Suzuki DF115 stumbles and slows -- need a new prop?

[pcbayou]

My Suzuki DF115 4-stroke began slowing down and "stumbling" at 5000-5500 rpm a few weeks ago. On my last boat trip, the stumbling started around 4000 rpm and now is happening around 3000.M

My mechanic and dealer said I may need a new prop with a higher pitch, because the current prop is turning too fast and causing the over-rev limiter to kick in. (The max RPM is 5000 to 6000). They also said a 19 pitch prop , rather than the current 17, may be better suited to that boat and motor. The boat is an 18-foot Palm Beach center console, fairly similar to Century, Cape Horn and Bayliner, for those unfamiliar with Palm Beach.

My prop is old and a little nicked up. But I'm wondering if it's normal to have propeller problems at that lower rpm level.

Any thoughts are appreciated.

[Moonlighter]

That does not sound right, what you are being told, based on your description.

The DF115 rev limiter does not cut in until about 6400rpm, so if your engine is misfiring between 5000-5500 then it is very unlikely to be related to the rev limiter.

In fact, you NEED to prop these engines so that they will reach 6000rpm at full throttle otherwise they dont perform well. Mine revs to nearly 6200rpm with the boat lightly loaded. I run a Suzuki 14 x 20" stainless prop, on my 18ft HD alloy Surtees Workmate cuddy (you wont have seen these boats in the US). Its a really good match.

I think that something else is wrong, unless there is more to the story?

Before this issue, what was your boats max rpm at full throttle? Was it going well until this sudden change? Was anything else done to the motor or the boats fuel system?

However, based on what you have told us so far, i would be looking at the simple things, like fuel and spark. Dirty fuel/water in fuel can cause these kinds of symptoms. Dirty or blocked injectors dont help an engine run well.

You ARE running a good quality water separating fuel filter, such as a Racor style filter, between the tank and the engine, arent you?? (They are absolutely essential on these efi motors). If so, have you changed the element lately?

It could also be a simple thing like a failing spark plug or bad spark plug leads.

Does your Suzuki dealer/mechanic have the Suzuki SDS system, and if so have they run the diagnostics and tests that can be done on that system?