im really sorry for the length of this post but theres a lot to this and many symptoms. i dont expect any of you to know everything about this problem but anything would help, thanks ------->
I just rebuilt an older Merc outboard 90Hp straight six two stroke. After taking it out in the water during the break in period I noticed that cyl. 3 wasnt firing correctly (piston's crown was clean while all others had a little carbon). I think it was firing at first and then spark died and all the clean gas moving around in the cylinder just cleaned it off (because I could clean other pistons with just alcohol and q-tips). The engine idles ok, however sometimes I can pull the plug wire off spark plug 3 and I cant hear a difference, and other times I can. And when in water I open the throttle, for the first few minuets it really gets up and moves the boat quite nicely. but after that it drops in performance (quickly). I have switched coils around and I cant say for sure, but I dont think thats the issue. Doing an impedance test that I got from the manual the switch box did fail (I think its 110 Mohm to 140 Mohm) I got a 100 Mohm on one of the three test combinations all of the others I think tested 120 Mohm. I flipped the switch boxes from 1-2 to 2-1 and that seemed to make a difference. I took it out on the water and it ran harder for longer but still died out over time. Now the reason i'm not saying that its the switch boxes for sure is because during the time that cyl 3 didnt fire, I took off the carbs and poked at the reeds, put them back on and within minuets the piston had a little carbon on it. Please help, its giving me a huge headache going back and forth from my shop to the dock. oh and i would also like to point out that the carbs are rebuilt and cleaned and a new fuel pump was installed. i tested the engine at night, shining my super intense LED flash light into the carbs i can see the vapor trail coming off of the jets (and all three carbs have a very nice equal amount of gas flow). i cotton swabed the bad cylinder for gas and yes there is fuel in there, its just unburned! and all cylinders have almost equal compression at 130 psi. with new spark plugs every time i test it.
i think that narrows it down a bit.
I just rebuilt an older Merc outboard 90Hp straight six two stroke. After taking it out in the water during the break in period I noticed that cyl. 3 wasnt firing correctly (piston's crown was clean while all others had a little carbon). I think it was firing at first and then spark died and all the clean gas moving around in the cylinder just cleaned it off (because I could clean other pistons with just alcohol and q-tips). The engine idles ok, however sometimes I can pull the plug wire off spark plug 3 and I cant hear a difference, and other times I can. And when in water I open the throttle, for the first few minuets it really gets up and moves the boat quite nicely. but after that it drops in performance (quickly). I have switched coils around and I cant say for sure, but I dont think thats the issue. Doing an impedance test that I got from the manual the switch box did fail (I think its 110 Mohm to 140 Mohm) I got a 100 Mohm on one of the three test combinations all of the others I think tested 120 Mohm. I flipped the switch boxes from 1-2 to 2-1 and that seemed to make a difference. I took it out on the water and it ran harder for longer but still died out over time. Now the reason i'm not saying that its the switch boxes for sure is because during the time that cyl 3 didnt fire, I took off the carbs and poked at the reeds, put them back on and within minuets the piston had a little carbon on it. Please help, its giving me a huge headache going back and forth from my shop to the dock. oh and i would also like to point out that the carbs are rebuilt and cleaned and a new fuel pump was installed. i tested the engine at night, shining my super intense LED flash light into the carbs i can see the vapor trail coming off of the jets (and all three carbs have a very nice equal amount of gas flow). i cotton swabed the bad cylinder for gas and yes there is fuel in there, its just unburned! and all cylinders have almost equal compression at 130 psi. with new spark plugs every time i test it.
i think that narrows it down a bit.