Low RPM 2009 75 HP Four Stroke EFI

[mland]

I just recently purchased a used 2009 Crestliner 1650 Sport Angler with a '09 mercury 75hp four stroke efi. When I test drove the boat prior to purchase it started right up, and planed out quick enough. Well, quick enough I thought for the lowest rated motor recommended for this boat. In that test run we ran it up to 32-34 MPH (boat speedo) with 3 people in the boat and a full tank of gas. I've never owned a four stroke, and was amazed at how quiet it was. I was somewhat unimpressed with top speed. But, given it was the lowest HP rating for the boat my expectation was somewhat low. First chance after purchase I took it for a run with the GPS. Unfortunately the GPS reading is more accurate, and that measured a top speed of about 28 mph. Absolute quickest I've run is 30 mph with the wind and waves at my back; Me and a 15 year old on board and a full tank of fuel. So, that got me to start messing with prop and checking rpms. Tach reading at WOT was 4500 rpms, well below the range specified in the manual of 5000 to 6000 rpms. Ugh. The prop that the boat came with was somewhat junk, so I swapped out with a new one that also came with the boat. Pitch did not change (13.25 x 17p, 3 blades), but it was in new condition, unblemished. Needless to say the change was unnoticeable. I did run it for about 35 minutes at 3/4 throttle (one way), and then again back for another 35 minutes at 3/4 throttle. On the return I noticed a bit of a tick (tickticktickticktick, continuous). Given it was only my 3rd time out with the boat, I wasn't sure if it was there before and just hadn't noticed it. So, I then checked the oil and wow.... oil came gushing out of the dipstick. I was really mad about the mess I just made, because I've never seen the inside of a 6 year old motor look this clean. It was spotless. Anyway, after cleaning up the oil I proceeded with an oil change. The old oil seemed very runny, more the consistency of water than oil. Pretty sure this was fuel in the oil, but my nose is not the best (likely though). After filling to recommended level (about half way up the crosshatch) I took it back out for another run. No change. It starts right up, planes reasonably well as before, but max rpm stops at 4500 and max speed was right at 28 mph (full tank of gas, and one kid as before). After bringing it back in a rechecked the oil. Looks clean and not runny at all, but oil level is up to the top of the crosshatch vs. midway after the fill. Not really sure where to go with this next, I do have a mercury service shop close by that's really good. Curious if anyone has thoughts on this or has had similar experience with the 75hp merc four stroke?

 

[galamb]

I see used four strokes somewhat like a Vegas weekend. You might get a couple good rolls with the dice but most times you just come home with a little to a lot less money.

Four strokes need service and some of the most important service is in the first couple of years. That's when they should be taken to have the check sheet run through (not many guys have the valves checked/adjusted after the first season running as an example). And motors don't like to sit unused.

And you have to make a couple of assumptions here unless you have complete service history with receipts. First, the boat wasn't "perfect", if it was it would not have been sold. Second, you can't count on it being taken care of. Guys/Gals buy boats on "easy payment terms", use them a fair bit the first season, then by year two its a few long weekends, then by year three it's just a hassle to take them out. So they sit in the driveway or the garage until the better half makes them get rid of it.

Unused boats don't get maintenance. Most will not spend the 100 or so bucks an hour to have the regular maintenance done on something they only used "a few times". And particularly with 4 stroke outboards the service schedule is the "minimum" that should be done. These are not like dad's 2 stroke that will come back to life with some fresh gas and a new set of plugs. These are potentially expensive toys.

So I would have it gone over thoroughly by a Merc tech skilled in working on 4 strokes as opposed to some guy like me that works on them in my garage - that's fine for most two strokes but once you get into inject 4 strokes with sensors etc a guy with a screwdriver and a multi-meter is just a touch out of their league.

You might be running a tad too much pitch for the boat you have it on, but the dry weight of the boat itself is only 1000 pounds. Add the motor, battery, gas, etc and your rpms even with a 17 pitch sounds a little low which may indicate some other issues. With a 90 horse I would expect a top speed on that rig of just over the 40 mph mark. So your 75 should get it in the high 30 (36-38 at full throttle with a couple people on board could be expected).

As things currently stand, dropping to a 13.25x15 pitch would still leave you below 5000 rpms and that just doesn't sound right - the boat, if sound, is just not that heavy that a 75 should labour like that.

So the motor may "look clean" because it sat around more than it was used and that could well result in it needing a good bit of maintenance to get her up and running like she should be.

And your note about "how quiet it is", that is only because you haven't had it opened right up. While the sound is "different" than a 2 stroke, and yes, below the mid-3000 rpm range they are "quiet" in comparison, most models, at full throttle send the db meter to the same number as their 2 stroke counterparts of the same horsepower - likewise, the "better gas mileage" claims likewise are limited to running a lower rpms.

When you open these up they burn every bit as much gas as a 2 stroke.

Good luck with the motor, but do have it checked out by someone competent. There is little you can do in your driveway with these save changing the oil/plugs yourself once it's been restored.