Top end Performance

[Tiras]

1992 4.3 MERCRUISER 1.85GR and a 14 x 21 prop. 256 hours on the motor and it sounds good. Motor sounds great but it seems like it should have more. I'm only seeing a 3800 RPM output at 35 MPH WOT. I'm going to verify the RPM with a tach to make sure I'm reading right. Speedo I'm assuming it right. Old boat may have sticky tach syndrome. Pulled the carb and rebuilt it. Put on a clear 3/8" automotive fuel filter. Changed the fuel hose and left the original style electric fuel pump. Added a high energy ignition coil.

Any suggestions of what else to look at before I change out the fuel filter?
Should I put in a better fuel filter system?
Could it still be fuel delivery or something else?
Plug wires? Plugs?
I could install a fuel pressure gauge but what would the fuel pressure be at WOT?
I'm wondering if there's an easy way to check fuel delivery at WOT to know if I have what I need?

 

[thumpar]

Re: Top end Performance

Did you get it trimmed out? Is the hull clean. 21 pitch is getting up there but should be getting more RPM depending. How big is the boat?

 

[HT32BSX115]

Re: Top end Performance

1992 4.3 MERCRUISER 1.85GR and a 14 x 21 prop. 256 hours on the motor and it sounds good. Motor sounds great but it seems like it should have more. I'm only seeing a 3800 RPM output at 35 MPH WOT. I'm going to verify the RPM with a tach to make sure I'm reading right. Speedo I'm assuming it right. Old boat may have sticky tach syndrome. Pulled the carb and rebuilt it. Put on a clear 3/8" automotive fuel filter. Changed the fuel hose and left the original style electric fuel pump. Added a high energy ignition coil.

Any suggestions of what else to look at before I change out the fuel filter?
Should I put in a better fuel filter system?
Could it still be fuel delivery or something else?
Plug wires? Plugs?
I could install a fuel pressure gauge but what would the fuel pressure be at WOT?
I'm wondering if there's an easy way to check fuel delivery at WOT to know if I have what I need?

Howdy,


Welcome aboard!

There's several things you can check but most important is a KNOWN tachometer and a known speedometer.

Pitot speedometers are notoriously inaccurate as are tachs.

Try using GPS (phone, handheld etc) for speed measurements.

Get a different tach.


Once you get a good tach and you know your speed, have a look at the following:

http://forums.iboats.com/mercruiser...tdrives/adults-only-no-exceptions-288430.html

Focus particularly on #1 and #14


Regards,


Rick

 

[Scott Danforth]

Re: Top end Performance

First, Welcome to Iboats

Second, remove the NON MARINE FILTER

Put on a clear 3/8" automotive fuel filter.

. That is a fire waiting to happen

Third, go thru the list here: http://forums.iboats.com/mercruiser...ives/list-possible-causes-low-wot-295070.html

you will need to verify your RPM with a calibrated shop tach. the Tach in the dash is merely a suggestion. THEY ARE NEVER ACCURATE.
you will need to verify your speed with a GPS The speedo in the dash is less of a suggestion than the tach. THEY ARE NEVER ACCURATE.

since your only getting 3800 RPM. my first thought would be over propped. However having been on many 4.3 liter boats that have had a bad plug, my first recommendation would be plug and wires because you may be running on only 5 cylinders.

Also check the exhaust flappers.

 

[Tiras]

Re: Top end Performance

Got my fuel rated compound gauge and I'm heading to Lowe's to get my setup that I need! Heading to West Marine in the morning to get a new fuel filter. I'll let you know what I find out! Thanks guys for all your help in getting pointed in the right direction.:yo:

 

[djmakko]

Re: Top end Performance

Water separator may be a better idea than a plastic inline filter .

 

[bonzoscott]

Re: Top end Performance

If your next fixes don't work, go to a 19 pitch prop. 4.3 with a 21 pitch would have to be a short light boat. If it's not missing or bogging down, it's not fuel or spark.