guys told me .
one more indication, the probleme came slowly, i had to readjust the idle rpm two times, it was dropping and loosing the maximum rpm at the same time
thank's again for your help

[TMD]

Light brown or tan spark plugs is a very fine line between excellent combustion/overheating and running lean.

I would rather see you plugs come out a bit oily.

Check your complete fuel system for an air leak.

TMD

[richardg]

I found this site while researching a problem with my 1989 115 Yamaha. It sounds like we have similar problems. My #3 cylinder is not firing. I switched the coils around and it had no effect, still #3 won't fire. As I was testing the impact of removing spark plug wires while idling around 1100 rpms, #1 and #2 cylinders caused significant drops in rpms, where #4 caused a noticable drop in rpms, but it didn't have the same level of impact as 1 and 2. I had to replace the CDI unit 2 years ago so I doubt (hope) that it is not the problem. I did some reading in my service manual and there are two coils under the flywheel. One coil serves cylinders 1 and 2 and the other cylinders 3 and 4. I am wondering if that coil could be weak? Could a damaged coil provide a weak spark to one cylinder and not the other?

Thanks in advance for any response.

Rich

[cyrillefrenchy]

thank's TMD for your answer, i checked many times the color of the plugs, they all come out light brown - tan after a full blast ride for about 1 minute and oily-greasy-wet after an idle or low rpm ride.
they all look about the same,that is why i do not understand why the cylinder #4 is not working as much as the others.
RICHARDG,i already replace the cdi unit 1 year ago, the cylinder#2 was not firing at all, there was no power comming out of the cdi to the coil#2
. that thing is expensive, by the way.
now about the coil under the flywheel,? i doubt because i always have spark.
is it possible to have spark but less powerfull only on 1 cylinder to cause my probleme?????
i have no tool to measure that.
thank's all for your help

[richardg]

I will keep testing various alternatives and post if I have any success. Good luck.

[TMD]

You can check for a weak spark.

Place a bolt into the spark plug boot and hold it 1/2 an inch from the block, spin the engine over... you want to see a nice crisp blue color. If its not, its weak. Compare it with the others.

TMD

[cyrillefrenchy]

thank's TMD for your tip, i will try as soon as possible, it's very hard to see the color of the spark during a bright sunny day,but i can hear it very easely wher i hold the wire at 1/2 inch from the ground or the plug. i usually use a spark light tester or a timing light.

[ROB]

I have your answer!I have the same motor 1997
year. I had the exact same problem with #4.
Drove me bannanas just like what your finding out.
Did all the things you did with no results.
IT WAS THE RECTIFIER. Easy to change, bolts on starboard side of motor in my case.
I ordered the part from this site.
Central Florida Yammaha helped get the correct one.I would bet you dinner thats your problem.
Good luck.

[cyrillefrenchy]

hello ROB, thank's for your help but what is the rectifier , where is it located exactly, what is it for.???
i am going to make research while i wait for your answer.

[cyrillefrenchy]

ok guys, i found what and where is the rectifier.
but is it also the regulator to regulate the battery recharge?
if yes, what is the relation with the #4 cylinder not working correctly???

[jb123]

i would run the motor at night and look for any bleeding at any of the wires in the back of the powerhead. secondly make sure you get a good spark jump on each plug,some techs will have a jump tester. have you checked your fuel pumps and cleaned carbs in the last few yrs ????