4 stroke Yamaha 50 WOT problem

[IanP]

We have nearly new 50hp Yam 4 stroke with standard prop, which starts and runs fine if you open throttle slowly, until you reach about 3000 rpm. If you go beyond 3000 rpm the engine becomes momentarily quite lumpy (not to the point of stalling - but a very noticeable stutter), but settles down after a few seconds, but only reaches max 4500 rpm. Will run all day at 4500 rpm with no problem but last couple of inches of throttle do nothing at all to revs. Every time accelerate through the 3k rev range same problem, but the faster you open the throttle the rougher the engine accelerate, but once it eventually reaches the 4k range it settles down again and would cruise all day without a problem, albeit never going beyond 4500rpm.<br /><br />If it was a car I would say it was an ignition problem, but this apparently checks out fine!<br /><br />Fuel pressure is apparently fine, but have tried new tank and fuel lines anyway. Bulb also replaced and pumps hard. Carbs also apparently check out fine. Cable/action is fine and physically fully opens the throttle.<br /><br />Engineer thinks overpropped, but boat is only 16ft open console and weighs less than 500kgs inc. engine, so I would have thought a standard prop would see more than 4500rpm on such a light boat. Engineer is going to try a new prop, which I don't doubt will help with rpm's, but I am concerned he might be overlooking something. Engine under warranty so really want to get it right!!!<br /><br />Any advice greatly appreciated.<br /><br />Many thanks<br />Ian

 

[DJ]

Re: 4 stroke Yamaha 50 WOT problem

What pitch is the "standard" prop?

 

[IanP]

Re: 4 stroke Yamaha 50 WOT problem

Not sure, but will find out.

 

[IanP]

Re: 4 stroke Yamaha 50 WOT problem

14" we think

 

[IanP]

Re: 4 stroke Yamaha 50 WOT problem

The prop is in fact 11 1/8 x 13 -G and also seems to be the standard prop for the 60HP engine.

 

[IanP]

Re: 4 stroke Yamaha 50 WOT problem

An update - engineer accepts unlikely to be prop and therefore wants to take a further look, as he suspects it is fuel related. <br /><br />Can anyone confirm that if the bulb in the fuel line pumps okay and goes hard it is unlikely to be an air leak? Pumping the bulb when at WOT makes no difference to the rpm and bulb remains quite firm throughout.<br /><br />Any thoughts greatfully rec'd.<br /><br />Thanks<br />Ian

 

[IanP]

Re: 4 stroke Yamaha 50 WOT problem

It now seems the problem is more serious, although nobody yet knows what! As engine is less than 2 months old and done less than 12 hours - is it reasonable I should allow them to undertake repairs or should I demand a new engine?

 

[IanP]

Re: 4 stroke Yamaha 50 WOT problem

If anyone can offer advice I would really appreciate it - the situation has progessed as follows:-<br /><br />Engineers could find no problem with fuel and therefore insisted it was prop. They changed the 11 1/8" x 13" for a 11 1/2" x 11". Engine still did not reach even 5000 rpm with 3 people on board and speed at WOT was same or slower (about 18 Knots at 4900). The stuttering disappears with smaller prop and she jumps onto plane. Engineers are now suggesting an even smaller prop!! This is 16 foot long open sportsboat we are talking about, which without engine only weighs about 400 Kilos (850 lbs). My argument with them is that according to all the Yamaha bulletins, boats of a similar size/design and weight reach max rpm with 14" pitched prop, even with a 40hp. Surely an 11" pitched prop on such a light boat should have been screaming, not struggling to reach 4900 revs.<br /><br />It therefore seems to me that this engine is not producing sufficient power to spin the correctly sized prop so I am convinced that reducing the prop size still further is just a cheap way of masking the original problem, rather than fixing it! <br /><br />They have so far refused to replace engine, as they want to try the smaller prop. <br /><br />Any ideas really would be very much appreciated.

 

[jim dozier]

Re: 4 stroke Yamaha 50 WOT problem

Put that engine in a test tank with a test wheel. I'm not a Yamaha mechanic but there are specific test wheels for that engine. It should turn a specified rpm in a tank with a test wheel. If it doesn't there is a problem with the engine, end of story. I'm surprised the dealer hasn't done this already.

 

[IanP]

Re: 4 stroke Yamaha 50 WOT problem

I think this was one of the first things they tried, but using a test prop. The problem is the test prop has such a fine pitch that it doesn't put sufficient load on the engine. Is a test prop the same as a test wheel?

 

[jim dozier]

Re: 4 stroke Yamaha 50 WOT problem

A test wheel or test prop is a wheel or prop like device that puts a specific load on a motor thats standing still in a test tank. The standard prop from your boat would not allow enough rpm at a standstill. A properly selected test wheel allows a particular motor running as it should to develop a target rpm. If the engine doesn't make that rpm, its down from its rated power and something is amiss. If they have the proper wheel its a simple test. If they have the wrong wheel you have the wrong dealer, find another one.

 

[BillP]

Re: 4 stroke Yamaha 50 WOT problem

Did you change the spark plugs or are you assuming they are ok? Checking spark with the plug out only gives half the story. Under compression the plug can break down and misfire. Even new plugs can be defective...cracked insulator, etc. My 50hp 4S mariner ate the original plugs at about 100 hrs. At 300 hrs it was on the third set. I changed brands and for whatever reason the "other" brand has gone 300+ hrs without a problem.<br /><br />A prop isn't likely to make a 4 stroke studder either.

 

[IanP]

Re: 4 stroke Yamaha 50 WOT problem

It transpires that first engineer tank tested the engine, but only using a small test prop. They are now going to do the job properly with a larger prop - so thanks for advice!<br /><br />As we suspected an ignition problem at the outset we changed the plugs ourselves, but this made no difference. Are the HT leads etc. replaceable on an outboard just like they are on a car? I suppose we could try this also.<br /><br />I think what I really need to know is whether a 50hp 4 stroke should be able to achieve more or less maximum rpm using a 13" pitched prop, bearing in mind this is only a 16ft sportsboat weighing less than 1500 lbs, with engine, fuel and two people aboard.<br /><br />We are not too bothered about top speed, or for that matter acceleration - but we do want to be sure that engine output is correct to avoid undue longterm stress problems.<br /><br />Someone must have a similar setup and it would be interesting to know what size prop they are using.<br /><br />Any input greatly received.<br /><br />Thanks<br />Ian

 

[Breamo]

Re: 4 stroke Yamaha 50 WOT problem

Ian,<br /><br />I have the same engine on a 15 foot aluminium boat - weighing about 1000 lbs. Much lighter and smaller than yours. And the prop is 11-3/8 X 12. Performance is as per specification. I can rev to around 5600 RPM at WOT. As your rig is considerably bigger and heavier than mine, it may well be a problem with pitch too long as the motor is struggling to push the rig along. <br /><br />Cheers, Breamo

 

[BillP]

Re: 4 stroke Yamaha 50 WOT problem

I think your "engineer" is right about the studdering not being related to the prop. On a two stroke it would be likely from loading the plugs with oil but not on a 4 stroke.<br /><br />Another question...did you check the tach for accuracy?<br /><br />These are just random thoughts dredged up from experience and thrown out on the table. I'm not a mechanic.