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Engine over reving at Wide Open Throttle

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Ogre One

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Ok so i have a tach installed on the motor. while idling it is around 750rpm +/- 50rpm. Boat drives very smooth. it is a 145 deltacraft. from what i remember boat plans around 4000 and than can very easily hit 7000 rpm at wot. I took a look at the stamped plate on the motor says in between 4500-5500 rpm for wot, so i keep it around 5500 and no more. What needs to be done here i know i have a stainless steel prop. is it normal for the wot to be that high or do i need to go to another sized prop or adjust something(linkage)
I tried to see if there was a stamped number on the prop but couldnt find anything, i havent taken the prop off... are the numbers stamped inside it?
Carb jets are 56 and 35 if i remember correctly, i recently did a carb rebuild because it wouldn't idle. Also replaced the ignition switch because my key would jam up
What would you guys recommend? the motor sounds great around 5500rpm btw

Thank you in advance.
 
Sounds like you need a higher pitch prop. But.. then again extra 1500 rpm is a lot. prop hub could be slipping at higher rpms? Or prop is cavitating somehow. Not trimmed up too high? My number was very hard to see it was lightly stamped on the inner sleeve where the hub is. Had to use a scotch brite pad to clean it enough to see it. Right there on the surface facing out.
 
thanks for the reply. it is only in the second from the top slot, before it was to low and water would splash inside the boat. moved it up an inch or so and now the spray is way less. i am thinking a prop is in order. i am going to try and measure the prop once i get home. and i will take it off and see if there is a number on it. i would go ahead and buy one with a different pitch but they are kinda pricey so i want to make sure it is correct first.
 
did it rev up as bad when it was in a lower slot?where is the load in the boat?forward loaded?you may just be cavitating at wot...where is the cavitation plate in relation to the boat?...it should be even with the keel...does the motor match the transom?21 inch transom and 21 inch lower unit?..is the tach correct?at 7k rpm the motor will be screaming at you ''i am going to fly apart''....
 
i have only had the tach installed while it was in the upper slot. tho the tach is kinda wanky. i am going to check the wiring again because it is working one second than the next it will sit at 0. if i push the key in it will work again.
maybe the signal is not all to well. maybe the wiring in somewhat working. because sometimes it will go all the way around the guage. Motor does match the transom. aside from the battery and the fuel tank that is all that is in the back of the boat.
And the vent plate is maybe an inch or so from the bottom of the boat. it is as close as i can get it. i cannot move the motor up anymore.
 
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yea i will have too. Dont like dealing with the remote control as everything in jammed in there with very little room, but it is what it is
 
Verify your tach. If you are able to tune a guitar and you have a tone reference small enough to take on the boat (harmonica, recorder flute, iphone + piano app etc.), you may use the tone of the engine sound to determine the actual rpm.

(Musical notes referred to standard pitch a = 440Hz)
D# = 4670 rpm
E = 4940 rpm
F = 5240 rpm
F# = 5550 rpm
G = 5880 rpm
G# = 6230 rpm
A = 6600 rpm
A# = 6990 rpm

Don't laugh at me, it's a quick no-cost test.
 
I was going to mention the fact that at 7000 rpms that thing should be sounding like its about ready to explode...but if theres nothing to compare it to, then how would you know? As for the musical notes...wouldn't that be different per engine?....that's interesting - I play guitar....im going to have to test that one.. :confused:
 
Believe me or not, it works. It appears there are not many who put technical stuff and musical stuff together.

Doubtless the repetitive combustions in the motor produce a repetitive sound waveform. The waveform may be of any shape, but its fundamental frequency depends only on the number of bangs per time.
At low rpm the combustions are not regular enough and the fundamental frequency is too low in order to determine the pitch. But it is possible around 5k when the fundamental is in the middle bass range.
 
I can see it...not the actual sound or tone...but the frequency of the combustion of each cylinder, It does produce a frequency or vibration which is what a lot of guitar tuners utilize. especially those cheap clamp-on your headstock type. I was questiong its accuracy depending on the number of cylinder but that doesn't matter, only one will be firing at at time and each fire is a revolution. The more I think about it...its starting to add up....but then again im not done with my coffee yet either...
 
I'm still having a hard time with this, but then again I'm not musical by any means. Doesn't mean it doesn't work, but I'd like to see someone try to read 7000 revs doing 95 mph on a rough lake :)
 
You need a good tachometer, wired properly, with the dilal on the back of it set to #6 before you can set that engine/prop up with the boat you have. Without a proper functioning tachometer... you'll achive nothing.
 
Yeah...back to reality.....got a bit off course there....that was my first thought on the matter, fix the tach! He's probably already got it fixed and on the water fishing while were discussing soundwaves and guitar tuners. :rolleyes:
 
Progator135 said:
He's probably already got it fixed and on the water fishing while were discussing soundwaves and guitar tuners. :rolleyes:
Click to expand...

Progator.... Yeah, I hear'ya. Come to think of it, I have two guitars just sitting in my bedroom closet... haven't picked either one of the up since about 1993. This has me serious thinking about doing a little struming and picking. :)
 
Tach question while you guy's are chattin about it. I've got a 1990 Johnson 90HP J90TLASB and would very much like to ad a tach. How do I find out pulses per or poles so I can order the correct tach?
Fresh rebuild and new to the boat so I have no idea where WOT is. Sounds good but really need to verify!!
 
Unless it's been changed recently, that would be six (6) pulses. If I'm mistaken, I"m sure some other member will jump in here with a correction.
 
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