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Jeff G.
Jeff G., Marine Mechanic
Category: Boat
Satisfied Customers: 4724
Experience:  42 years experience, Certified Master Technician, All Major Outboards, Sterndrives, Hi-Performance
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I have a 1999 yamaha 150hp carbed outboard. I took it out

Customer Question

Customer: I have a 1999 yamaha 150hp carbed outboard. I took it out for the first time in 4 months or so. It cranked fine and idles fine but would only run at about 75%. It took twice as long on the hole shot. I finally got up to plane and ran about 5 minutes to my fishing shot. It was running around 4200 rpm ***** ***** but normally runs 5500 rpm at WOT. I fished for a few hours and hole shot was bad again but after i got to WOT still 4200 rpms all the sudden it surged up to 5500 and back to 4200 or there abouts for about 2 minutes then motor ran like normal for the next 10 minutes until I left.
JA: Thanks. Can you give me any more details about your issue?
Customer: Im Lance
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Customer: Im Lance not ***** *****
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Submitted: 1 year ago.
Category: Boat
Expert:  Jeff G. replied 1 year ago.

Hi Lance, sounds like you may have a bad cylinder or two. Usually when a engine has sat we look at the fuel system. However your losing 1300 rpm and then coming and going make me suspect the ignition.

If you have or can get a timing light then we can use it to diagnose the engine.

Put your timing light on the top cylinder, start the engine, look at the flashes from the timing light.

If you have good flashes move the timing light to the next cylinder. Do this for all the cylinders. You can do this with the engine backed into the water still on the trailer. Repeat after 2500 rpm. If you have just one bad cylinder, move the ignition coil on that cylinder to another cylinder, retest. If the problem moves with the coil the coil is bad. If it stays on the same cylinder after the swap, the problem is elsewhere.

Post back your results and we can go from there.


Customer: replied 1 year ago.
I can have a bad cylinder even though it ran perfect for 10 minutes?
Expert:  Jeff G. replied 1 year ago.

The problem won't be the cylinder itself but the ignition on that cylinder. You could have a coil that is internally broken and when it heats and cools works and then doesn't. Common problem. Using an inductive timing light just allows you to see which cylinders are firing. The problem could be anywhere in the ignition system, which is why we will take it one step at a time.

Customer: replied 1 year ago.
ok. Makes sense. Could ethanol cause this reaction?
Expert:  Jeff G. replied 1 year ago.

Ethanol could cause fuel problems. But usually it works or doesn't.

Customer: replied 1 year ago.
One other thing. I bought the boat a 10 months ago and wasn't sure if the remote oil tank was working so I just put oil in my tank essentially running 25:1. Could that have caused this problem? Maybe fouled a plug in the process?
Expert:  Jeff G. replied 1 year ago.

Possibly but not likely. Pull the spark plugs and if they are black, clean or replace them. I would recommend going back to 50 to 1.

Customer: replied 1 year ago.
Ok. I will do as you suggested in ghe first post. Will it hurt to use the boat as is if it continues to run fine? As I mentioned earlier it basically ran bad, then intermittently bad, then ran great for 10-15 minutes.
Customer: replied 1 year ago.
Reason I'm asking is I'm planning on going fishing and won't have to to look into it until next month
Expert:  Jeff G. replied 1 year ago.

If it is a bad coil you will be down on power. This can strain the engine. If the fuel is partially clogged then real damage can occur, You are taking a chance if the engine isn't 100% correct. So I would try to find the problem before any real use.

Expert:  Jeff G. replied 1 year ago.

I'm off now if you have further questions on this I'll respond in the morning, I'm in est here.

Customer: replied 1 year ago.
I can fish close to ramp if it need be but if it's running perfect is it ok?
Expert:  Jeff G. replied 1 year ago.

If it is running perfect you can run it normally. I would run it very briefly then check the spark plugs, if they have a normal burn, then you should be OK. It is possible you had a little trash in the fuel system that went through. If the acceleration is a little flat, check the firing and don't run the engine wide open.

Expert:  Jeff G. replied 1 year ago.

Let me know what the engine does when you run it.

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