1986 25 Yam. low RPM

[cheesehead4]

1986 25 Yam. low rpm.
Engine has great hole shot. (Beats my 25 John. every time.) Engine will not come up to speed. It seems to rev to a certain rpm and just throttle itself down to a steady rpm. (I can maintain his speed at 3/4 throttle.) All linkages seem to be fine. Carb is full stroked. (why does the carb butterfly have a split spring loaded section?)
Found that thermostat was stuck open with debris. Cleaned it out and eventually installed a new one. Engine now comes up to temperature.
Close to 120 psi in each cyclinder. Carb has been been completely disassembled and cleaned. Fuel lines are open and clean. Filter is clean and open. Squeezing fuel bulb has no effect on engine rpm. Vent is open. Boats in our group are all the same. 16 flats. What rpm should this engine have at WOT? It's got me stumped. Any help appreciated. Thanks.

 

[Ray Neudecker]

Re: 1986 25 Yam. low RPM

Best of my memory, that is the choke flap you are refering to. It allows the engine to pull a little more air than if it was a solid piece.
I have no expeience with the electronics since they have been very reliable. The engine should turn close to 6000 rpm's at full throttle. Those carbs can be a real SOB to clean. If not done by stripping and soaking at least overnight or using an ultrasnic cleaner, it probably needs redoing.

 

[cheesehead4]

Re: 1986 25 Yam. low RPM

Ray,

I am leaning towards a possible prop issue. My friend changed it last year and may have not put right one on. Just a guess. Any idea what would have been standard on this engine.

 

[cheesehead4]

Re: 1986 25 Yam. low RPM

Where do I find a tach for this engine? Just something inexpense but accurate enough for test purposes. Thanks

 

[deejaycee_2000]

Re: 1986 25 Yam. low RPM

prop for the 25 yam is ideally a 13p - 15p .....

 

[cheesehead4]

Re: 1986 25 Yam. low RPM

For some reason he bought both a 9 and a 13. The 13 was on the boat. We put the 9 on today and his speed is equal to the rest of us. I still have not found a tach for test purposes. But for now things seem to be ok. Did he just not have enough HP for the 13? Props confuse me. I'm used to gear ratios and pulleys etc.

 

[deejaycee_2000]

Re: 1986 25 Yam. low RPM

quicksilver makes a digital rpm meter that just clips on the coil wire to measure the rpm ..... the more pitch the less the rpm but more top end speed and lower hole shot, the less the pitch, the more rpm, more hole shot but less top end speed .......