DF90a sluggish acceleration

[mvstill]

Hey folks, I've got a 2014 Suzuki DF90a that I inherited that's not running optimally. It probably sat for a year or more without being winterized. We got it last summer and immediately noticed it's sluggish at lower RPMs. It's mounted on a Pioneer Venture 175. We can cruise it around at low speeds and do alright. But if you go from zero to wide open throttle it just chugs and slloooooowly accelerates. Going from 3000 to 3500 RPMs takes forever, but once it hits 3500 it suddenly gets a burst of power and will start to plane out and does fine from there.

I know that's not a lot of information to go on but I'm new to the boating world. I've replaced the fuel/water separator filter and drained all the old fuel. I've also replaced the internal fuel filter. What are some other things I could look into cleaning/replacing?

Google searches keep pointing to a faulty neutral safety switch or possibly the Idle Air Control valve. Any thoughts on those or other ideas?

Thanks in advance!

[Moonlighter]

The IAC does one thing only, and that is to control the engine at idle speeds, to make it idle smoothly and stable. It has nothing to do with how the engine accelerates or runs at speeds higher than idle etc.

If it was the neutral safety switch, it would prevent the engine revving past about 3000rpm, and no further. It wouldnt suddenly switch off as the boat accelerates and cause a surge of acceleration.

So I would completely discount both of those theories as fanciful.

Something else is happening.

Have you changed basics like spark plugs?

Have you checked to make sure the electrical connections are clean and tight (undo, clean, tighten up, dont just look at them), and that the start battery is in tip top condition?

When it gets going, what is the max rpm that you can achieve at full throttle, trimmed out as far as practicable? The engine should be able to hit 6000rpm. If it doesnt then the whole issue may be that its overpropped.

[mvstill]

OK, thanks! I changed the plugs as well. I'll go re-seat all the connections to be sure. I've got access to a battery tester so I'll double-check on that as well.

Maybe you're on to something with the RPMs. I think at WOT we're only getting up to 4500ish RPMS. I know we're never hitting 5000. And that's regardless of whether it's loaded down or if it's just me on the boat.

We took it out last weekend to test it out after changing the plugs/gas/filters and noticed that when the water was choppy we weren't ever able to get up to a plane. It just chugged along at sub-3500 RPMs. Once the water smoothed out it was able to hit 3500 and then it just shoots up to 4000.

[Moonlighter]

What size is the prop that is currently fitted?

So, typically the 90’s run a 14” dia 3 blade prop.

Without knowing your boat its hard to say what prop is likely to suit it. Maybe a 3 blade 14 x 20 might be close. It will depend on the boat weight and especially if the hull bottom is clean or covered in weed and barnacles or not LOl!

Here is a link to a boat test on Suzuki’s website that may help. There are quite a few other boats tested there so you will probably find something similar to yours listed so you can see what props they run, and how that compares to your current prop. At least it should tell you if you are in the ballpark or not. And that will give you an idea of whether the issue is the prop or the engine not running right.

http://www.suzukimarine.com/product%...7%20LS%20DF90A

Link to DF90a details with the list of boats they have tested with that engine
http://www.suzukimarine.com/product%...012/df90a.aspx

[Moonlighter]

But you will see from all those tests that they prop these engines so they will at least hit 6000 at full throttle. If they wont hit something close to that, then assuming that ther isnt something wrong with the engine, then its overpropped snd needs a smaller pitch.

There is also a sticky thread in the top section of the forum that deals with engine heights and propellor selection, its well worth a read. Because an engine that is set far too deep will cause poor performance too.

[mvstill]

After comparing what I have to the tests Suzuki has done, I don't think it's overpropped. The prop is a 14" 3 blade, but I'm not sure on the pitch. This boat is freshwater only and the hull is super clean. I don't think anything else significant has changed (same prop, no damage, clean hull, no extra gear/accessories) but the performance has certainly decreased, so I'm guessing it's an engine issue. I suppose I should have mentioned that it previously didn't have this issue. This isn't a new setup.

Assuming it isn't overpropped, any thoughts on where to begin? An auto mechanic friend thinks it could be fuel delivery, possibly a clogged injector. Is it worth it/safe to run a fuel injector cleaner through?

[mvstill]

SOLVED: It wasn't the prop. It was one of three things. 1) New spark plugs. 2) Drained the VST (no water in it, but still) or 3) ran injector cleaner.

Doubting it was the injector cleaner as it fired right up immediately and ran as it should. It didn't need any time to run the cleaner through. Not sure about draining the VST. Is that even a possible cause? The plugs looked fine. Had about 100 hours on them and they weren't fouled or cracked or anything but I feel like they were the most likely of culprits.

Anyways, thanks Moonlighter for your assistance