2003 70 dropping clyinder at 1/2 throttle

Can anyone point me in the direction with the problem of a cylinder dropping (bottom cylinder) at 3/8 to 1/2 throttle or more. I thought it was the coil because when I turned it off then re-started it would run fine the rest of the day. Now the problem is constant and before I tear into it I'm looking for any ideas.

[boats.net]

The only way to find problem is to run the ignition test on motor out on the water when it is dropping cylinder.

Thank You for your response. I purchased the ign. coils from you. Boats.net will get all my business. This afternoon I pulled the carbs and found #3 had 3 large semi-floating particulates in the bowl. Since I rotated the carbs to drain out the gas nothing was embedded in the main jet when I opened up the bowl but, I fear the solids were lodging in the main jet sporadically. I cleaned the bowl and checked the other 2 carbs but no solids were found. When I built the boat I installed a Racor clear/bowl. The stuff I found was much too big to have passed through the factory strainer in the cowling. Have you ever seen Yamaha leave stuff like this after manufacture? Looked almost like dried gasket sealer. I will read up on the electrical test tonight if my manual covers it if not I will have to order a factory manual from you. First thing in the morning is to put it in the river. Thanks again

[boats.net]

The stuff you found in carb is probably old gas that has dried, if thats the case the carbs need to be removed and cleaned.

[Operaman]

Was it black? Like a fuel line disintegrating? I found some black filings in the engine filter, after the Yamaha water seperator. I am considering changing the fuel lines in the engine and thinking about the one from the tank. It is black and most likely original, 1989.
Ethanol is having it's fun with me too.

I have to also see about a possible dropped cylinder at mid-range. Late start for me this year, still on the hard. 4600 runes fine, then can drop a few hundred rpm, then after a while go back up. Someone here recommended to check for spark arcs, run at night. I have run with my hand at idle running along the coil wires, nothing. If I run it up to WOT, it seems to get there when this happens, runs smooth too, but it is 225 V-6 and that is hard to hear if you have a cylinder drop out.

Good luck.

Take a timing light with you and check for fire on that cylinder when it is acting up if the carb cleaning doesn't cure the problem.

The three cyl CDI's do sometimes have intermittent failures and drive mechanics through the wall. They will run fine a few minutes and then start dropping a cylinder.

Hopefull your problems are a fuel restriction.

Operaman, you most likely have a fuel restiction. it needs to be corrected before you sieze the motor.
On that model, bypass the fuel disconnect and connect hose directly to the input barb. Install a water seperating filter and bypass the engine mounted filter.
If that doesn't cure the problem, it may be fuel pump replacement time (do not rebuild) and last would be the carbs.

[Operaman]

Ray, got it. Will do as recommended, Thanks

[Operaman]

Operaman, you most likely have a fuel restiction. it needs to be corrected before you sieze the motor.
On that model, bypass the fuel disconnect and connect hose directly to the input barb. Install a water seperating filter and bypass the engine mounted filter.
If that doesn't cure the problem, it may be fuel pump replacement time (do not rebuild) and last would be the carbs.

Ray. Fuel Disconnect bypassed.Origianl and looked pretty bad, pin was bent. Still did it, but was an improvement. Changed out fuel hose from tank to water seperator. If I could post pics you would be amazed. The outer part of the hose was rotted off at the tank and for about 2 feet after. Original and not so easy to change out. Now have alcohol resistant Shields type III.
I also grabbed 2 new fuel pumps and changed them out. Took a test ride and it seems to have been corrected.
I will let you know how it goes.
BTW, post more often! Thank You Ray!