Boat no longer goes top speed

[Romeo21779]

We have a 1991 Rinker 260 Fiesta Vee. It has a Mercruiser 454 (7.4 L) and a Bravo One outdrive. Up until recently the boat did 40 mph at WOTM at the proper RPM, don't remember off hand what the RPM was.

Now the boat goes about 33 mph at full throttle and the RPMs are lower. We changed the plugs and fuel filter and it gained about 3 mph, up from 30 before the plugs and filter. Nothing else has been changed.

Any ideas on what to check to try and get this resolved? Thanks in advance.

BTW, the boat did hit ground but it wasn't anything bad since it was a small sandbar and the prop is stainless. No damage other than a little paint missing from the skeg. But the boat did work perfectly fine after that.

::EDIT::
Mods, please move the thread if it's in the wrong section. Thanks.

 

[TowRoper]

Re: Boat no longer goes top speed

your talking about a 4 mph dif, did you have more people or an extra 6pack on this last outing?

 

[Romeo21779]

Re: Boat no longer goes top speed

The boat used to do 40 with 5 adults onboard, it barely hit 33(GPS for all speeds) with 2 adults onboard.

 

[Don S]

Re: Boat no longer goes top speed

http://forums.iboats.com/showthread.php?t=295070

 

[TowRoper]

Re: Boat no longer goes top speed

OK hmmmm i would check the prop even if it is SS its not indestructible if all is good there i would move onto the carb and look at the new plugs to see what the fuel mix is showing

 

[Romeo21779]

Re: Boat no longer goes top speed

What's the proper way to check for prop damage? I'm assuming it would have to be removed but that's a little harder to do since the boat is in the water.

And what should i be looking for when pulling the plugs. When we changed the plugs, on about half of them, the anodes were black and the other the anodes were white.

 

[TowRoper]

Re: Boat no longer goes top speed

just a simple visual to see if any blades are bent or cracked no real need to remove it

 

[Romeo21779]

Re: Boat no longer goes top speed

In that case, the prop looks fine, no chips or rough edges. I checked it right after the incident and kept an eye on it thereafter.

 

[CaptnReinell]

Re: Boat no longer goes top speed

What's the proper way to check for prop damage? I'm assuming it would have to be removed but that's a little harder to do since the boat is in the water.

And what should i be looking for when pulling the plugs. When we changed the plugs, on about half of them, the anodes were black and the other the anodes were white.

if you have white anodes you got water in the cylinder somewhere!!
which means your running on half of the cylinders.

 

[Romeo21779]

Re: Boat no longer goes top speed

Just to be on the same page.

The tips of all the plugs were black with the exception of the circled area in the picture which was white on half of the plugs and black on the other half.

BTW, the engine doesn't/didn't sound like it was misfiring or running rough.

 

[Apollo75]

Re: Boat no longer goes top speed

Well, the big dogs have chimed in --- I listen first before I through my two cents in --- probably what it is worth too.

I would check that list --- then depending on the intake manifold --- some plugs may run leaner than the others if there is a flow problem --- the manifold supplies half the cylinders from one side of the carb and the other half from the other side. If one side of the carb or jet etc. has some blockage half your plugs may be white running lean while the other half are dark from fuel.


OFM

 

[TilliamWe]

Re: Boat no longer goes top speed

Is the bottom of the boat dirty?

 

[BigBK]

Re: Boat no longer goes top speed

Is the bottom of the boat dirty?

I agree,
This would be my guess is the boat stored in the water? if so 5-7 mph would be typ for a dirty hull.

 

[Grand Larsony]

Re: Boat no longer goes top speed

Or did you have the canvas up\down when taking the different readings? I lose 5+ kph for a dirty bottom plus another 4-5 kph for having the canvas up.

 

[45Auto]

Re: Boat no longer goes top speed

Do a compression check .................

 

[Romeo21779]

Re: Boat no longer goes top speed

I guess it has been a while since we washed the bottom of the boat and the top was up during that outing. Never really though that the top would have that big of an affect on speed.

I'm guessing it would still be a good idea to clean out the carb after trying the boat with the top down and a clean hull.

The plugs were coming out white in a random order, btw. It wasn't that one side of the engine had all white plugs and the other had all black. Would that be carb or manifold related?

 

[TilliamWe]

Re: Boat no longer goes top speed

Ding Ding Ding, we have a winner. Clean the bottom and watch your top speed magically return to it's former self.

DO NOT continue to operate the boat with a dirty bottom. You are lugging the engine at all speeds and you WILL damage the valves.

This is the reason for anti-fouling bottom paint, btw.

 

[Romeo21779]

Re: Boat no longer goes top speed

Thanks for all the advice.

Boat is going to be pulled out of the water for cleaning this weekend.

Any ideas on why some of the plugs were white while others were black? I'd like to get that sorted out as well at this point. It seemed like every other plug was a different color.

 

[TilliamWe]

Re: Boat no longer goes top speed

You could have already damaged your valves. Do a compression check, if you like, as 45Auto suggested. If you have a few really low cylinders, you probably have tulipped valves. Then you are in for some cylinder head reconditioning. Have fun, and good luck.

 

[Romeo21779]

Re: Boat no longer goes top speed

After all said and done, the mystery is solved. We made to the last channel marker of the marina and the fuel pump seized up completely. So it's time to pull the old one off and pickup a new one on monday.