Mercury 175 Rebuilt. What Is This Noise? Link To Video

[Capnmoench]

Hello all, I got this 97' Mercury 175 that was freshly rebuilt. I did the break in and am still burning off a tank of mixed since I didn't know how much fuel was left in the tank when the old motor blew. It's an 80 gallon tank so it's taking awhile. It seems to run strong and is pretty smooth despite the mix fuel coughing and roughness when cold, but at certain throttle settings I get kind of a roar and buzzing sound. The motor is pretty quiet on plane or if I gently nudge it to full throttle, but if I get on it, or drop the throttle fast it's got this "rawr" sound that concerns me. It's gotten better after a few trips and I have to try to make it do it by goosing it, but I try to avoid running it at a setting where it makes the noise. I'm running 87 octane and was thinking maybe I should have gone with 89 with the mix. Unsure if it's detonation or if it's normal. Maybe I'm just paranoid about any noise since the last motor took a crap. Throw on some headphones and give it a listen. Any advice or feedback would be much appreciated. Thanks.

https://m.youtube.com/watch?feature=youtu.be&v=OUmmRHyrN4Q

[roadkill636]

Is the air box off?

[Capnmoench]

The air box is on.

[David Borg]

Are you running an electric pump?mine does that at low rpm when i run more than 4psi fuel pressure.

[Capnmoench]

No, just the factory pump on the motor.

[Euroski]

87, I don't run 87 in my mower! Especially since it's 20year old engine.

[Capnmoench]

I didnt think the stock compression of an older Mercury would warrant a higher test. The low grade where I originally came from in MT is 85. We ran our 1993 Evinrude on that for 20 years. Every once in awhile we'd give her a dose of 87 which was the mid grade there.

[Euroski]

I guess where you boat is higher elevation and I have heard octane ratings are lower. We have 87/89/93 octane.


I didnt think the stock compression of an older Mercury would warrant a higher test. The low grade where I originally came from in MT is 85. We ran our 1993 Evinrude on that for 20 years. Every once in awhile we'd give her a dose of 87 which was the mid grade there.

[Capnmoench]

Yeah, same here in WA now where I live.

[Capt.Insane-o]

Sounds like a Mercury v6. With it's inherit resonance and all.

[Capnmoench]

That's exactly how I would describe it. A resonance. This is normal? The only other boats I was greasy hands familiar with were a 115 Ever-rude and a 25 Yammie 4-stroke. Neither made that ruckus. I am just a bit paranoid after spending 6 months replacing the transom and stringers on the boat, breaking a bottle of champagne on the bow and after only 5 trips on the water, having it die suddenly on plane. Alarm system made no noise. (Found out later after slapping a new PH on that it makes some loud 3 beeps when you turn the key on. Hmmm. Never did that before). Then the motor started fine and I got back on plane. Died again. Made the mistake of plowing back at 2000RPM and when I went about 5 minutes, "tap tap tap knock knock KNOCK KNOCK" shut down. 3 months of searching and I find a rebuilt PH on another boat for less than a bare powerhead. Now I have that on and my woman asks me why I never smile in pictures when we are out on the boat. She says I should be happy that it's finally done. I say, "I'm listening to it, waiting for something to break!"

[Capnmoench]

A resonance and a buzz.

[RSWORDS]

Sounds like a v6 merc working hard to plane off. Wide open butterflies and low rpm.

[tunnelmike]

Sounds like too much trim or pitch to have to work that hard to plane. There will be a rpm that they will be " louder" than other times. I don't hear anything that alarms me. Anyone else? Mike.

[H2OPERF]

Agree with above.. what rpm does it turn wide open? sounds like its really working to get on plane, maybe why it blew up before.

[Capnmoench]

It was a 150 that blew up before. Thinking it was a cooling issue with that one. Right thermostat was all corroded. This is a 175. It turns 5300 at WOT and has no problem stepping on plane. I'm troubleshooting a vibration on the hose in another thread at around 1500 RPM. Might be related.

[rkmirage]

Its over propped, 5300 wot is too low, drop a couple inches of pitch and it should turn 5800 and plane much easier.

[Capnmoench]

I read the max RPM to be 5000-5600. I was thinking I was right on the money. I know you guys run em' hot, but I'm just one of those weird guys with sticks hanging on the outside of my boat. AKA a fisherman. I like the performance and economy of the 21 pitch. 37MPH at cruise and 4300-4500RPM. 43.1MPH top end on a 3000lb walk around cuddy. Also, I switched to a 21 from a 17 pitch that was originally on the same 25" leg I reused from the 150. It made the same ruckus with the much lower pitch and it's actually quieted down to where there's only a couple points at the power curve it makes the noise instead of throughout the entire curve up to plane if I gun it. I used to have to be gentle bringing it up on plane to avoid it. I'm running 2:1 gears on a 175HP powerhead. People ask me what the hole shot is like. I don't know what this "hole" they speak of is. Even with a full 80 gallons, two full 15 gallon fishwells and 4 people aboard.

