My Top Speed Dropped from 45MPH to 35 MPH

[milkyway]

1999 Alpha 5.0L Thunderbolt V

I have not gone wot for sometime, not even last year, but when I did last week and then today, I could only go 35Mph on calm water. Used to be 45mph and this was the specifications. Bottom paint is good and smooth. FYI - I installed new rotor and sensor assembly 2 weeks ago and I am not sure if this is even relevant. And it seems like fuel consumptin has gone up. Thanks.

 

[milkyway]

Re: My Top Speed Dropped from 45MPH to 35 MPH

As added information - boat planes out nice and quick and engine runs smooth, starts well, new plugs and wires this season, compression was checked last month and ranged 175 to 180. It is only the top speed that concerns me. One thing also is that I get some beeps from somewhere and this happens everytime I slow down from cruising speed of 25mph. Happened again today when slowing down from 35mph. Tach, however, is not consistent and I don't pay attention to it. Moves up and down though engine does not display change in rpm.

 

[Don S]

Re: My Top Speed Dropped from 45MPH to 35 MPH

Get a tach, either a shop tach or a new tach and find out what the actual (accurate) rpms are. Without the rpm data, the rest of the information means nothing.

 

[Boatist]

Re: My Top Speed Dropped from 45MPH to 35 MPH

How much extra gear and weight have you added since last time it did 45 MPH??
Was 45 with full fuel load of empty??
Is it the same prop as before and what condition is it in??
Have you checked your Timing? Have you adjusted the valves?
Best speed is with the bow high.
Are you running the same type fuel, some fuel today contain Alcohol which do not produce as much power.

 

[milkyway]

Re: My Top Speed Dropped from 45MPH to 35 MPH

Will report back digital tach reading next week.

My gear load has not changed. In fact, I don't carry full tank, usually half now, but before it was always full with 63 gallons and it ran 45mph. Prop is same condition -scraped some sand and clay but that's only cosmetic change, no dents or bents.

 

[QC]

Re: My Top Speed Dropped from 45MPH to 35 MPH

Where you getting your speed number?

 

[milkyway]

Re: My Top Speed Dropped from 45MPH to 35 MPH

From the dashboard speedometer it reads 35mph at wot . At the same time GPS/charplotter reads 30.1 knots speed at wot.

Othes forum recommends checking the ignition module if it's getting certain 8 volts from the knock sensor- around that topic/area.

 

[milkyway]

Re: My Top Speed Dropped from 45MPH to 35 MPH

Is there any other checking tha I can do on the trailer until I get a correct rpm reading on water? I don't expect to take her on water for next few days.

Don - When checking the rpm, can I just run the engine on neutral and WOT, or should I run it loaded? If neutral is ok, I think I can put a large water container and dunk the impeller holes, remove the prop, and run it wot.

 

[Boatist]

Re: My Top Speed Dropped from 45MPH to 35 MPH

If you run it WOT without a prop or in neutral You will blow the engines is seconds. Engine will excees red line and tag all the valves with possible holes is pistons.

 

[milkyway]

Re: My Top Speed Dropped from 45MPH to 35 MPH

You must be right, Boatist. No wot on trailer.

While I find the time to take the boat out and use my Actron digital tachometer, could there be anything about the carburetor, a problem with it, that can cause this? How about the ignition system? Just thinking ahead. I just feel that the engine will not show 4400-4800 rpm at wot.

This is my 3rd season and my engine oil still has no and does not smell like it has fuel in it. Thanks to everyone for the tune up help.

 

[tommays]

Re: My Top Speed Dropped from 45MPH to 35 MPH

There was a question if you ever got the timeing set correctly and check for full advance ?


Tommays

 

[milkyway]

Re: My Top Speed Dropped from 45MPH to 35 MPH

I got the timing set at 10BTDC per specs.

Don't know how to check for full advance. is this where you just run the engine to the desired rpm and watch if the rotating timing mark will advance to specified timing?

 

[tommays]

Re: My Top Speed Dropped from 45MPH to 35 MPH

Part of setting the timeing is checking if you get full advance at the correct rpm

i dont know what that is on your motor BUt if its not reaching it you will not get full power

"for example 32 deg at 2500 RPM" JUST AND EXAMPLE !!!!

Tommays

 

[milkyway]

Re: My Top Speed Dropped from 45MPH to 35 MPH

Ok I got it. Tommays.

Looking a my last month's post, I used the advanced timing lite to set the timing at 10BTDC. I stopped right there. I did not do any further to check if the timing would advance with higher rpm. I have the rpm and avanced timing chart at home and will check this tonight. I suppose with the advanced timing light, all I have to do is increase the timing lite's degrees, say 30 degrees, and run the engine at the correct rpm to match this, and the rotating mark and the reference mark should be aligned.

Now I remember why I did not check for advanced timing - I was working on muffs on trailer and I could not raise the rpm too much for the muff might fail. I will use a big bucket and dunk the impeller to solve his problem and be able to check advanced timing.

One step ahead, please - what could be the main problem if the timing does not advance properly with the specified rpm?

 

[milkyway]

Re: My Top Speed Dropped from 45MPH to 35 MPH

I have read something about mechanical, vacuum and total advance. Too much for me to absorb. I will post the results of my advanced timing and I will take if from there.

 

[milkyway]

Re: My Top Speed Dropped from 45MPH to 35 MPH

Took the boat out and ran WOT with a service tachometer. Top speed was 31 knots or 35mph per GPS with 4100 rpm full throttle. Specs WOT is 4400-4800. Before taking her out, i did three things:

1. Cleaned flame arrestor
2. Cleaned antisiphon valve.
3. Checked timing. Timing changed from last setting -probably due to replacement of sensor wheel and sensor assembly.

I would appreciate any suggestions.

 

[Reel Poor]

Re: My Top Speed Dropped from 45MPH to 35 MPH

While you had the boat out would have been a great time to check for total timming advance.

Still need to know what it is totaling at? ? ?

 

[milkyway]

Re: My Top Speed Dropped from 45MPH to 35 MPH

My engine is 1998 Merc 5.0 carbureted with 220HP. Specified wot rpm is 4400-4800.

The comparable boat is a 2000 Maxum with 5.7 EFI and 260 HP. On the performance review, this boat which has the same hull design and dry weight ran 32 knots at 4000rpm and 40 knots at 4800rpm.

My boat ran 31 knots at 4100rpm, with half tank, some water, 2 anchors, and maybe 20 more pounds of other accessories. The speed in knots seems comparable with the other model - in the 4000-4100 rpm range.

If I downgrade to a lower pitch of propeller, this will increase my rpm and therefore increase my speed? And since my wot is below specifications, I should first try this? I have used the dashboard tach and speedometer and it must have given me incorrect readings over the last 3 seasons. And I have changed prop size 2 times but I can't remember what pitches changed.

 

[tommays]

Re: My Top Speed Dropped from 45MPH to 35 MPH

You seem to keep avoiding the timeing issue which will have a BIG inpact on performance if its not correct


Tommays

 

[milkyway]

Re: My Top Speed Dropped from 45MPH to 35 MPH

Tommays - I was not able to check the advanced timing coz I keep bouncing on the water and got concerned about disconnecting any wire and shorting anything. I had to stop after doing a wot rpm check. I also couldn't get a big and deep container to do it on trailer.