Low RPM
08-20-2003, 04:54 PM
#1
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Low RPM
1989 Checkmate 251 Convincor w/454 (330hp) spinning a 23 Mirage prop. Early in the year it would hit 4100 RPM at WOT (two people, full tank of gas). Last time out only hit 3700 rpm (three people full tank of gas)
Does this sound like more than a prop issue? Suggestions???
Does this sound like more than a prop issue? Suggestions???
08-20-2003, 05:16 PM
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First early in year it should be hitting 4700-4900 rpms. Since summer heat takes a hit on rpms I would say with summer with the prop you have you are now way out of power band.
As to what prop I think some of the others will be able to help you better. I would say you need at most a 19 would get you another 600 rpms, better takeoff and maybe a little more speed. I had a 89 226 islander with a 454 & it had a 21 Mirage.
As to what prop I think some of the others will be able to help you better. I would say you need at most a 19 would get you another 600 rpms, better takeoff and maybe a little more speed. I had a 89 226 islander with a 454 & it had a 21 Mirage.
08-21-2003, 04:05 AM
#4
In the red, when i bought my 90 251 convincor (330hp)it had a 21" mirage on it. That would spin at around 5000-5100rpm. I went to a 23" Mirage plus and that spun at about 4700. Sounds like there is something else wrong.
I've got a 425hp 502 in it now with a 24" hydromotive QIV (not labbed) that is spinning around 4800.
I've got a 425hp 502 in it now with a 24" hydromotive QIV (not labbed) that is spinning around 4800.
08-21-2003, 09:02 PM
#5
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We need more data!!!
Red is a friend and we have been discussing this
can you confirm the speed was 57 on gps
with a 23 pitch prop that should net about:
(dividing by......... carry the 2....subtract the...... Ding!...)
4500 rpm
i say there is a problem with the tach
3700 rpm with a 23 pitch can only get you 48.3mph with 10% slip
anyone ever have a tach act this way?
Red is a friend and we have been discussing this
can you confirm the speed was 57 on gps
with a 23 pitch prop that should net about:
(dividing by......... carry the 2....subtract the...... Ding!...)
4500 rpm
i say there is a problem with the tach
3700 rpm with a 23 pitch can only get you 48.3mph with 10% slip
anyone ever have a tach act this way?
08-21-2003, 09:40 PM
#6
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If you have lost speed as well as rpm then I would think that you aren't making the same amount of power. If the speed dropped that much you may need some tune up work. When you replace the spark plugs check the compression. It should be around 155 in all of the cylinders. You may need a valve job. Make sure the ignition timing is correct (8 degrees at an idle) and that all of the fuel filters are replaced. You didn't overfill the oil did you? The prop isn't dinged is it? Growth on the bottom of the boat?
Good luck
Dennis Moore
Good luck
Dennis Moore
