Mechanic

This exact thing has happened to me twice. My duck boat has a 87 Yamaha 30 with 3 cylinders and 3 carbs. Both times it went from WOT to shaking violently and wouldn't rev above about 10%. Heat light came on. Pulled the carbs and cleaned them and it ran great for another 2 years before it done it again. NOTE... Carbs appeared to be clean both times to the eye. I was not able to see the blockage either time. I usually run non E fuel and marine Stabil.

If the gas leak thing doesn't fix it, this is what I would try. It's what worked for me.... after I had tried replacing plugs, fuel pump diaphragm, plug wire end caps, a whole can of Sea Foam, and compression check. Remember Keep it simple......

those old mercs have a diaphram fuel pump that when air gets in it will not function properly. If the fuel is not the problem, inkdabber is probably right. If you have fuel and loose power it is most likely from loss of fire. take a plug out and connect it too the plug wire and ground the metal end of the plug to the motor where there is no paint. You should see it spark as you turn it over and try each plug. You might want to use a rubber handled pliars for this. If you have fire to each plug then you still have a fuel problem in the carbs, but I think it is loss of fire or compression. A broken reed valve would do ti also..

Ok so, if its at all possible. I need someone to walk me through a compression test. I am extremely afraid right now that I have lost a cylinder. I attempted to do a compression test today. I don't have this tool myself, so I went to autozone and rented it and had my friend help me with tools etc to get working on it. Took off the cover, removed the plug (top one only as bottom is near impossible to get to without removing a lot, didn't have time today) , removed the spark to Cyl 2, removed the plug on cylinder 1, plugged up to tool, friend held gauge while I was at the key, turned the key until it tried to start 6-8 times, No pressure. At all. Didn't even move a little bit. So I blame my inexperience with the tool. I wish I could check the 2nd Cylinder but I can't figure out how to get it apart enough to work on it. I need to find a manual for my motor I believe.

Next time I try this I'll do a step by step pic/video to ensure Its not my process that's the problem. I hate to rush things but I'm trying to get this sorted to go on the trip on the 18th. This is going to be tight! But I'm going to give it everything I've got!

I dont take my motor to anyone but Wilson Marine in New Berry. They are up front with how much it will cost and sometimes with cost of repairs. They are not washy washy. THEY KNOW BOAT MOTORS front to back and up and down. I highly recommend them.

I believe you don't have to remove the other plugs, only the one at the Cyl you are testing. If all are removed you won't have any compression at all on the motor. Pull the coil wire or all the plug wires or pull the kill switch so the motor doesn't start while testing.

What Stump Hunter said.

Where you able to get this resolved?

I hope this link is ok but go here and you can view your service manual online.Boatinfo - The library, containing manuals, brochures and other technical documents from the last hundred years!

Nope. I will attempt this again this weekend. Thanks for that link. I'm having a hell of a time navigating it, I'll keep at it and see if I can figure it out.

Quote:

---

Originally Posted by DownCountryAngler

Nope. I will attempt this again this weekend. Thanks for that link. I'm having a hell of a time navigating it, I'll keep at it and see if I can figure it out.

---

pull up the page........ pick your motor manufacturer on the left and then right below the manuals in about the center of the screen you will see a little easy to miss slider that when moved right or left will show all the other manuals available..... Many people including myself miss the little slider that allows you to see all manuals, first time I went there I thought they only had 3 mercruiser manuals

Thanks for all the help!!! Did the compression test today! Cylinder 1: 60psi Cylinder 2: 120psi :( Bye bye cylinder 1. *Sigh* Time to find another motor :\