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15 hp over RPM?

C

Cjalbert

Member
Hi Guys,

I am very confused, I have a 2007 G3 Eagle 175 with a 89 15hp on it with a TMS tach and it states that I am at idling at 12-1400 RPM, and my WOT is like 8400 which is way over the 6500 top end??/ MY boat is severely underpowered....(HP restricted lake). I bet it is about 1500 lbs, I am currently running a 9.25X11 prop, Shouldn't this be leaving me way under RPM? Doesnt this sound like I am idling at 6-700 rpm and only getting to 4200 RPM? I am afraid to blow this thing up? I simply got the tach and wrapped the wire around the spark plug wire and there it is....I am running 7.75 MPH at the 8400 but I don't stay at that RPM for obvious reasons, any thoughts would be appreciated.

Thanks Chris
 
On the back of the tach is there a pulse switch turn that switch to 6 and then check the tach with a tach timing light you can probably borrow from your local auto parts store. Not sure on what the pulse setting should be on the tach just try different settings until it reads correct?
 
Hi Scott thanks for the input I do actually have a long shaft motor on it and it does require it, I would say that the tachometer that I purchased was off of Amazon for $15 so it's probably a cheapo model of the tiny tack. It just sounds like it's kind of streaming now so I was afraid to bring it up another 2000 RPMs without hearing someone else say that they agreed with my thought process
 
​Are you sure the prop hub isn't spinning? <8 MPH sounds awfully slow for that kind of RPM. Does it plane the boat at all?
 
Unfortunately that tach that you had listed has two wires mine only has one, With the one that I had purchased I wrapped the attached wire around the spark plug wire and the directions on the unit say that it is factory set to a 2 stroke, this is what is really confusing me? Everything in my mind tells me that I am on the wrong setting....but I am a carpenter by trade and a marine mechanic by need....I am going home tonight to run through these settings as I actually have two of these I will change one and leave the other to see how it does.

And again thanks for the input!

Chris

hm010-2_1.jpg
 
No, The boat does not really plane....I think it would need a bit more to really "get up", I have a hydrofoil I was going to try as well.

The Prop looked really good so I never considered that the hub might be spun....I will check that.

Last specific question pertaining to this tach - would you agree that setting #2 is the correct setting for an outboard....That is 02(once per revolution)? If so I am really stumped, cuz thats what it is set for now.

Thanks Chris
 
You sure have something wrong there....spun hub perhaps.
I have an '86 15hp Johnson that will put you in the back (the wife didn't hear me say "hold on" and came tumbling off the front bench to my feet in the back when i went full throttle from trolling speed. I can't repeat what she said to me) Clocked with my GPS is 32mph on 14' boat.
 
Cjalbert said:
No, The boat does not really plane....I think it would need a bit more to really "get up", I have a hydrofoil I was going to try as well.

The Prop looked really good so I never considered that the hub might be spun....I will check that.

Last specific question pertaining to this tach - would you agree that setting #2 is the correct setting for an outboard....That is 02(once per revolution)? If so I am really stumped, cuz thats what it is set for now.

Thanks Chris
Click to expand...


I believe that motor has one coil with 2 plug wires..series wound coil..both plugs fire at same time so 2 times per rev.
 
It has two coils go ahead and set it at 01. For a 17.5 ft flat bottom you would be alot better off with a 50-70hp to hit a nice plane its rated for 90hp max. With a full tank of fuel and two adults a 15hp might barely get it up out of the hole. It also has a 20 inch transom is your motor a long shaft? Is the cavitation plate level with the bottom of the transom? If you have a short shaft the prop is probably sucking air (ventilating)
 
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Confirm the tach is reading correct if your only turning 4200 WOT rpms that makes alot of sense for a 17.5 ft boat you are way over propped. what is the size of prop you are running you probably need 4 inches less pitch to bring the rpms up to around 5000 wot.
 
Hi Guys,

I changed the tach setting to 03 which was supposed to be every other rotation but it seems to have put me on the right track. I now idle at 600-700 and at WOT I was at 4200RPM, and 7.75 MPH on the GPS, this was with the newly added HYdrofoil, mounted 1/2" below the bottom of the boat, (which might be to low because I have a CMC pt 35 which sets me back 5-1/2") anyhow I had a 9.5X10 prop around so I swapped it out and at WOT it went 5000 RPM and 9.10 MPH on the GPS. So it is a too much pitch issue now that I have vetted the Tach issue. I will continue stepping down a pitch until I get to 6500 RPM at WOT. Thank you for the input/info I sometimes I just need to bounce my thought process off someone,
 
Hi Guys,

I changed the tach setting to 03 which was supposed to be every other rotation but it seems to have put me on the right track. I now idle at 600-700 and at WOT I was at 4200RPM, and 7.75 MPH on the GPS, this was with the newly added HYdrofoil, mounted 1/2" below the bottom of the boat, (which might be to low because I have a CMC pt 35 which sets me back 5-1/2") anyhow I had a 9.5X10 prop around so I swapped it out and at WOT it went 5000 RPM and 9.10 MPH on the GPS. So it is a too much pitch issue now that I have vetted the Tach issue. I will continue stepping down a pitch until I get to 6500 RPM at WOT. Thank you for the input/info I sometimes I just need to bounce my thought process off someone,
 
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