2005 Suzuki DF70 prop & rev limit issue

[Bob1Crab]

I have a 2005 Suzuki DF70 70HP 4 stroke with approximately 50 hours on a 17' Cape Craft CC that ran fine but after a year or so the over rev light started coming on at WOT after a few minutes and powers the motor down unless I keep the boat at 4500 RPM or lower. I currently have an aluminum prop 13.5 X 15 and a SE Sport 300 cavitation plate that were put on by the previous owner. The previous owner said the boat came with a 17" pitch prop that he felt wasn't enough for the hole shot, so he went to the 15 pitch and added the whale tail. The boat has always topped out a little low at 5000 RPM, by specs should be 5200-5800. Unlike the previous owner I'd rather have more speed on WOT and hole shot is not important. Either way I can't figure out the over rev issue. I've always had about the same mid to heavy weight load before and after this issue and I'm inclined to buy a 17" prop like the boat originally had and ditch the whale tail.

Two questions:
1- What's causing the rev limit light to come on at 5000 when the optimal WOT range is 5200-5800?
2- Would a 17' pitch increase WOT RPM? (I understand it should increase MPH on heavy loads)

 

[Silvertip]

Re: 2005 Suzuki DF70 prop & rev limit issue

You have the theory wrong. More pitch = higher speeds and lower rpm but that is load dependent. Heavier loads require less pitch and higher rpm. Higher speeds can be achieved with "lighter" loads. As for the engine you need to have it checked. The prop has nothing to do with the light coming on. The engine has a computer that is constantly monitoring engine RPM. It wouldn't matter which prop you had -- 5800 RPM is 5800 so anything above that should trip the alarm..

 

[Bob1Crab]

Re: 2005 Suzuki DF70 prop & rev limit issue

Spoke to the former owner yesterday & he said the boat topped out at around 5400 (just right) when he first put the 15" pitch prop on and was surprised I've never exceeded 5000 since I've had it. I know I put more weight on the boat than he ever did with my family, dogs, & coolers so I thought weight load might be an issue. I've also been told that my prop might be beat up to the point that the pitch has changed or that the hub could be slipping. I live on the water where getting in & out at low tide can be brutal on the prop. Could there be any truth to the hub slipping or pitch change theory? Although the motor is 6 years old it has very low hours and isn't due for a service so I'm thinking of taking the prop to a specialist to check it out. My boat hangs behind my house and the ramp is a long trip so taking just the prop in first is a little easier although I can see a full service is on the horizon.

 

[Silvertip]

Re: 2005 Suzuki DF70 prop & rev limit issue

You are missing the point. The tachometer doesn't know or care what prop you have or if anything is slipping. The engine electronics (the Computer) knows how fast the engine is running and it is setting the rev limiter. In fact it would activate the rev limiter even if you didn't have a tach. Changing props or repairing props is not going to make this work the way it is supposed to. For all you know, the tach is wrong, and the engine actually IS running in excess of 5800 rpm.

 

[Bob1Crab]

Re: 2005 Suzuki DF70 prop & rev limit issue

Just had a long conversation with a Suzuki service tech and he said the prop could be part of the problem. Thanks for trying to help but it looks like you're missing something.

 

[clanton]

Re: 2005 Suzuki DF70 prop & rev limit issue

Check the Tach, with shop tach. If the tach is correct and you still have problems connect a laptop with the Suzuki program, check for codes, over heat, low oil pressure. The rev limit should not lower the rpms much, only hold it close to the limit. Rev limit lite may indicate other problem, have to check manual.

Silvertip is right on.

I`m a tech, I think, can`t remember, anything after being hit in the head with a Harley chain. LOL

 

[Silvertip]

Re: 2005 Suzuki DF70 prop & rev limit issue

I think your tech is missing something or misinterpreted your description of what is happening.. The only thing on that engine that monitors rpm is the engine ECU. If the engine is running less than the rev limited rpm 5800+ in this case, the ECU will not reduce revs unless there is some other issue such as overheat, low oil, etc. The prop cannot have anything to do with this UNLESS there is too little pitch and the engine actually IS over reving. In that case IF the tach was accurate it would show it. Besides, the tach is not where rev limiting is detected. As I pointed out earlier, the rev limiter would set even if you didn't have a tach. Buy a new prop if you really feel that's the issue. But I contend you'd be wasting your money. A slipping hub cannot show 5000 rpm and result in an engine overrev. An incorrect tach however may be showing only 5000 but the engine actually is reving beyond 5800. that would take two failures however. Prop and tach.