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Author Topic:   RPMs, 90hp Yamaha

Hack posted 09-06-2001 03:21 PM ET (US)
My montauk has a 1986 90hp Yamaha, ive been reading the posts about speed vs rpms, my WOT is only 4500, i tried 3 different props this last weekend and same difference, i have had this boat/engine for 8 years now, no problems, but now im wondering, top speed is 40 mph. Should this engine deliver more rpms?
any info is very appreciated!
Cya l8r
Bigshot posted 09-06-2001 03:33 PM ET (US)
What size props? WOT is 5500 on that, you should be at least 5200 with light load. I run a 17" ss prop and get 5500. Has it always read this low? Your tach might be screwy too or you also might not be getting WOT due to a cable misadjustment.
Hack posted 09-07-2001 11:04 AM ET (US)
I have a 17 ss on it right now, could the tach be off? it is the digital display. i read in a previous post that someones cable was on upside down? i cant see how this can be on this setup, at WOT, the thottle handle does not have to be all the way pinned(is this right?) i have about 2.5 inches of play(cable is still tight) maybe that is my problem, but gps reads 40 mph at 4500 rpm. do you think i have a problem? this boat has been like this for at least 10 years.
Bigshot posted 09-07-2001 11:30 AM ET (US)
Think your tach is off. What is your GPS around 3700-3800? WHere do you cruise. I do about 28mph @ 3700 with a 17"ss prop. To check cable, remove cover and pin the throttle(at helm). Then try and move throttle on carbs. If no or little movement, your WOT.
JFM posted 09-07-2001 11:31 AM ET (US)
My 2000 yamaha 90 has the standard 3 blade prop WOT is 5450, but at 5200 the boat goes 40 MPH and won't go any faster at 5450. Hey Bigshit what kind of prop should I change to. Regards, Jay
JFM posted 09-07-2001 11:35 AM ET (US)
Sorry Big shot I hit i instead of o. Sorry if I offended anyone. Regards, Jay
jimh posted 09-07-2001 01:13 PM ET (US)
Check the propeller shaft speed that would be necessary to run at 40 MPH with your prop pitch. Allow for perhaps 5% slip, and adjust for the gear reduction. From this you can deduce the crankshaft speed, which may tell you something about your tachometer reading.
--jimh
Bigshot posted 09-07-2001 01:16 PM ET (US)
jay Are you running SS or alum? I have the Jo-standard SS Yamaha prop on mine 17"(kind of cleveresque. I tried a 18" polished & ventilated merc 3 blade and it sucked big. I am thinking of cupping mine and seeing what gives, maybe polishing it for S&G. Right now I am experiencing carburation problems so....
JFM posted 09-07-2001 02:53 PM ET (US)
Bigshot, the prop is painted light grey. I don't know what it is made out of. The Yamaha website says almn. ans SS are options. So I think maybe painted almn. In the past all my props were brass before the outboards I had. Why don't they use brass any more. Regards, Jay
Bigshot posted 09-07-2001 03:13 PM ET (US)
To costly to make and repair? I had an old merc that had a brass prop or bronze? Was not alum nor SS.
JFM posted 09-07-2001 03:37 PM ET (US)
Jimh, thanks for the info. I don't know how to do what you suggest. I have to admit I'm a dummy when it comes to that kind of stuff. I used to pay to have all my work done. When I was younger 32 years ago I was a grunt at a boat yard and just did wood work and painting. You guys on this site have given me the confidence to do many of the things I paid for in the past and have saved me $$$. Thanks, Jay
Hank posted 09-08-2001 12:53 AM ET (US)
Hack
Your Tach seems to be off. Your WOT speed of 40 MPH is more appropriate at around 5200 RPM.
Follow Bigshot's suggestion and try another data point. Run at 28 MPH and see if you come close to his tach reading of about 3700 RPM.
Some data I have from an old ('84) magazine shows the 90 Yam /Montauk combination runs dead on Bigshot's numbers of 28 MPH at 3700 RPM.
