90hp Merc 2 stroke - Over Revving

[snsbrian]

I recently purchased another used boat and up until now haven't had to deal with over revving. I have a 17ft alum DC with a 1995 90hp 2 stroke merc. The boat came with a 13 x 19 prop, which I just replaced with a 13 x 21. The total weight when driven is about 1700 lbs. With the new 21 pitch prop the boat will rev in excess of 6200 (supposed to be 5400 max). With the 19 pitch I stopped at 6500 before I realized how high it was. The speed at 5400 is below 30 and I don't know what is good/bad/normal. The motor runs ok, but I have only its current condition as a reference, Any help will be appreciated.

-Brian

 

[Texasmark]

Re: 90hp Merc 2 stroke - Over Revving

Brian, I run a 17' alum bass boat with the '02 version of that engine with the 2.3 gearbox which I'm sure yours is also. I'm probably running the same weight rig. On/about july 25 of this year, I responded to a post that was titled something like "more on ole hole shot", all lower case as I recall. I included pictures of my hull and my prop. Pull up my archives.

I bought the boat with a Ballistic 17P SS prop on it with the boat being 2 years old. My first time out, I punched it out of the hole and it nearly snapped my neck. It was at redline just after planing out....obvious reactions: how did the prev. owner use this boat in this condition, and wow, this sucker is wayyyyy under propped.

I swagged another Ballistic SS prop and chose 21P. That helped but didn't solve my problem. I bought another Ballistic SS and this time went for the 24P which I have been running for 2-3 years now. It runs my little boat at 5600-5800 1-2 people in it at 48.5 to 50+ gps on a cool day and calm.....my avatar is a pic of it's wake.

The engine is jacked up one hole from sitting on the transom which puts the anti-vent about 1" above the 3rd (lowest) step in the hull). The transom has a high tilt to it and I run the engine at max trim out when running at WOT.

I run 30 mph at somewhere around 3000-3500 as I recall.

You have something wrong and I'd like to help you find it. Pics would help, especially of the engine in the vertical position showing it and the bottom of the transom from a side view (sitting on your trailer) and rear view showing the same vertical engine position, lower rear transom area, and prop.

Where did you get this new prop? Is it new? A spun hub would surely be a possibility if you purchased a used 21P. Your numbers are so bad that the problem will just jump out at us.

Come back and while you are doing that, I'll see if I can find the thread with the pics I took and clip them on.

Mark

P.S. I just ran back through the archives and found my pictures. I answered the post 3 times and the pics are attached to my last answer:

Date and title of orig post: "Same ol more holeshot", Posted July 25 @ 3:47 pm.....currently on Page 10 of this forum "Prop Questions and Topics".

My response to that post with pictures: July 26 @ 8:58 pm.

 

[SparkieBoat]

Re: 90hp Merc 2 stroke - Over Revving

you need a new RPM gauge..sounds like to me//. a 19P at 6500 rpms is flying

 

[Texasmark]

Re: 90hp Merc 2 stroke - Over Revving

you need a new RPM gauge..sounds like to me//. a 19P at 6500 rpms is flying

Yeah, ......or a new rpm gauge (tach). And that probably is the answer if he bought a NEW 21P.....and since he just purchased the boat, problem could be a simple as it is set to the wrong pulse rate (tweak is on the rear).

Mark

 

[craze1cars]

Re: 90hp Merc 2 stroke - Over Revving

Assumptions:

2.30 gear ratio, 5,400 rpm, 21 pitch prop = theoretical top speed of 47 mph. If you are going "less than 30" at that RPM, that means you have nearly 40% slip.

This is not a realistic figure at all...it should be somewhere between 5% at absolute best and 20% at absolute worst.

I will suspect one of three things: 1. Sparkie is correct and your tach is reading incorrectly. Confirm your tach with another known good tach. 2. Your speedometer is way off. Confirm with a GPS unit. 3. Your motor placement is incorrect, and you're sucking air into the prop when you should not be....possibly trimmed out way too far, or motor placement too high. This would cause ventillation, which creates extreme slip.

I suppose a fourth possibility is that your gear ratio is not 2.30. But I believe it is. Can you confirm this spec?

Photos of your motor placement would be helpful as per Texasmark's post...

Your boat should have no problem reaching speeds in the mid-40's, if properly set up.

 

[snsbrian]

Re: 90hp Merc 2 stroke - Over Revving

Wow, this forum works...
I've attached some pictures that I hope help..The plate seems to be level with the bottom of the boat. With the 19p prop on first I compared it with my gps and the speedo seemed to be right on, but since the new prop I haven't compared the two, but will next time out. When I bought the new 21p prop I also purchased a new hub also. I would also say I don't think I over trim it, most the time I have kids on it so doing wheelies makes the kids laugh, but not the dads. Another thing I'm wondering is that my motor is a 3 cylinder, and what if I had one cyclinder having carb problems and not running strong which I believe is going on. (but don't plan on working on it until the winter). Could this cause the motor to still pull the RPM's, but not make enough power for speed.

One other thing that is in my knowledge base with this is that last year on the second trip I had a situation where the boat seemed to run smoother and the speed was up around 35 which has happened since. I may have been a little foggy that night, but just another part of this equation to ponder.

Thanks again everyone for the help, it was a great way to wake up and check my message.

-Brian

 

[SparkieBoat]

Re: 90hp Merc 2 stroke - Over Revving

most likely you have a faulty RPM gauge..possible it is on the wrong pulse setting (on back of gauge). looks like you could raise that motor up a hole or 2..raise it to the bottom holes then see if it ventilates. if it ventilates, lower it 1 hole until it stops venting..make sure you have the motor trimmed all the way down when starting out...you must fix/replace tach before you can do this. you should get 40 MPH easily, maybe 45.... a manual 10" jack plate would probably get you another inch higher, than with out it and maybe another 2 or 3 MPH too, plus better MPG.

 

[snsbrian]

Re: 90hp Merc 2 stroke - Over Revving

Thanks SparkieBoat.... I looked at the back of the tach and it was set on 4. There was some info on a website that said it should be set at 5I set it at 5, and plan on testing it, along with my GPS speed in the morning. Its raining up here in Michigan, or trust me I'd be out right now. I've suspected the tack the last few trips, but was playing it safe with the rpms until I did the research. Everyone here has saved me hours of research before I would have found the adjustment on the back of the guage. Growing up in SE Michigan we have a trait of willing to work hard and doing everything ourselves...so this problem was going down eventually. I can only hope its this easy, but I still need to verify the correct pulse rate.

 

[Texasmark]

Re: 90hp Merc 2 stroke - Over Revving

The gear ratio is 2.3:1. With that much of a v at the transom, yours it not a straight forward approach. Sparkieboat may have the right answer. Won't hurt to try.

I had a temperature sensitive CDM (fires the plugs) on my engine. Real hard to tell when one cylinder is cutting out unless you are coming out of the hole and the engine is really working hard. Mine failed when engine was first operated for the day.

Mark