Mercruiser Runs Rough Under Load

[davemoore]

Hello everyone... newbie here, our first boat, learning along the way and this forum and web site has been a great help.

Our boat: 18' Marlin 1983, Mercruiser 470 I/O. Was running rough first couple times out so I replaced plugs (which were gapped almost .010 less than they should have been) and set points (which were gapped about the same - almost half of what they should have been). Sitting on the trailer, engine ran a lot smoother and revved much quicker.

However, once we put it in the water yesterday, it still ran kinda rough as soon as I put it into gear... kinda like a muscle car with a racing cam... once we were up to crusing speed it smoothed out and ran fine, but at 1000-1500 in gear it's really rough... makes the whole boat shake.

Also, looks like the trim switch isn't working... down works fine, but up does nothing. Trailer button works fine. Saw some other posts here about that and will try the troubleshooting advice.

Could the shaking under load be caused by a bent prop? If so, are we just not noticing it at cruising speed?

Thanks for any help.

Dave

 

[davemoore]

Re: Mercruiser Runs Rough Under Load

That was my original plan, but when I got in there, I noticed that the cap, rotor, and points look brand new. And the wires are in excellent condition too. But the plugs looked like they'd been in there forever; in fact, I almost couldn't get one of them out... seriously stuck... and all four were quite black.

The timing is one thing I didn't check, and I don't know what it should be... Guess I'm off to subscribe to the online manual here for my boat.

Thanks.

 

[davemoore]

Re: Mercruiser Runs Rough Under Load

Good point Bubba1235. I'm off to check the timing too...

I wish I knew what distributor cap and rotor to ask for... I assume it's the same as some 4 cyl. auto engine's distributor.

No boat supply here in the area and I don't want to wait for mail order... I guess I could just take it in to the auto parts store and have them match it up...

Thanks,
Dave

 

[Don S]

Re: Mercruiser Runs Rough Under Load

It's a Delco distributor. If you have a NAPA, they usually carry Sierra marine parts.
I'll post the timing, dwell, and other tune up specs here in a few minutes.

 

[Don S]

Re: Mercruiser Runs Rough Under Load

Here you go.

 

[davemoore]

Re: Mercruiser Runs Rough Under Load

Hey - thanks Don!

Really appreciate the help.

I will call Napa now...


Dave

 

[davemoore]

Re: Mercruiser Runs Rough Under Load

Ok, so I went to our local Napa Auto and got new cap, rotor, condenser, points, coil and wires, put it all in, and wow! Fired right up! Running VERY smooth, can't wait to get her out in the water again. I think this motor is very sound, and runs cool even at 3500 RPM for 10-15 minutes on the lake.

Thanks Don for the specs - I also discovered that they're stamped on a label right on top of the valve cover!

Figured out why the trim up isn't working - there is a limit switch on each side of the outdrive with a pair of wires coming out of them. The starboard side wires aren't even connected to anything and the port side are skinned up and questionable. Now I have to figure out where to connect these to.

And I have a couple of questions, that may be really dumb, but being a newbie, how else do I learn? #1: What is the rule of thumb about running the motor with the outdrive up? Should it NEVER be done? I noticed that when I had it running with the unit up, at about 1500 RPM there was a really loud "rattling" noise in the upper unit, but when it's down, it wasn't there. Of course, I had the shifter in neutral. With the drive down, no matter what RPM's (800 - 4000), the outdrive is quiet.

#2: While in the water, to what degree does the outdrive move when you trim up? I assume all the way down is 0 degrees. I don't know the answer to this because my trim switch doesn't work, but from what I've read it seems like it's FAR less than when you "trailer" the outdrive.

I also discovered that the prop is in really bad shape. Lowered the outdrive and put it in gear, ran it up to about 2000 RPM and checked it, and it's wobbling pretty bad, but the shaft looks perfect.... so that's good...

Thanks for your patience and all the help here guys!

Dave

 

[davemoore]

Re: Mercruiser Runs Rough Under Load

Bubba1235:

Finally got my hands on a timing light and checked the timing - it was about 10* BTDC so I adjusted it to 8. With all the new ignition parts and having the timing adjusted, it purrs like a kitten now. Can't wait to get her back out on the water. Thanks for the suggestions.

 

[davemoore]

Re: Mercruiser Runs Rough Under Load

Hey Bubba1235:

Just wanted to say THANKS for pushing me to do a full tune up... we took her out again yesterday and wow!!! What a difference! It screams now! (compared to before)

I think I may have not purchased the right plugs though... they didn't have the exact part number and the guy tried to cross over to Motorcraft, and I don't think they're exactly right so I'm going to Napa to get the Champion RV9YC that are called for in the specs.

I'm also going to recheck the timing and points, because there was one time I had a little "run on" and several times it was hard to start up.

I'm going to get some Seafoam and run it through the engine too.

But what a difference in performance already... thanks for the tip!

Dave