[FMP]

I didnt think the stock compression of an older Mercury would warrant a higher test. The low grade where I originally came from in MT is 85. We ran our 1993 Evinrude on that for 20 years. Every once in awhile we'd give her a dose of 87 which was the mid grade there.

85, what? No way! That must be the land of 301 2bbl Pontiacs.

[Capnmoench]

Haha! I think it's still that way!

[Capnmoench]

Yup. 85.5 is low grade. Eh herm... regular.

[rkmirage]

I read the max RPM to be 5000-5600. I was thinking I was right on the money. I know you guys run em' hot, but I'm just one of those weird guys with sticks hanging on the outside of my boat. AKA a fisherman. I like the performance and economy of the 21 pitch. 37MPH at cruise and 4300-4500RPM. 43.1MPH top end on a 3000lb walk around cuddy. Also, I switched to a 21 from a 17 pitch that was originally on the same 25" leg I reused from the 150. It made the same ruckus with the much lower pitch and it's actually quieted down to where there's only a couple points at the power curve it makes the noise instead of throughout the entire curve up to plane if I gun it. I used to have to be gentle bringing it up on plane to avoid it. I'm running 2:1 gears on a 175HP powerhead. People ask me what the hole shot is like. I don't know what this "hole" they speak of is. Even with a full 80 gallons, two full 15 gallon fishwells and 4 people aboard.The engine sounds fine to me, but I would drop 2" of pitch. Always prop it to turn max rpm with light load. Mercs like to spin up, they don't like to labor.

[tunnelmike]

I think your fuel consumption would be higher with the motor in a bind most of the time. Getting a higher top speed isn't the best way to prop that kind of rig. I would try a 19 to see of you can get closer to 6000. 5300 seems low. Mike

[David Borg]

I think your fuel consumption would be higher with the motor in a bind most of the time. Getting a higher top speed isn't the best way to prop that kind of rig. I would try a 19 to see of you can get closer to 6000. 5300 seems low. Mike

:iagree:

[Capnmoench]

Agree that my economy might be improved. I'm hesitant to push the limit until I can get a true tach reading. I set it up in the driveway, and looks to be accurate but it's hard to do by yourself running back and forth and trying not to over accelerate the motor. I got the woman to drive it and it was accurate to 3500-4000RPM on the water, but it was getting a bit rough, she wussed out on going any faster because she's only driven a boat a couple times, the optical tach was having a hard time getting a reading in the morning sun, I was getting some water splash in the motor, and all of the sudden a low oil warning beep from sitting bow high with me in the back of the boat for so long trying to get a reading. I need to find a mech with a dynomometer to check it or can do when I take it to a lake instead of the never calm (except in the early morning) Pungent Sound. It looked to be right on from 1000-4000 +/- 50 RPM. Also, I just bought this prop. They offered to exchange it for free if I needed a different pitch, but I was like "5300? That's right in the middle! I'm perfect and a genius to go +2 pitch to make up for my non-standard 2:1 gearing." and promptly threw the box and receipt away.

[W2F a V-King]

I think your fuel consumption would be higher with the motor in a bind most of the time. Getting a higher top speed isn't the best way to prop that kind of rig. I would try a 19 to see of you can get closer to 6000. 5300 seems low. Mike

^^ Agree!!

It's like driving a stick shift car lugging the motor...Your in 4th gear cruising at a low speed, and really need to be in 3rd gear.

Prop for the max rpm and you will not lug the motor.

jmo

[outasite]

That noise sounds pretty normal,I think it may be harmonics of the incoming air,reeds and tuner coming together at certain RPMs

[Capnmoench]

Deleted my last post about jets. They are the correct # I could see it after I cleaned them. The 6 looked like a 5.. I'm starting to think that it is the harmonics at the certain RPM. Only other thing is the vibration on the hose I thought might be related. The carbs are filled with white crap from ethanol I can only assume. I'm working on cleaning them out and I'll take it for a spin after I get them back together and on. By this time, I'm so loaded up on it, I need to do a high speed burn to clear everything out and start fresh.

[Capnmoench]

Update from cleaning the carbs on the other thread here: http://www.screamandfly.com/showthread.php?330197-Mercury-175-Running-Rich-Rough-to-2000-RPM/page3. It has solved this issue along with the rough running. Update on the prop when I finally got some smooth as glass water is 5500 RPM at 42 MPH. Been talking to Ken at Propgods since he comes highly recommended. I have a lot of slip, more noticeable in the less dense, warmer freshwater. Thinking a SS prop is in order when I save up some ducats. I'll update that process on my other thread here: http://www.screamandfly.com/showthread.php?330467-Beaking-Free-at-More-Than-1-4-Trim-New-Prop-Suggestions. Thanks!