DT65 throttle variation

[JohnOrrett]

Hi, was out in my brother in law's 17 ft dory style boat last week on an estuary. First time in the water since he purchased it.

Engine started and ran well, but at about 4000 revs while on the plane, the boat started porpoising a bit, and the revs dropped down to 3000 (safety limit I assume?). We tried altering the tilt and trim, but every time we increased the revs and the boat started bouncing a bit, the revs dropped.

I'm not sure if is because the prop was coming out of the water too much as it was bouncing around a bit - a local suggested the engine might have a worn rubber bush behind the prop, or whether the bouncing was affecting the throttle lever making it slip. I tried adjusting the friction setting at the front, but it made no difference.

It didn't do it every time, and the engine never missed a beat, but the drop in revs is confusing.

Anyone had a similar experience or any thoughts

Thanks,

John

[Solarman]

What year is this motor?
So much can cause porpoising?

When porpoising does motor come out of water completely? What rpms does it go up to when out of water? If out of water often, is it getting too hot/ overheating?

Such porpoising could be due to hull design, weight placement, motor position, or too much for design?

If over-revving too often damages could occur, maybe prop should be changed?

Post back when able.

[JohnOrrett]

Hi Solarman, the engine is a 1994. The problem is not over revving, but the revs dropping when bouncing (porpoising was the wrong description really). Hull design is good for sea fishing and handles swells etc. well. The boat needs people at the front to keep the bow down, but it still bounces when riding waves at speed.

As mentioned, the revs dropping down to 3,000 looks like a safety feature reading the handbook. If the throttle cable or something was slipping, I would expect to see variances in the rev drop, but it always dropped to 3,000. Engine was never out of the water and no overheating. We tried various trims too.

Maybe a fuel or an oil transfer issue or something with the bouncing, but never missed a beat and no visual or audible alarms.

At a steady pace, it is fine.

[Solarman]

OK. Then i suspect it may be a fuel problem, if it doesn't skip or miss when bounces. Possibly floats, or linkages in carbs bouncing too much? Does it run normally when in smooth water and not bouncing?

Have you looked at carbs to see if anything is extremely loose that may be causing this?

Post back when able.

[JohnOrrett]

It runs really well on a smooth river, no dropping of revs, nice and steady, 4,000 revs plus. I'm thinking that the bouncing is doing something to the engine, whether it be carbs, oil starvation or something, as it always drops to 3,000 revs. Will have a look at carbs over the weekend, but dont envisage finding much wrong with them.