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Author Topic:   Mercury 90 HP 4 stroke gear slip problem
John Iwaniec posted 08-21-2005 02:40 AM ET (US)   Profile for John Iwaniec   Send Email to John Iwaniec  
I just used my 2004 17 Mauntauk for the first time today and was thrilled beyond belief from the difference of my old 1986 version Mauntauk and this new beauty. The four stroke Mercury 90 ran smooth as glass except for one noticible problem. When In put it in gear it makes a noticible clonk which is a little loud but acceptable, the thing thats bothering me is it feels like the gears are slipping with a very noticible bump bump before it catches and goes. Once its up and goin she's fine but I kept bringing it back down to around five knots and giving it the gas again and it always went thru the same bump bump before it caught and took off has anyone else experienced this problem?
bigjohn1 posted 08-21-2005 08:02 AM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
Big John - my old spearfishing buddy from Okinawa! Welcome to the forum and greetings from Guam. I don't think I ever told you I bought the very same boat when we moved here.
You will get plenty of feedback on this as it seems this CAN be a common problem with that engine/prop combination. I don't experience this problem personally but I attribute that to my running an aluminum prop. I wanted to run aluminum around the reef while anchoring in tight in case I ever hit the reef in the shallows. I am about to install a 4-blade stainless and expect that I may have some of the same issues you have. This issue has been discussed alot here in the past two years or so and I think the consensus is to make sure your engine is not set to idle too low and to make quick and deliberate shifts. If you lost my e-mail address, check my profile and its there. E-mail me any time to share ideas or just to yack. That new 170 MONTAUK is a great boat that you are going to love. We regularly troll out to about 20-25 miles and its really good for only a 17 footer. Its not the Kerama Islands around here, but we still land fish:-)
Maximus posted 08-21-2005 08:35 AM ET (US)     Profile for Maximus  Send Email to Maximus     
BigJohn is correct...the clunk is most likely a result of the heavier SS prop. I've gotten used to the clunk at this point. Make your shifts delibrate.

For all the newer 170 owners...FSM on the 90 4s Merc:
This was posted in the GAM, cross posting here for newer eyes. There may be a procedure for adjusting the shift mechanism.

fishinchips posted 08-21-2005 07:27 PM ET (US)     Profile for fishinchips  Send Email to fishinchips     
Make your shift quickly and firmly.
Do not do it slowly.


John Iwaniec posted 08-24-2005 04:13 PM ET (US)     Profile for John Iwaniec  Send Email to John Iwaniec     
Thanks for the comments, good to hear from you John, I had heard you bought a Whaler. So you're saying its the SST prop thats the problem, I can understand this for the shifting Clonk part but what I'm experiencing is a bump bump after the shift has been made and I'm moving along at around five knots and then give it the gas. Its during the transition from going slow to going faster that I feel the bump bump right before it seems to catch and go. Another peculiar noise is when I shift into neutral the engine purrs quietly and then starts a sort of hissing noise, whats with that? is this another common noise, I don't hear that on my buddy Brian Robinsons 40HP 4 strokes.....
bigjohn1 posted 08-24-2005 05:39 PM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
John - I'm by no means the expert but I think its the heavy mass of that prop which is causing most of your symptoms of surging. I could be wrong but that is what most have said on this via various boating forums. As to your hissing noise, I haven't a clue. I have the 115efi instead of the 90 so maybe one of the 90 4-stroke owners can chime in.
whaler1234 posted 08-25-2005 04:04 AM ET (US)     Profile for whaler1234  Send Email to whaler1234     
That hissing noise is normal. There have been other similar posts re: the 90hp four stroke sounding like a supercharger blower. I've asked Mercury as well and it seems to be common. I just live w/ it now. The noise actually helps me know I'm in neutral since that neutral notch is extremely hard to find.
bigjohn1 posted 08-25-2005 08:13 AM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
Hissing....what about the carbs sucking air into tiny air passages at idle? I don't hear this with my efi but my money would be on the carbs and the air intake process. As to why you only hear it at idle, I can only guess that once the engine is in gear, the increasing idle has the carb butterflies opened and air passes through in more un-restricted fashion, hence less high-pitched noise (hissing).
Moe posted 08-25-2005 08:30 AM ET (US)     Profile for Moe  Send Email to Moe     
To check your shift linkage adjustment:

With the gear shift fully forward, make sure you can turn the prop clockwise with a click every so often but that you can't turn it counterclockwise.

Shift back to neutral, making sure you don't go past it into reverse, and make sure you can turn the prop in both directions.

Shift into reverse fully and make sure you can turn the prop counterclockwise but not clockwise.

Shift back into neutral, making sure you don't go past it into reverse, and make sure you can turn the prop in both directions.

It's also probably worth pulling the prop and inspecting things:

Make sure the prop nut was on snuggly (not overtightened though).

Make sure the rear thrust washer was seated in the prop.

Disassemble the hub and inspect the components.

Make sure the prop was well seated on the front thrust washer (and that it wasn't omitted).

Turn the prop shaft and make sure it isn't bent.

Inspect for any signs of the prop rubbing on the gearcase.

Just a few thoughts...


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