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Old 06-21-2016, 08:50 PM   #1
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Fuel Filter Reduce WOT?

Would a plugged secondary fuel filter make you lose 300 rpm from normal WOT?
I changed the Racor 75900 primary with no rpm change.
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Old 06-21-2016, 09:11 PM   #2
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My limited experience with plugged filters is that the performance continues to degrade quickly. IOW, it gets worse, the lower rpm level doesn't remain constant. Certainly hurts nothing to change it and cost is pretty minimal.

When was the last time you cleaned or inspected the hull and running gear?

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Old 06-21-2016, 09:11 PM   #3
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Old 06-21-2016, 09:23 PM   #4
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Quote:
Originally Posted by Pgitug View Post
Would a plugged secondary fuel filter make you lose 300 rpm from normal WOT?
I changed the Racor 75900 primary with no rpm change.
Also you can kick up some debri in the process of changing the primary that the secondary could have picked up.
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Old 06-21-2016, 09:47 PM   #5
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I have seen several instances where reduced fuel flow reduced top rpm (in other people's engines).
I have vacuum gauges after my Racors and pressure gauge after the lift pumps. It makes it easy to tell when filters are getting dirty. Always with enough advance warning to plan the filter changes.
If you have dual primary filters that can be switched running, put a 2 micron in one and a 30 in the other. Run on the 2 until it starts to plug, switch to the 30. Do this a couple times and you'll have super clean fuel and tanks. I buy clean fuel and I run on 2 micron primaries all the time. Use biocides and conditioners.
I carry 2000 gallons and it only took one 2 micron change.
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Old 06-21-2016, 09:50 PM   #6
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With Racor pressure gauges, what RPM should the engine be running when you check the gauge?
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Old 06-21-2016, 10:22 PM   #7
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With Racor pressure gauges, what RPM should the engine be running when you check the gauge?
I like the gauges that have have drag indicators. They will show the highest vacuum that you have achieved. That way you can check the gauge with the engine off before or after to have run the engine.
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Old 06-22-2016, 05:37 AM   #8
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"and you'll have super clean fuel and tanks"

Clean fuel will not strip decades of grunge from inside a fuel tank , unless the boat spends much of its time in quite rough conditions .
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Old 06-22-2016, 08:14 AM   #9
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With Racor pressure gauges, what RPM should the engine be running when you check the gauge?

WOT should be 2800 but all I get is 2450.
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Old 06-22-2016, 08:54 AM   #10
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WOT should be 2800 but all I get is 2450.
In addition to above suggestions, a few more preliminary things to consider:
  1. Is this a new happening?
  2. If 1 is yes, what did you do "last" in the way of maintenance or just messing around?
  3. When was last full RPM under load achieved?
  4. If 1 and 2 are yes, retrace steps in 2.
  5. Have you verified RPM with a photo tach?
  6. Yes, buy a Racor vacuum gauge and install it.
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Old 06-22-2016, 09:17 AM   #11
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You could also have a fouled/damaged propellor, or the wrong pitch propellor if it has been this way since you owned the boat.
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Old 06-22-2016, 09:39 AM   #12
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Sunchaser asks all the right questions. If this is a new phenomenon then look to something like a fuel filter. If you are just noticing now, then it could be a bunch of things including being overpropped by a bit. FWIW, I don't hit max rpm either, I am shy by about 200rpm. This was done at the sea trial. Not sure why, I could be over propped slightly.
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Old 06-22-2016, 01:49 PM   #13
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Sunchaser asks all the right questions. If this is a new phenomenon then look to something like a fuel filter. If you are just noticing now, then it could be a bunch of things including being overpropped by a bit. FWIW, I don't hit max rpm either, I am shy by about 200rpm. This was done at the sea trial. Not sure why, I could be over propped slightly.

At the sea trial we hit 2725. So I know that a proper pitched prop would read 2850. But now I am at 2400. A few weeks ago I was 2450. So I am guessing it's the secondary fuel filter since I changed the primary Racors.
Today I did change the secondary and flushed a few larger dirt particles out of the old filter. So next time out I will see if we have gained some ground.
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Old 06-22-2016, 02:33 PM   #14
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Well I can tell you that when I had my ex boat with the new Cummins 6BTA, I once had a reduced rpm situation due to some filter clogging.
It was a very subtle rpm reduction, but non the less due to a filter that needed changing. (which I did later that day after we anchored up for the night).
So that certainly CAN be a cause.
I did not have a vacuum gage on that system so can't help you out with a number.
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Old 06-22-2016, 02:35 PM   #15
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Photo tach readings would be useful?
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Old 06-22-2016, 02:39 PM   #16
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Quote:
Originally Posted by Pgitug View Post
At the sea trial we hit 2725. So I know that a proper pitched prop would read 2850. But now I am at 2400. A few weeks ago I was 2450. So I am guessing it's the secondary fuel filter since I changed the primary Racors.
Today I did change the secondary and flushed a few larger dirt particles out of the old filter. So next time out I will see if we have gained some ground.
Let us know what you find out. Also, if you don't already get a vacuum gauge on your Racors.
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Old 06-28-2016, 11:30 AM   #17
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I have a gauge between my Racor 75900 pair. It has a drag needle that has never gotten to the red.
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Old 06-28-2016, 11:53 AM   #18
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I have a gauge between my Racor 75900 pair. It has a drag needle that has never gotten to the red.
Great. I would look at replacing the filters though if the gauge ever gets to about 7mmHg.
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Old 06-28-2016, 12:10 PM   #19
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Quote:
Originally Posted by Pgitug View Post
I have a gauge between my Racor 75900 pair. It has a drag needle that has never gotten to the red.
You may want to talk to a Cummins dealer regarding the safe limit as the gauge warning has no relationship to the vacuum pump limits. For my "C" series Cummins my dealer said the Cummins lift pump had a significantly lower vacuum tolerance before the pump would fail. Don't remember the number, but once I see the needle move on a full tank of fuel, it's time to change the cartridge. It's changed annually regardless. Compared to fuel, the cartridge costs nothing (<$10). The "B" series is more vacuum tolerant, but it would be wise to know Cummin's limit.

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Old 06-28-2016, 12:17 PM   #20
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The one time I couldn't reach WOT, the first thing I checked was the Racor. Switching to the second Racor resulted in immediate solution.

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