30 Yamaha cuts out after half trottle

I've got a older 30 hp - 3 cyl which has run excellent for the last 10 yrs. But now we've got a problem that we just can't figure out.
When I start it, it works great, idles nice, and runs good up to half trottle. After half trottle is start to cut out like it's running out of oil. But if I disconnect any one of the spark plugs boot, and run it on 2 cyl - I can go all the way to full trottle with no cut outs.
We have changed everything electrical in it, CDI box, stater coil, plugs, carbs, fuel pump, fuel supply line, new primer ball, new gas, drain the carbs & filter... It doesn't matter what I changed it still cuts out after half trottle. I and every small engine shop here are baffled.
It acts almost like the low oil safety is coming on, we have tried to disconnect the low oil sensor and hi-temp sensor, nothing changes.
I do have a Pirranah prop on it, but can't figure that this would be a problem?
If anybody has any suggestions, I'd be very grateful. Its been a good motor and would hate to get rid of it.

Re: 30 Yamaha cuts out after half trottle

It helps if you give the modle # of the motor so people know which motor you are talking about.
Sound like you have not pulled and cleaned the carbs which is one of the first things to look at along with other fuel related parts and delivery system if it has good compresson and spark on all cylinders.
Have you checked the alarm to make sure it is working?

Re: 30 Yamaha cuts out after half trottle

99Yam40,

The motor is a Yamaha 30 precision blend short shaft - model 30MSHQ,
serial # 6J8 S 106571, I don't know the year but I'm guessing '71 from the last two digits on the serial # ?
I did clean the float bowls this spring and passed a brush through the orifice holes but that's it, along with a good blowing with the air hose?
What baffles me, is when I remove a spark plug boot off, I can go to full trottle without it cutting out, and I've tried to do this on all cylinders - doesn't matter which one, when I'm on 2 cyl only it doesn't cut out, as soon as I go back to 3 cyls it starts to cut out.
I know the low oil light works because when I was out fishing this spring it lite up when I was low on oil, there is no alarm, just a dummy light. And I even tried to disconnect the low oil safety switch - thinking that maybe it was the oil reservoir float that was giving me the problems, same thing?
I even changed the spark plug coils one at a time, thinking I had a bad coil, no difference.
This is just baffling everybody around here.

Re: 30 Yamaha cuts out after half trottle

you need to find out what parts of your alarm system work and what does not just to see if one is coming in.
You need to clean the carbs real good ,not just half way. It sounds like a fuel restriction maybe main jet, needle and seat, filter , air leak. pick up tube etc.
Pressure and vacuum test the fuel system, check compresson, and check timing and spark

Re: 30 Yamaha cuts out after half trottle

30MSHQ is a 1992 the bet I can find by modle #
do you have a manual? you need one?have you done any PPM on this motor over the years?does the bulb remain firm? does it help to pump the bulb?
Just replacing parts is not the way to go test things to find the problem

Re: 30 Yamaha cuts out after half trottle

99yam40,

I'm asuming that ppm is preventive maintenance? If it is then not really, we change the oil in the lower unit and change the plugs yearly - my moto - if its not broke don't fix it.
As for pressure and vacuum test, haven't done that either, don't have a clue or the equip to do it - that's going to have to be a shop thing.
As for a manual - Yes your absolutely right, I'm really going to need one, especially if I'm going to try to re-clean the carbs.
I've got a spare top end from an outfitter, so I have all the extra parts for it, so I'll start by cleaning the carb packs on that one, and switch them.
The bulb doesn't stay firm after startup also, and it does seem to help a little if I keep pumping it. What would this indicate - pump problems or filter problems?
Thanks for all your help, much appreciated.
Andre

Re: 30 Yamaha cuts out after half trottle

99yam40,

The other thing I just thought of, but don't know if it would help, is I'm running it on a jerry can with an adapter (can't recall what they call it) but its so you can interchange between jerry cans. I had blown my cruise a day a couple years back, and these are more practical for us up here in the North. Are those cruise a day pressurized somehow, or have something special about them, I mean they do have a bleeder valves on them same as a jerry can?

Re: 30 Yamaha cuts out after half trottle

Where are you located? I do not know what a cruise a day is and never heard that term before. Most tanks need to be vented to keep from forming a vaccum and not being able to move fuel.
If the bulb is getting soft and it helps to pump it, I would look for a restiction in the tank pick up or lines,air leak in fuel lines, bad pump or plugged filter, etc.
A pressure / vacuum guage can be picked up at Sears or a auto part store and teed in the fuel line at different places to trouble shoot and see if the is a restricion , blockage, and see if pump is working properly

Re: 30 Yamaha cuts out after half trottle

Buddy just had very similar problem - shop found the manual choke lever spring was missing! Hope that helps.