Hank

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Author	Topic: RPMs, 90hp Yamaha
Hack	posted 09-06-2001 03:21 PM ET (US) My montauk has a 1986 90hp Yamaha, ive been reading the posts about speed vs rpms, my WOT is only 4500, i tried 3 different props this last weekend and same difference, i have had this boat/engine for 8 years now, no problems, but now im wondering, top speed is 40 mph. Should this engine deliver more rpms? any info is very appreciated! Cya l8r
Bigshot	posted 09-06-2001 03:33 PM ET (US) What size props? WOT is 5500 on that, you should be at least 5200 with light load. I run a 17" ss prop and get 5500. Has it always read this low? Your tach might be screwy too or you also might not be getting WOT due to a cable misadjustment.
Hack	posted 09-07-2001 11:04 AM ET (US) I have a 17 ss on it right now, could the tach be off? it is the digital display. i read in a previous post that someones cable was on upside down? i cant see how this can be on this setup, at WOT, the thottle handle does not have to be all the way pinned(is this right?) i have about 2.5 inches of play(cable is still tight) maybe that is my problem, but gps reads 40 mph at 4500 rpm. do you think i have a problem? this boat has been like this for at least 10 years.
Bigshot	posted 09-07-2001 11:30 AM ET (US) Think your tach is off. What is your GPS around 3700-3800? WHere do you cruise. I do about 28mph @ 3700 with a 17"ss prop. To check cable, remove cover and pin the throttle(at helm). Then try and move throttle on carbs. If no or little movement, your WOT.
JFM	posted 09-07-2001 11:31 AM ET (US) My 2000 yamaha 90 has the standard 3 blade prop WOT is 5450, but at 5200 the boat goes 40 MPH and won't go any faster at 5450. Hey Bigshit what kind of prop should I change to. Regards, Jay
JFM	posted 09-07-2001 11:35 AM ET (US) Sorry Big shot I hit i instead of o. Sorry if I offended anyone. Regards, Jay
jimh	posted 09-07-2001 01:13 PM ET (US) Check the propeller shaft speed that would be necessary to run at 40 MPH with your prop pitch. Allow for perhaps 5% slip, and adjust for the gear reduction. From this you can deduce the crankshaft speed, which may tell you something about your tachometer reading. --jimh
Bigshot	posted 09-07-2001 01:16 PM ET (US) jay Are you running SS or alum? I have the Jo-standard SS Yamaha prop on mine 17"(kind of cleveresque. I tried a 18" polished & ventilated merc 3 blade and it sucked big. I am thinking of cupping mine and seeing what gives, maybe polishing it for S&G. Right now I am experiencing carburation problems so....
JFM	posted 09-07-2001 02:53 PM ET (US) Bigshot, the prop is painted light grey. I don't know what it is made out of. The Yamaha website says almn. ans SS are options. So I think maybe painted almn. In the past all my props were brass before the outboards I had. Why don't they use brass any more. Regards, Jay
Bigshot	posted 09-07-2001 03:13 PM ET (US) To costly to make and repair? I had an old merc that had a brass prop or bronze? Was not alum nor SS.
JFM	posted 09-07-2001 03:37 PM ET (US) Jimh, thanks for the info. I don't know how to do what you suggest. I have to admit I'm a dummy when it comes to that kind of stuff. I used to pay to have all my work done. When I was younger 32 years ago I was a grunt at a boat yard and just did wood work and painting. You guys on this site have given me the confidence to do many of the things I paid for in the past and have saved me $$$. Thanks, Jay
Hank	posted 09-08-2001 12:53 AM ET (US) Hack Your Tach seems to be off. Your WOT speed of 40 MPH is more appropriate at around 5200 RPM. Follow Bigshot's suggestion and try another data point. Run at 28 MPH and see if you come close to his tach reading of about 3700 RPM. Some data I have from an old ('84) magazine shows the 90 Yam /Montauk combination runs dead on Bigshot's numbers of 28 MPH at 3700 RPM. Hank