Really Bad Performance - 15hp Yam. 4 stroke

[Rudderman]

I have a 15hp 1998 Yamaha four-stroke electric start engine.<br />This is the first time I've used it since I bought it from the dealer. I have a small 10 foot plywood speedboat (a planing hull) and when I tried it out today, the max speed I could get was 5.7mph, measured on gps. Other people with a similar boat and engine combination are getting 25 / 30mph. What the heck is wrong with my setup?<br />My prop has a stamp on it that says 9 1/4 X 10 1/2

 

[Johnshan1]

Re: Really Bad Performance - 15hp Yam. 4 stroke

I think we need more info, what are you running for RPM's at that speed?

 

[DJ]

Re: Really Bad Performance - 15hp Yam. 4 stroke

Two things.<br /><br />1. Where is your cavitation plate (plate above prop) in relation to the bottom of the boat?<br /><br />2. Is it running on both cylinders?

 

[Rudderman]

Re: Really Bad Performance - 15hp Yam. 4 stroke

I'm sorry, I have no way of knowing the rpms, but at WOT, the engine sounded like it was say half way between tick over and mid throttle.

 

[DJ]

Re: Really Bad Performance - 15hp Yam. 4 stroke

I think the engine is sick. Check for spark on both cylinders.

 

[Rudderman]

Re: Really Bad Performance - 15hp Yam. 4 stroke

The cavitation plate (the one just above the prop) is 2 inches below the bottom of the boat.<br /><br />Yes its definately running on both cylinders<br /><br />Its a short shaft motor and the transom height of the boat is 15 inches

 

[DJ]

Re: Really Bad Performance - 15hp Yam. 4 stroke

It is set too low.<br /><br />For some eason, you are not acheiving wide open throttle. With the cowl removed, watch the movement of he trottle.

 

[Rudderman]

Re: Really Bad Performance - 15hp Yam. 4 stroke

Is it possible that I have the spark plug leads in the wrong way round? I removed the spark plugs to check their condition last week and I may have swapped the HT leads by mistake (I don't think so though)

 

[DJ]

Re: Really Bad Performance - 15hp Yam. 4 stroke

I doubt it. But you can switch them and see.

 

[Rudderman]

Re: Really Bad Performance - 15hp Yam. 4 stroke

Here is a carb photo at WOT<br /> http://www.sum-it.biz/squirt/website/fullthrottle.jpg <br /><br />Here is another photo at idle<br /> http://www.sum-it.biz/squirt/website/idle.jpg <br /><br />Here is a photo of my boat and engine together <br /> http://www.sum-it.biz/squirt/website/trailer1.html <br /><br />Two things I've been wondering about...<br /><br />There is a little spring sticking out just beside where the choke lever connects to the carb. Is it supposed to be like that?<br /><br />Above the idle adjust stop screw, there is a brass coloured screw...what does the brass screw do / how should it be set?

 

[Rudderman]

Re: Really Bad Performance - 15hp Yam. 4 stroke

I checked both cylinders for spark, by removing the plugs, grounding the ends against a nut on the engine, and cranking the electric starter.<br /><br />Here is what I discovered:<br /><br />One of the plugs was wet when I took it out (although not dirty). When I try to get a spark using this plug, it does not spark across the gap where its supposed to, although sometimes there is a spark where the plug is resting against the nut on the engine.<br /><br />The other spark plug was dry and a light tan colour. It sparks correctly across the gap, I checked it on both HT leads and it works on both.<br /><br />Is this what is causing my engine problems? How would the engine run at all if one of the plugs wasn't sparking? The engine does not obviously sound like its only on one cylinder.

 

[Rudderman]

Re: Really Bad Performance - 15hp Yam. 4 stroke

Well I put in brand new plugs and it fixed the problem, the engine runs great now.<br />Unfortunately the engine fell off the back of the boat at about 20mph (oops). Luckily I saw it happening and grabbed it before it sank. <br />Before the accident, I was getting 26.7mph top speed. Here is a pic for anyone who is interested<br /><br /> http://www.sum-it.biz/squirt/website/allen2.jpg