1989 Mercury 90 Only Reaches 4,000-RPM

A conversation among Whalers
enggass
Posts: 33
Joined: Mon Sep 18, 2017 3:42 pm

1989 Mercury 90 Only Reaches 4,000-RPM

Postby enggass » Sun Oct 08, 2017 7:42 pm

Today I had a chance to get out my 1989 Mercury 90-HP. The maximum RPM was 4,000 and boat speed was 25-MPH. It was windy and choppy.

[Later more information is provided. The recommended wide-open-throttle engine speed for this 1989 Mercury 90-HP is said to be 5000 to 5500-RPM, the boat is a Montauk and assumed to be a 17-foot model, and the anticipated boat speed is thought to be 42-MPH--jimh]

[What are your] thoughts?

A 17-pitch propeller.

Thanks.

Don SSDD
Posts: 164
Joined: Fri Oct 23, 2015 6:58 am
Location: Nova Scotia

Re: Engine concern - what to look for?

Postby Don SSDD » Sun Oct 08, 2017 8:51 pm

Check plugs, see if you are firing on all cylinders.
1986 Outrage 18 with 2001 Honda 130 HP
Former Owner 1991 Guardian 19 with 1994 Evinrude V4 140HP
Former owner 1987 Montauk with 1998 Mercury 90HP
Nova Scotia

jimh
Posts: 7129
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: 1989 Mercury 90 Maximum RPM

Postby jimh » Mon Oct 09, 2017 7:10 am

Need more data:

--what is the manufacturer's recommended speed range for this engine at full throttle?

--what is the gear ratio?

--is this limited data you report supposed to be something abnormal?

--what boat is this data about?

enggass
Posts: 33
Joined: Mon Sep 18, 2017 3:42 pm

Re: 1989 Mercury 90 Maximum RPM

Postby enggass » Mon Oct 09, 2017 7:32 am

Montauk 17
Should be [5000 to] 5500-RPM
42 MPH top speed

jimh
Posts: 7129
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
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Re: 1989 Mercury 90 Maximum RPM

Postby jimh » Mon Oct 09, 2017 7:39 pm

Use an inline spark check tool to verify you have spark on all cylinders.

How many cylinders does this engine have?

Is the engine a two-stroke-power-cycle engine?

Does this engine use carburetors?

Has the boat speed previously been 42-MPH at WOT?

What is the gear ratio?