my merc keeps cutting out.

little doc · 05-27-2006, 09:09 AM

Hey everyone,
I thought someone out there likely knows more about the 2 cycle internal combustion engine than me so...
I have a 90 Hp merc that starts easily and runs well up to about 3000 rpm. After that, depending on the day it seems to cut out and die at variable rpm up to 5000. I have been told that redline is 5000 to 5500, and I dont go past that.
I had it in to the shop earlier and they said I was over reving the engine, and the cut off was kicking in. At that time it only did it at 5000 rpm. My problem is the threshold of cutting off seems to be getting lower all the time. At low revs the motor runs great, and it starts easily. The problem is only at higher rpms and is like the engine stops getting gas. If I throttle back, the engine will keep running, and if I keep the throttle up, the engine will completely die. I have been told by guys at work that this is a bad coil. All I know is that I haven't got a clue.
Any suggestions would be appreciated
Tight lines.
Doc.

gerry · 05-27-2006, 09:28 AM

It sounds like the "stator" to me. I had a problem with mine a while ago, only mine wouldn't run at low rpm's. The stator has a high rpm side and a low rpm side. On mine the low side was bad. I would have to use a rope on the flywheel and pull it very fast with the boat at about half throttle to get it to start(very dangerous!!)Yours sounds like the high side is bad. A shop can test this for you.It could also be a fuel problem. Does it start right back up after it dies?

little doc · 05-27-2006, 09:31 AM

Yes , It alway starts easily, and stays running at low rpms.
The fuel line has a water separator in line and there is no water in the filter (I thought that may be the problem). The gas is not brand new, but I thought I would also have starting issues if the gas was old.

gerry · 05-27-2006, 09:37 AM

It sounds electrical to me then. Mine is a 94 Force 70 and the stators are probably the same.The mechanic told me they were notoriously bad for going out. The replacement is a better higher amperage part. I would have it checked for sure.

HntnFsh · 05-27-2006, 01:18 PM

I'm going with the stator also.

little doc · 05-27-2006, 04:10 PM

Thanks everyone,
It seems I need to make an appointment with the local mechanic. Some things I am well equiped to fix, but this is not one of them.
Doc.

gerry · 05-27-2006, 05:42 PM

When mine went bad, I had the boat shop order the part and I replaced it myself.Had to pull the flywheel off but that was about the hardest part. It saved about an hour of labor cost.

DK · 05-28-2006, 12:01 PM

Try pumping the priming bulb in the fuel line when its cutting out and see if it stops, had a 89 merc 80 hp that did the exact same thing, replaced the fuel pump diaphram on the motor, never had a problem after that,

HntnFsh · 05-28-2006, 12:10 PM

You might consider buying a factory Merc manual for that motor.They are spendy.But are very good manuals.They explain in easy to understand terms how to test that stuff.And what tools to use.

They are well worth the $ and will save you money in the long run.

gerry · 05-28-2006, 07:47 PM

As DK said, I thought a fuel problem, but the fact that it starts right back up made me think of something different. It would be something to check though (a lot cheaper too). A bad primer bulb will give problems like that also, but squeezing it will give the engine enough gas to rev back up.

Like Hntfsh said, a manual is money well spent. I bought a Seloc brand manual for mine and if it saves you an hour of labor cost, it's paid for.I think there around $30.

Jsail · 05-29-2006, 09:36 AM

Quote:

I have a 90 Hp merc that starts easily and runs well up to about 3000 rpm. After that, depending on the day it seems to cut out and die at variable rpm up to 5000. I have been told that red-line is 5000 to 5500, and I don't go past that.

A little off the subject but .........
If the motor will go past the red-line (5000 to 5500) then your prop is to small. Think about getting a prop with more pitch.
My limited experience with boat motors is that a electrical problem will be consistent. A fuel problem could be variable. Fuel lines and components (bulb and connectors) don't last for ever. An easy check is to hook up the fuel line to the motor and pump the bulb up. Let it sit for 15 to 20 and see if the bulb is still firm. If it's not you may not be getting enough fuel to the motor at higher RPMs.

little doc · 06-01-2006, 12:09 PM

Update: I pumped the bulb while the engine was sputtering and was revivied. I put some fresh gas in the other tank and the problem was solved. The issue seems to be a flow limiting bit of junk in the line between the port tank and the switch that chooses which tank I get my gas from. It appears I will be running the port tank dry and then checking the lines followed by pulling the tank and having it cleaned.
Thanks for all the input everyone, this is a great place.

Jsail, My motor will rev to 5800 if I let it, but I never have gone over 5k after finding out the max it will go with the current prop. If I wanted to get a steeper pitch prop, would one size be enough or do I need to go up 2 sizes. Also what about increasing diameter? What does that do for rpm, etc. Is dia. determined by the motor or is it determined by the boat?

Jsail · 06-01-2006, 12:55 PM

I'll explain my understanding or the prop issue, but the guys at Stevens are great at answering and accommodating this issue.
The prop size (diameter) should be a constant for each motor. That doesn't mean that if you bought it used that they had the right prop at the time.
The pitch issue is described as power and speed. Lower pitch for faster take off, pulling skiers and for heavy boats. The lower pitch (flatter) is for higher top speed. The higher pitch will take more power to move water so it will lower the top RPMS a little.
If you look on the hub of your prop there will be 4 (3 on a kicker) numbers like 1519 or 1517. If I remember right the first 2 numbers are the diameter and the second the degree of pitch.
I would first find out if you have the right diameter and if thats right go to the next higher pitch.
I don't know if they still do it but, Stevens used to have loaners to help determine the right prop.
Glad to hear you figured out the other issue.

gerry · 06-01-2006, 06:10 PM

Good deal. Another thing to carry is a spare bulb in the boat. I've had them go out and had to prime about every 30 seconds to get back to the ramp.

Starfish · 06-01-2006, 07:28 PM

Jsail, I was with you until you said the lower (flatter) pitch was for higher top speed. Actually the higher (steeper) pitch is most often for higher top speed, but only up to the point that you are still able to turn the proper WOT rpm's with that prop. The pitch refers to the number of inches the prop will theoretically move forward for each revolution, so for the same RPMs a higher pitch prop will move forward faster. Think of it as a higher gear in a car, it's faster on the highway but tougher to accelerate from a stop.

Jsail · 06-01-2006, 07:36 PM

OK, makes sense to me.

nobrownline · 06-01-2006, 07:53 PM

My 90 hp merc just did the exact same thing.Shop replaced the fuel pump and its as good as new