Problems with new motor

Okay guys. Here it is. I have a Grizzly 1648 and used to have a 25 hp mercury on it. It got on plane fast and topped out around 25 mph. Last summer I bought a 40 hp Nissan and put on it. Ever since I’ve had it, it hasn’t been fast, will barely get the boat on plane…after a very long time…and runs a lot slower than the 25.

I’ve had a new carb put on it…no change.
I dropped the pitch on my prop from a 3 blade 12 pitch to an 8 pitch 4 blade.

Everything about this motor checks out. But when you put it under a load, it never reaches WOT. Actually, it only feels like it’s running half throttle once you’re on plane.

So, yesterday I noticed that the anti-cavitation plate is about 3 inches below the bottom of the boat and it’s my understanding that it should be running above the water on plane….which it isn’t.

My question is: Could this be the problem? I’m going to make a jack plate and raise the motor to see. But, if this isn’t the problem, where do I go from here???

Yes, this is likely the problem. You are dragging 3 inches of lower unit through the water. This is what is slowing you down. Plus, it is probably causing increased hull drag.

-J.

3″ is a lot of drag.I would set the anti cavitation plate even with bottom of boat to start with.Also a tach is almost essential for finding proper pitch prop.

That boat should fly with that motor,that is a 3-cylinder motor?I would have it checked out thoroughly again.An 8″ prop would overrev that motor,even with gearcase that low.

Is this a two stroke? Are you sure you’re firing on all cylinders (literally)? Dropping a cylinder will manifest exactly as you are describing.

My motor was well under the boat as well, but didn’t have an effect on overall speed. Just was a beast to get on plane.

First thing I would check would be is if the shaft length of the new motor is correct for the boat? If the transom on the boat is meant for a 20″ shaft motor and your new motor is a 25″ shaft you will have very poor performance like you stated.

A couple of things to check:

1. Trim. Is this a power trim setup on the 40 HP?

If power trim/tilt, are you trimming the outboard all the way down and do you have the trim pin set at the right level to get on plane? Then are you trimming up to the correct running trim?

If not power trim, do you have the trim stop pin in the correct position so that the boat is riding correctly when on plane?

2. What is your transom height? Is this new motor a long shaft where the Merc was a short shaft?

Your anti-cavitation plate should be set at or just below the bottom of the hull. If it’s below that level, you either have the wrong shaft length or you need to adjust the transom height, but 3 inches to me sounds wrong. That’s a lot of adjustment to make under normal circumstances with the correct outboard shaft length.

3. Are you sure the motor is running right. BK is right, this sounds like a situation where 1 cylinder is dropping out or not firing. This can be more difficult to spot than most people would think.

Grouse

Wow. Thanks for the fast responses.

To answer some of your questions:
This motor has a 23″ shaft length and I have a 20″ transom. I looked up how to get this measurement and measured it. I was actually surprised because I didn’t know there was anything in tiller steer except 15 and 20″.

This is a 2 stroke motor…Nissan NS40C. The guys at the boat shop did a thorough check on everything and said that compression is excellent (135) and, mechanically, everything is as it should be. They were the ones that asked if I had the right prop, which led to me buying the one I did.

The prop I bought is actually the one recommended for this motor under a “heavy load”. I got the specs off the Nissan/Tahotsu website. BTW, a heavy load is considered anything over 1500 lbs. Since I usually have at least 2 guys in it and I’ve customed out the inside (adding a lot of weight), I figure I’m probably pushing 2000 lbs.

So….since this is a 2 cylinder, I’d assume that if one cylinder was “dropping out”, it would be noticeable. Engine runs smooth, even on plane. The issue is, it runs like you’re at half throttle.

I’ve read some posts that say running the motor too deep has a significant affect, whereas others say that it’s hardly noticeable where RPMs are concerned. I don’t know, I just want it to run right.

Am I wrong in assuming that a 40 hp 2 stroke should outperform a 25 hp 2 stroke?

One more thing. This does not have power tilt/trim and I’ve played with the trim adjustments A LOT. No change whatsoever.

When you rev the engine in neutral, you can feel the difference in power between it and the 25 I used to have. Feels like a beast in comparison.

When you put it in gear, idle speed or a little faster, you can feel a VERY noticeable difference between it and the 25. Moves the boat MUCH easier and quicker.

It’s when you shower down on it to take off that you get the big let-down. Takes a while to get up on plane and, even once you’re up, the sound of the engine doesn’t change. It’s literally like you’re holding the throttle in the half-way position.

The guy I bought the motor from had it on an aluminum V-hull boat that matched my boats dimensions and weight almost exactly and this thing flew on his boat. Unfortunately, I didn’t look to see where the anti cavatation plate was in relation to the bottom of his transom. I’d assume it was in the correct position, meaning he may have had a deeper transom than I do. If memory serves, I think his transom was “lifted” where the motor mounted on, but I could be wrong.

Scott Beech wrote:

Wow. Thanks for the fast responses.

So….since this is a 2 cylinder, I’d assume that if one cylinder was “dropping out”, it would be noticeable. Engine runs smooth, even on plane. The issue is, it runs like you’re at half throttle.

Not necessarily. Two strokes can run just fine on one cylinder. Really simple test would be to run the motor (in a barrel or with muffs) and take an INSULATED pliers and pull one plug wire at a time to see if it dies.

Newb comment here, but does correct gas lines have any influence? Perhaps same fittings but smaller lines only allowing so much fuel to get through?

I’ll check for the dropped cylinder. Who knows? Could be.

As for the gas issue, nhamm; I’ve wondered about this from day one. I didn’t get his gas can when I bought the motor because it was one of the old metal types that had seen better days.

I bought a new gas can (plastic type), hoses, and fittings. I did have the fitting that hooks into the motor because he cut it off and gave it to me.

I’m wondering if there’s a way to test for this. I was thinking that I could maybe put an inline filter (clear) and see if it stays full while opening up. I’m not sure this would work or not though. Any ideas to test to see if it’s getting enough gas?

Engine would probably cut out if it wasn’t getting enough fuel.

Was there any change in RPM from the 12/3 prop to the 8/4 prop?

Another thing to check is to verify that throttle linkage is advancing all the way.

If anything.I would get a service manual.Those motors are pretty simple in design,and tough.

There’s no way to be sure, but this really sounds to me like you are firing on only one cylinder full stop or one cylinder is cutting out or misfiring at higher RPMs. There’s just no way a 40 should be this big of a dog.

Use a spark checker to check both cylinders for spark. Inspect spark plugs for signs of uniformity, any major difference between the appearance of the plugs could tip you off. I’m not a fan of pulling plug wires.

Other possible issues could include air leaks, etc, but I would think that would be noticeable at all RPM levels. In theory, some kind of carb bockage could be possible as well, but again I’d think that this would cause other issues.

Certainly check the throttle linkage and be sure it appears to allow for WOT position. This should be easy to check visually.

Grouse