3.0L Mercruiser RPM issues

[Berniesm88]

Looking for a some assistance with my boat. For reference, its a 1996 Glastron SSV175 with the 3.0L Mercruiser and Alpha One drive. So i purchased this boat new this winter, stored it in my heated garage until spring. Spring time came around and I did all the correct PM items; oil change, fuel filters, and even replaced a few items that I thought needed to be swapped out after 20+ years; Complete carb rebuild, new T-stat, checked compression 140-150 all cylinders. Boat fires right up on the muffs but when i went to set idle, tach was acting funny. It was reading far to high for what sounded right on the engine, so i ignored it, set the idle by ear and went to the lake.

This is where things got more interesting, started right up off the trailer, idled fine, took off out the harbor (Lake Sunapee, NH) and the tach started bouncing, reading way to high (5000 RPM's) when just going 10-15MPH but motor was not sounding like it was not running high at all. Wrapped on the dash board and it settled. Got up on plane and things seemed to be off Tach was showing 3800 RPM's but I was only going about 25 MPH (Confirmed with GPS). Any ways figured tach was off as there was gobbs of throttle left and the motor sounded fine. Went for a cruise never took the tach over 4800 (WOT limit for my motor is 4400-4800) but at that shown RPM boat was only going about 30 MPH. Again there was tons of throttle left and the boat was only doing about 30MPH.

So I took the boat home, put on a brand new 21P prop (the old one was 19P and wasn't terribly damaged, but figured I'd rule this out). I also moved the tach selector switch forward and backwards a few times and set it back to position 1 which says its for a 4 cylinder. Went back out the next weekend. Wanted to make a full throttle run.

On the water, boat didnt really seem to act much different. maybe a 1-2MPH faster with the new prop. but same cruise RPM of almost 4000 RPM's to go 25-30 MPH. Tach was no longer bouncing. So I said F**K it and ran the throtlle all the way till it was maxed out and made a run. Engine sounded fine, but tach reading was crazy high. Confirmed GPS speed was 42MPH (speedo said 45) but RPM's climbed to almost 6K. I moved the selector to position 2 (wrong position for my motor) but seems to jive much better. made another WOT run, tach showed 3900 RPM's and speed was 42 MPH. Cruising speed of about 30mph now showed about 3K rpm. Which with the 21P prop seems more reasonable. Motor also sounded fine, i mean it was running at WO but still sounded good. Thinking I might put the 19P back on if i can figure this issue out.

So where would you suggest going from here? How would I verify the engine RPM, would something like the Tiny Tach work for my motor? How would i go about hooking it up. Finding a factory replacement tach is seeming difficult, best i can find is an entire sealed dash assembly. Any advice would be welcome, Thanks!

 

[havoc_squad]

If you intend on working on boat motors often enough, buy a digital timing light gun with a tach mode that gives a 4 stroke and 2 stroke mode.

I have the Actron digital timing light, it did the job for timing and gave me accurate RPMs.

 

[QBhoy]

Check the earths, connections at alternator too. Roughly you should be around the 3000 rpm mark at about 20ish mph with that set up and prop. Usually at least. Know plenty of similar generic boats like this with same set up. The well sorted prop examples (usually 21” 3 blade like laser 2) will do early 40’s gps and occasionally the odd 45 tops at about 4800ish rpm. Lowest planing speed is almost always around 3000 rpm give or take 100 rpm or so and about 20ish or just over mph if lucky.
Just as a guide. Hope that helps.

 

[QBhoy]

Also. Perhaps more than anything else. The tachometer on most boats is usually variably not accurate. It will maybe show close on occasion, but almost always tell tales for the most part.
Mine is about 150 rpm out at low rpm and about 200/300 rpm out at full throttle when comparing it to the real time data given by smart craft monitoring.