Top end sucks

[meek77]

Top end sucks

hey all,<br /> top speed for my boat should be around 45-50, but with the new GPS i put in, its showing i'm topping around 40. I also noticed that when i took off the positive crank ventilation tubes that alot of vapor was comming out, this could be blow by, not sure yet. Anyway, would blow by cause the slow top speed? Last year the boat ran a lot faster. If its not that then any other advice would be great, thanks.<br />P.S. 86' 5.7L merc 260, with roch 2 barrel.

 

[rbezdon]

Re: Top end sucks

Re: Top end sucks

I am going to do my best to do an Don S imitation, "Do a complete tune up, points, plugs, condenser, cap, rotor, wires, timing, change all filters including fuel and water seperator, clean and inspect fuel delivery system and run a compression test." I hope I covered all of Don's favorite issues. After you complete the tune and get the compression tests let us know how the spark plugs looked and what the numbers on each cylinder are. Keep track of which plug came from which cylinder.

 

[meek77]

Re: Top end sucks

Re: Top end sucks

already did most of that this year. Parts replaced in the past two weeks: plugs, wires, cap, rotor, coil, battery, alternator, starter motor, belts, oil, , fuel filter (twice), oil filter, added star tron to fuel (enzyme break down), cleaned carb out but did not disassemble ( i think it has a flat spot in it). I've done more than my yearly maintanence so it has to be something else.

 

[rbezdon]

Re: Top end sucks

Re: Top end sucks

run the compression and it you can leak down test. That is the next most logical step. This can quickly rule out many possible issues or point you in a direction, and yup, Don is big on maintenance. If it hasn't been done for a while , di it and then look for problames. Seems like you took his advice.

 

[QC]

Re: Top end sucks

Re: Top end sucks

Also, it apparently is assumed that the load is the same, the bottom condition is the same, water and weather were the same, the propeller is the same, the trim settings are the same etc. etc. etc. ANY changes to the above negate last years performance numbers . . .<br /><br />Also, you say "new GPS". Where did you get your previous speed info? What is/was your WOT RPM?

 

[meek77]

Re: Top end sucks

Re: Top end sucks

My old GPS was a handheld magellan and I just installled a Garmin 182C on the dash. I assume that both GPS units are accurate. WOT is around 4000, same as last year. And yes conditions are practically the same, i always keep trim at the same level when going WOT. I run in the same bay as always, and on good days.

 

[Olds Eddie]

Re: Top end sucks

Re: Top end sucks

Marine speedometers read slow. My gps says 57 while my boat speedo is pegged on 60. Also if there is current present in the water you are running on it will make the gps read different when you are running with it or against it.

 

[jmccall001]

Re: Top end sucks

Re: Top end sucks

As an aside, make sure the GPS is set for mph not knots. 40 knots is 46 mph. Gps measures the speed made good. if there is a two knot current, a tide and headwind, you may show 40 mph and the speed calculations by rpm may show 50.

 

[QC]

Re: Top end sucks

Re: Top end sucks

If you are running the same propeller and getting the same RPM then I question your speed numbers. The RPM and propeller combo result in speed . . . so the same prop at the same RPM should get close to the same speed. With a heavier load RPM should drop and visa versa. Something isn't right if the RPM is truly the same. <br /><br />This is unrelated to the question of camparing last year to this year, but it sounds like the RPM is low. You should be in the 4200 - 4600 range.