1987 Carver 3207 Cruising speed not as per specs

[flo2]

I purchased my boat at the end of last season, did a sea trial at 17 Knots.
This year I cannot reach above 15Kn at 3000 RPM
I have Mercruiser 5.7L and both engines behave the same, if I push the throttle above 3000, the RPM starts going down.
Someone told me it could be the wrong pitch on propellers.
I did a survey at the beginning of the season and it shows I have propellers 18 x 18 as er the specs. Not sure if the inspector took the value in the specs or an actual reading on propeller.

Any info or comments on the above would be appreciated.

Mike

[Midnightsun]

Cannot understand the rpms dropping as more throttle is applied above 3000 rpm. Something is amiss with the engine/s, fuel delivery or timing advance. Someone with more knowledge in this respect will surely chime in.

[waybomb]

When were the multiple fuel filters changed? How about the anti-siphon valves - I do not believe in cleaning them; they are so inexpensive and generally hard to get to, just change them out with new. Clean the screens on your fuel tank vents.

Spark plugs/wires/cap/rotor how old?

Check total timing, not just base timing. Base timing is at idle, total timing would be at whatever rpm your spark controller stops advancing.

None of this is very expensive, and would rule out the basics.

[g36]

Dirty fuel filters can cause this for sure

[km1125]

Yes, if it ran good at one point, then it's certainly not the prop sizing. They don't change by themselves. They could get fouled or damaged.

Is the bottom clean?

I also think that filters or timing advance are your best avenues for troubleshooting.

[bud37]

Welcome to the forum Mike.....all good suggestions so far....only one question , since the sea trial and now, how much heavier do you think the boat might be.....eg sea trial with tanks low and now you have full water all your stuff and maybe more fuel....anyhow, good luck with your new boat....

[flo2]

Thanks for all of your replies.

When I purchased the boat, previous owner had the engines redone and at the time of the transaction engines had 70 hours.(apparently)
I did about 130 hours since.

I know for a fact that water tanks were empty and fuel tanks were at less than half at the sea trial so this might be the difference between the 17kn and 15 kn.

The bottom is clean, navigating in fresh water (Lake Champlain) which I would categorize as clear water.

As for the RPM dropping at 3000rpm, it is happening on both engines so perhaps the mechanics who did the adjustments possibly did both of them wrong.

Last weekend was quite rocking and I lost an engine so I replaced an inline fuel filter and a fuel/water separator but I am not aware of an anti-siphon valve, will have to look this up. Anyways, after replacing both filters, engine ran fine but still have the 3000 rpm limit.

Mike

[waybomb]

Anti siphon valves are at the fuel tank. All the ones I've seen look like a long brass house barb.

[flo2]

Hi to all,
I know it's been a while since my first post but I wanted to come back and inform on the issue I was having so it turns out it was a bad coil.
Why both engines had the same limit, I presume the one with the bad coil had the limitation and the other engine could not exceed that limit by itself.
I was told, I had an automotive coil and not a marine coil... looks like one is oil filed and the other is air filed and I don't see why it would make a difference but after replacing the coil, I can now bring it up to 25Kn

Life is good.
Cheers!

[bud37]

Thanks for posting the resolution, that I am sure will be very helpful for folks .......quite the difference....