I have a 2003 2120 sc with a 150 yamaha ox66 with 460 hours. With the exception of a few VST filter issues she runs great. My only concern is that at WOT I have only gotten her up to 4900 rpm (according to the yami gauge) I usually top out at about 28mph at this level. I have read that this engine should top out around 5500 rpm. I bought the boat used in 2009 and use the prop that came with her. I ran this season with the same plugs as last year and also went two years on the bottom paint (bottom was very clean when I hauled it this fall). She starts and runs great, I am just concerned if low RPM is an indicator of a bigger problem looming?
what factors affect rpm?
- Thread starter reefman
- Start date
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DaleH
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1 - Broken Tach or wrong 'point/counter' setting
2 - Wrong prop, the #1 reason
3 - Linkages worn, not getting to full throttle stop (you can check this with OB off)
4 - Carb butterfly plates not opening all the way
5 - Faulty coil, 1 plug not firing or breaking down @ max RPMs (check to see if all plugs 'look' the same; if one is cleaner than the others, that could be it). When powerpacks break down, they usually limit the RPMs to be much lower than what you're seeing. A bad plug could do it too, if broken - again, a quick check here can tell.
6 - Poor compression on one or more cylinders
7 - Fuel restriction (not sure if Yam use fuel draw vaccum alarming - it seems they copied everything else from the OMC design but this)
8 - Motor mounted too deep
My bet is #2, and I'd bet $$ > 50% of the boats leave Dealers setup with the wrong prop. Look at boat tests, NO BOAT MADE regardless of hull and motor brand EVER has met a published factory test. And the factory does not load them up with all of the personal crap we all carry, which can add up to hundreds and hundreds of pounds :shock: ! Also, I firmly believe most dealers mount the OBs a hole too deep, if not 2, on 'some' rigs. They set up boats to the 'average' user ... we're above average here
on CP!
PROP: Beg, borrow, or steal a prop with 2" less pitch and try it out. Or, contact Ken from http://www.propgods.com and get a test wheel from him, letting him know I sent you.
TACH: Buy a TinyTach (< $50), as it is the easiest/fastest way to confirm RPMs, see: http://www.tinytach.com/tinytach/gasoline.php. You can mount it temporaily and check it every few season or even use on other boats. I use mine on all my OBs, but my bros and I run 11 OB motors between us 4. They are quartz driven and are exceedingly accurate!
OTHER: A "sync & link" check on an OB (1-2 hours labor max) will verify #3 & 4. Maybe a visit to a reputable service house is in order, as they could verify 5 - 7 too (maybe in that same time, if the Tech is 'good').
Also look over the stern when running at a medium fast cruise ... if you can't see the ventitation plate mounted immediately above the prop ... your OB is mounted too deep. It should be at the plane of water surface if not above. I posted some DIY instructions on how to adjust the motor height by yourself, with a helper or 2, using the trailer as the leverage/lift mechanism; use the search feature.
Remember, lugging an OB motor is THE surest way to shorten its life ... less not adding oil.
2 - Wrong prop, the #1 reason
3 - Linkages worn, not getting to full throttle stop (you can check this with OB off)
4 - Carb butterfly plates not opening all the way
5 - Faulty coil, 1 plug not firing or breaking down @ max RPMs (check to see if all plugs 'look' the same; if one is cleaner than the others, that could be it). When powerpacks break down, they usually limit the RPMs to be much lower than what you're seeing. A bad plug could do it too, if broken - again, a quick check here can tell.
6 - Poor compression on one or more cylinders
7 - Fuel restriction (not sure if Yam use fuel draw vaccum alarming - it seems they copied everything else from the OMC design but this)
8 - Motor mounted too deep
My bet is #2, and I'd bet $$ > 50% of the boats leave Dealers setup with the wrong prop. Look at boat tests, NO BOAT MADE regardless of hull and motor brand EVER has met a published factory test. And the factory does not load them up with all of the personal crap we all carry, which can add up to hundreds and hundreds of pounds :shock: ! Also, I firmly believe most dealers mount the OBs a hole too deep, if not 2, on 'some' rigs. They set up boats to the 'average' user ... we're above average here
on CP!PROP: Beg, borrow, or steal a prop with 2" less pitch and try it out. Or, contact Ken from http://www.propgods.com and get a test wheel from him, letting him know I sent you.
TACH: Buy a TinyTach (< $50), as it is the easiest/fastest way to confirm RPMs, see: http://www.tinytach.com/tinytach/gasoline.php. You can mount it temporaily and check it every few season or even use on other boats. I use mine on all my OBs, but my bros and I run 11 OB motors between us 4. They are quartz driven and are exceedingly accurate!
OTHER: A "sync & link" check on an OB (1-2 hours labor max) will verify #3 & 4. Maybe a visit to a reputable service house is in order, as they could verify 5 - 7 too (maybe in that same time, if the Tech is 'good').
Also look over the stern when running at a medium fast cruise ... if you can't see the ventitation plate mounted immediately above the prop ... your OB is mounted too deep. It should be at the plane of water surface if not above. I posted some DIY instructions on how to adjust the motor height by yourself, with a helper or 2, using the trailer as the leverage/lift mechanism; use the search feature.
Remember, lugging an OB motor is THE surest way to shorten its life ... less not adding oil.
