Help Needed - Engine Issues

[Stacer560]

Hey guys, well after planning a nice day out for australia day, half way through the harbour my engine cuts out (01 Mercury 90hop 2stroke carby saltwater series). Not sure what caused it, so i restart my engine and back underway.

Anyway about 1km later, i cant exceed 4000rpm (usually get close to 6000rpm)

Then after a km on 4000rpm it clicked to me it may be no 2stroke oil. (i am sure i had plenty prior to my last trip). Being stuck under the harbour bridge bouncing around i had no time to get the engine cover off, so i quickly topped it up and on i went.

Seemed fine then not long after back to the same problem.

For the rest of the day could not exceed 4000rpm, sounds like it is running on 2 cylinders.

Has anyone here ever experienced this problem running out of 2 stroke oil, and what damage could i have caused to my engine before i go to a boat mechanic.

The oil sensor didnt go off either btw (if there is one). On my previous boat it use to beep like it was about to explode when it was low.

Please help and feel free to pm me. Thanks guys.

[nsaykali]

ouch! hopefully you didnt do too much damage...goodluck!

[a boat]

Hi, IF an engine runs without oil you will have damaged it, but from your explanation an engine without oil will not run at 4000RPM, it will seize and stop all together. I would not mind betting you have lost spark on one cylinder but would not hurt to check compression when doing a spark output test. Also for what it is worth you will be underpropped if you see close to 6K, your engine does not want to rev to more than 5500 at WOT.

Cheers,

Huey.

[Bleeding Green]

Hi, IF an engine runs without oil you will have damaged it, but from your explanation an engine without oil will not run at 4000RPM, it will seize and stop all together. I would not mind betting you have lost spark on one cylinder but would not hurt to check compression when doing a spark output test. Also for what it is worth you will be underpropped if you see close to 6K, your engine does not want to rev to more than 5500 at WOT.

Cheers,

Huey.

Hi, I see it often and have a fair idea as to what it means, but can you tell me what WOT stands for? Thanks.

[BFB]

WOT stands for "wide open throttle". This is the maximum revs your motor should get, or gets, under a normal load at maximum speed.

[a boat]

+1 to what BFB said. ALOT of boats are not propped correctly and as a result the owner is, at best not getting the most out of the combo and at worse can damage the outboard.

Cheers,

Huey.

[Bleeding Green]

Thanks for that.

A couple of questions along those lines then:

* are you doing the motor harm by running at WOT for long periods of time? I usually drop it back to somewhere around 4000 - 4500 revs once on the plane and in full swing.

* Is there any reason to look at updateing the prop? I'm guessing my prop is the original factory version. I would be interested in re-propping if it would help me pull a skier, but I wouldn't want to lose any of my top speed nor would I want to put the motor under unnecessary strain. Is it a case of give and take?

[a boat]

Hi, a properly set-up and propped 2-Stroke can run all day at WOT if you want to pay the fuel bills. I have read some 4-stroke manufacturers owners manual that advise against extended periods of WOT so read into that what you will. On most rigs the most economical range is 3500-4500 so you are smart to back off and run around like that.

No one can answer that without trying a prop. Rule of thumb we need to know what engine you have , what is your WOT RPM, is she at the right height and then we can point you in the right direction. You can not get best acceration and most top speed out of one prop. A lower pitch than you have at present will accerate better, but it will not offer the same top speed, UNLESS you are incorrectly prop now. Only on small engines do they come with props from the manufacturer. Lets take a 150HP of any brand for example, the manufacturer does not know what hull that is going to go on so a light weight ski boat will need a totally different prop than an offshore fighin rig and it is upto the skill of the selling dealer to fit the right prop and I am sorry to say in this industry there is alot of boats not propped correctly.

Cheers,

Huey.

[Bleeding Green]

There ahve been a few threads lately that indicate how critical props are. I am a first time boatie, but based on what I am reading in these threads there is no reason for me to play around with my prop.

But just to give you some idea:

* 70 hp yammie 2 stroke direct injected

* fitted by enterprise marine as a package when new in 2000.

* 475 quinnie millenium hull

* WOT at around 5500 rpm (but I will re-confirm that next time i'm on the water)

* At full throttle it really sucks the oil (and I assume that means the fuel too) confirming what you state above

* According to the fishfinder it tops out at about 40knots on smooth water with 1 or 2 on board.

* I notice a big difference to revs, speed and motor load (by sound) when trimming up while on the plane.

[BFB]

Many years ago when I had a smaller boat and did a lot of water skiing I used to have have 2 separate props. A smaller pitch one for pulling skiers. It had better pulling power out of the hole but lost top speed and had to be careful not to over rev the motor. Basically changing the prop is like changing gears in a car but remembering, unlike a car, you can not change gears on the go.

Therefore a smaller pitched prop will give you better acceleration but loose top speed. A bigger pitched prop can increase top speed but the boat will take forever pulling out of the hole. Thus using huge amounts of fuel.

The correct pitched prop is a compromise between power out of the hole and top speed.

This is why some ski boats are fitted with such large motors for the size of the boat. To obtain a good top end but, more importantly, to have the power to get the boat out of the water whilst trying to drag skiers through it.

[Bleeding Green]

From what I can gather the pitch is a calculation which takes the diameter into account. But I assume it would be possible to have props of differing diamters with the same pitch (the angles of each blade would vary).

If this is correct, with two props having the same pitch and differing diameters would they have exactly the same performace?

Cheers.

[BFB]

The pitch of a propeller is the theoretical distance a propeller will move through the water for one rotation, i.e a 19 inch propeller will move a boat 19 inches for each one rotation.

The pitch of the propeller does not take into account the diameter.