Blue Needs Help!

[THEFERMANATOR]

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.

[Mulv80]

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.

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

[Blue_Runner]

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.

[willy]

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

[willy]

Just be careful you can zapped by the high voltage and knocked on your keester.

[Mulv80]

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.

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.

[Blue_Runner]

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?

[Mulv80]

Originally Posted by Blue_Runner

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?

I would check for a bad plug wire first.

[Mulv80]

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.