RPMs won't go above 4000; were 5100

[DragonWagon]

Hi folks!<br />I'm hoping someone can help diagnose this problem. I have a 2001 Mercury 125hp 2-stroke outboard. I was running along at 5100 rpm (48 mph) when the engine rpm dropped suddenly. No warnings of any type. I stopped the boat, checked the prop, the oil level, and then made sure I had a good stream of water exiting. I didn't find any problems. However when I cranked the boat back up, it wouldn't run above 4000 rpm. I changed the spark plugs the next day and tried again but that didn't help. Any thoughts?<br />Thanks, DragonWagon.

 

[Hooty]

Re: RPMs won't go above 4000; were 5100

Welcome to iboats DragonWagon.<br /><br />I don't suppose you looked at the plugs you pulled out. They'll tell ya a lot. Sounds like you lost a cylinder. If you did, those plugs might have told which one. If a coil quit or a plug wire came loose, the plug that wasn't firing would have been wetter then the rest.<br />What you can do, while running on the hose, is ground each plug wire out until you find one that doesn't make the engine miss. That'll be tha one not firing. Don't just pull a wire off and let it dangle, ground it to the engine block. If ya don't. it's real hard on the powerpack.<br /><br />c/6<br /><br />Hooty

 

[DragonWagon]

Re: RPMs won't go above 4000; were 5100

Thanks Hooty. No loose plug wires when I took the cover off. I'll do the test tomorrow and post what I find out.

 

[Hooty]

Re: RPMs won't go above 4000; were 5100

Super! But be sure and ground the wires. They'll knock fire from your as* if you're not careful.<br /><br />c/6<br /><br />Hooty

 

[DragonWagon]

Re: RPMs won't go above 4000; were 5100

I checked for spark on all 4 plug wires using a High Energy Ignition Spark Tester from Advanced Auto Parts ($7.00, well worth it). All showed spark output. I tested the compression. All cylinders were at approximately 9. I tightened the bottom plug wire and got the rpm up to 5100 (under no load, prop not spinning). I'm just going to have to take it to the lake and try it out.

 

[graddy]

Re: RPMs won't go above 4000; were 5100

When you were going accross the lake did it sound like you were running out of fuel or did the engine die like you turned the key off?

 

[DJ]

Re: RPMs won't go above 4000; were 5100

Dragon,<br /><br />A 2-cycle four cylinder outboard will turn up high rpm's, with no load, easily. Even if running on a few of its available cylinders.<br /><br />Running an outboard that hifg, without load is NOT good for it.

 

[Dhadley]

Re: RPMs won't go above 4000; were 5100

9 lbs compression is not real good. Im sure you mean 90. Lol!<br /><br />Good luck!

 

[DragonWagon]

Re: RPMs won't go above 4000; were 5100

DTS- The rpm change was so sudden that I immediately knew some happened and shut it down right away.<br />djohns19- You are correct. I just took it up to 5100 rpm for just a second.<br />Dhadley- Big OOPS on my part. I was looking at the inside numbers on my Compression Tester. The actual readings for the pounds per square inch were all between 125 and 130. <br />To all:<br />I just got back from the lake. Fortunately it's 45 minutes away from my house. However, the problem is still there, which I expected since I only checked a few items and tightened up one plug wire. The boat will plane off and run 36 mph at 4000 rpm, but it takes forever to get there. It feels like I'm running with the anchor out, and yes, I did check that too!

 

[DJ]

Re: RPMs won't go above 4000; were 5100

Dragon,<br /><br />You are running on three of your four avialable cylinders.<br /><br />Now take a look at the spark plugs, they will tell you a story.<br /><br />If I remember right, this engine runs on two cylinders at idle and kicks in the other two at 1800. There may be something wrong with whatever makes that transition.

 

[DragonWagon]

Re: RPMs won't go above 4000; were 5100

I rechecked the spark plugs today- three of four were uniformly tan- the fourth (bottom) plug still looks almost new, white. I pulled off the airbox and cranked the engine. The carb for the fourth (bottom) cylinder is BLOWING air out the front. None of the other three were doing this. What would make it do that? From my slight experience, the carbs should all be sucking air in, not blowing it out. Any thoughts?

 

[graddy]

Re: RPMs won't go above 4000; were 5100

Bad reeds.

 

[DragonWagon]

Re: RPMs won't go above 4000; were 5100

Aside from the disassembly and reassembly, is replacing the reed set a relatively straightforward job, or is this a job that only the pros should attempt?

 

[DragonWagon]

Re: RPMs won't go above 4000; were 5100

Well pro or no, I replaced the bottom reed set tonight. Out the expected nine round reeds, I had eight. No wonder it was blowing air out the front of the carb. One hole was completely uncovered. I will test the boat tomorrow. <br />Thanks to everyone who helped me find the solution. I also bought a Mercury Service Manual that covers my engine. That was a necessity for this repair, for me at least.<br />Thanks again,<br />DragonWagon