Blue Needs Help!

That engine uses the YAMAHA CDI unit, and they are VERY reliable. A piece of trash sitting in front of one of your main jets would cause ALL of the issues you describe. Drain the carb bowls and look for debris. While you have the drain screws out, spray some carb cleaner in there. The main jets are located right behind the bowl drains, and they can be removed with the bowl drains out. Another thing you can try is to take it out and one at a time pull a plug wire off until you find the cylinder that doesn't change with the plug wire being off.
 
Don't pay to have the carbs rebuilt. Like I suspected you have a machine screw at the bottom of each carb bowl. Be careful when you take them out they are short fat screws and easy to drop. After you take the screw out a little gas will come out of each carb, you should have a white rag or something to catch it with. If theres debris in there you will be able to see it on the white rag or in a clear container. You can now see your high speed jet and can shoot some carb cleaner through that jet. Sometimes this works, sometimes it doesnt.

Before you do any of this, pull off your air silencer so you can see your carb throats. let the engine idle and see if you can see gas coming out of any of the carbs. If you can see gas its a sure sign that you have some debris in there thats preventing the needle from seating properly in the seat and they will need a rebuild.
 
Don't Pay

I have been diagnosed with carbophobia, but everyone above is right!
Do it yourself, I've been doing mine on my 200 YAMA for several years now and it is easy. I can't believe how much $$$$$$ I gave to local repair shops to do it before. Just take your time.

JJBroc
 
I have a good buddy with an early 90's Yamaha 150. He recently had the same problem and was able to clean the jets and get it back up to par. He lives less than 2 miles from me so I will be calling him soon.

Ferm, explain what you mean by this:
Another thing you can try is to take it out and one at a time pull a plug wire off until you find the cylinder that doesn't change with the plug wire being off.
Click to expand...
 
He is talking about listening for the engine rpm change as you remove the plug one at a time. It was an old trick to find out which was the bad cylinder without equipment. Some engines you will hear it better than others, some guys rely on the tach for the detection, the one that is not firing will have no effect on the sound or rpm.
Least that is what I believe he is talking about
 
I've touched the spark plug boot while the motor was running to make sure I had spark on all cyclinders. You can definetly get a good shock, its not like sticking a fork in the toasted but you can feel it for sure.:nut:

Like you said before Blue, if they are all new plugs you should be able to tell if ones not firing just by pulling them and taking a look.
 
I'll be sure to pad my keester real good before said test.

Here is the latest question dancing in my head - what if I determine there is one cylinder not firing? What to do at that point?
 
Blue, i got all kinds of ideas rattlin around in my head about your problem.

Heres one that just came to mind. If the problem started right after you changed the plugs, its possible one plug is defective?

Also, go pick up a cheap spark tester from the auto parts place. You connect it between the wire and the plug and will tell you if you have spark. They are cheap, I bought one at Harbor Freight for like $4.00.
 
WILLY had the right idea as to what I was getting at, except I would pull one plug wire off with it NOT running and then run it. This way you can pull the wires without turning into SPARKY. Only pull one wire at a time, but if you pull a wire off and you notice no change in the engines performance then you found the weak cylinder.
 
Ok, Mulv I know its not the spark plug itself because I replaced them before the Hatteras trip (5/22) and all was well until recently when it started acting up. Now I've changed all 6 plugs again and no difference.

Thanks for the tips. I'm resting a bit after work. Gonna let it cool off just a little before I go out there.
 
Ok, it is definitely 1 cylinder not firing. So now I suppose I have to determine whether it is fuel or spark right? What is the best way?

Clean as a whistle - the only one that obviously isn't doing its thing:
0616081846.jpg

Facing the rear of the motor it is the top right cylinder:
0616081847.jpg
 
Swap plug wires with another cylinder and see if the problem follows the wire. Then you can eliminate the plug wire from the equation.
 
Does the plug have any signs of fuel on it? If not I would suspect the carb for a problem. If it is wet, but clean then I would suspect ignition(or water). Remove that plastic cover from the rear of the engine and then swap that coil with another and swap the wire with it as well. If the problem stays on that cylinder then it isn't the coil or wires. If you suspect fuel, then remove the airbox cover and start the engine. Bring the RPM's up to around 2500 and check for airflow through the carbs. If air is flowing through it, then spray some WD-40 in the top carb port side(the carbs feed the cylinders on the opposite sides of the engine) and see if the RPM's increase. DON'T USE STARTING FLUID OR CLEANERS FOR THIS! They will wash the oil off the cylinders and burn a bearing or cylinder REAL QUICK! WD-40 has enough lubrication to it to prevent damage but still potent enough to fire the cylinder. Also pull the drain screws as a piece of trash over the main jet can shut down all of the fuel to that cylinder.
 
Slot and Ferm, thanks for the ammo. My buddy is supposed to bring over one of those gadgets Mulv mentioned to test for spark. If he does I will first check for spark and then perform the above mentioned tests.

If it turned out to be a coil, how much do those boogers cost?

Wish me luck!!

P.S. Ferm no sign of fuel on that plug, however it had not run in a day or two. Probably best to run it for a minute then check for signs of fuel.
 
Performed a spark test yesterday evening and I'm getting spark on all 6 so it has to be a fuel problem to that top right cylinder.
 
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