Mercury 90-HP Two-Stroke: Engine Speed Jumps at Idle to 3,000-RPM

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Author Topic:   Mercury 90-HP Two-Stroke: Engine Speed Jumps at Idle to 3,000-RPM

jmontani posted 07-05-2006 09:20 PM ET (US)
My 170 Montauk is not even a year old. The two-stroke Saltwater Series Mercury 90-HP motor ran great after they replaced the powerhead six months ago. This last weekend I was fishing all morning and ran back to the marina. When I came down off of plane and started to idle in at the "no wake" signs, the engine speed was jumping to 3,000-RPM from 1,000-RPM. The throttle position was all the way back just in front of neutral.
I put the boat in neutral and the engine speed continued to jump around. I shut down the engine and took a tow into the dock. The boat is now in for warranty work.
What could the problem be? The boat has about 20 hours on it since Mercury replaced the powerhead. It has not been a good first 11 months for this engine.
I appreciate any input.
Jack

bsmotril posted 07-06-2006 09:28 AM ET (US)
I've had this happen to me too. The stopper nut on the idle adjustment screw vibrated loose and the idle stop screw then vibrates in until it contacts the idle stop on the block. It keeps the throttle from closing down completely, hence the high idle speed. Easy do it yourself fix, back the screw out to where it belongs. Paint it with laquer or nail polish when set correctly to keep it from happening again. Don't try to adjust the idle speed with the motor running on the flusher. The exhaust must be submerged in water so the motor sees the proper exhaust system backpressure when adjusting the idle speed. BillS
sosmerc posted 07-06-2006 07:00 PM ET (US)
"jumping from 1000 to 3000 RPM"......
Really? If the throttle plates are closed (or nearly closed) and you are getting that much real fluctuation in rpm...I would guess you have a a problem that is affecting the timing...such as a bad trigger or CDM module. Loose magnets on the flywheel could do it, but that's pretty rare.
Keep us posted.
bsmotril posted 07-06-2006 07:11 PM ET (US)
Disregard my post, I missed the part about the RPMs fluctuating. I'd also look for a cracked or loose vacuume bleed line between the block and carbs. BillS
jimh posted 07-06-2006 10:51 PM ET (US)
I think you were very smart to get a tow into the dock. WIth your engine running at 3,000-RPM it would be very difficult to maneuver. It would also be potentially harmful to the engine to shift in and out of gear at that speed. You did the right thing to shut it off.
A two-stroke outboard motor like your classic Mercury 90-HP is really a very simple engine, and it operates on relatively primitive technology. Even though it is somewhat primitave, it should be quite reliable. It is unfortunate that your first 11 months of use of this Mercury outboard have been so filled with problems.
At this point you should rely on the dealer to make proper repairs. Working in your favor is that just about every Mercury dealer in the world is familiar with these simple engines and should be able to work on them. If the dealer you are using for support is not able to quickly locate and remedy this problem, he is probably not much of a dealer.
Are you using the Boston Whaler dealer to support the engine? In some situations the Whaler dealer may not be the best choice for support of Mercury outboards, as these two brands have not been married for very long.
jmontani posted 07-10-2006 12:21 PM ET (US)
Thanks for the replies.
It turned out that a screw came out of the middle intake on the back side of the engine and was prohibiting proper throttle. I am still trying to get the full story since the person that worked on the engine was not there this weekend.
The boat is fixed and appears to be workng well. (I ran it for a few hours on Saturday and 2 more on Sunday.)
As for the repair, I did not use a Boston Whaler dealer but and authorized service center on Texoma.
Jack

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Author	Topic: Mercury 90-HP Two-Stroke: Engine Speed Jumps at Idle to 3,000-RPM
jmontani	posted 07-05-2006 09:20 PM ET (US) My 170 Montauk is not even a year old. The two-stroke Saltwater Series Mercury 90-HP motor ran great after they replaced the powerhead six months ago. This last weekend I was fishing all morning and ran back to the marina. When I came down off of plane and started to idle in at the "no wake" signs, the engine speed was jumping to 3,000-RPM from 1,000-RPM. The throttle position was all the way back just in front of neutral. I put the boat in neutral and the engine speed continued to jump around. I shut down the engine and took a tow into the dock. The boat is now in for warranty work. What could the problem be? The boat has about 20 hours on it since Mercury replaced the powerhead. It has not been a good first 11 months for this engine. I appreciate any input. Jack
bsmotril	posted 07-06-2006 09:28 AM ET (US) I've had this happen to me too. The stopper nut on the idle adjustment screw vibrated loose and the idle stop screw then vibrates in until it contacts the idle stop on the block. It keeps the throttle from closing down completely, hence the high idle speed. Easy do it yourself fix, back the screw out to where it belongs. Paint it with laquer or nail polish when set correctly to keep it from happening again. Don't try to adjust the idle speed with the motor running on the flusher. The exhaust must be submerged in water so the motor sees the proper exhaust system backpressure when adjusting the idle speed. BillS
sosmerc	posted 07-06-2006 07:00 PM ET (US) "jumping from 1000 to 3000 RPM"...... Really? If the throttle plates are closed (or nearly closed) and you are getting that much real fluctuation in rpm...I would guess you have a a problem that is affecting the timing...such as a bad trigger or CDM module. Loose magnets on the flywheel could do it, but that's pretty rare. Keep us posted.
bsmotril	posted 07-06-2006 07:11 PM ET (US) Disregard my post, I missed the part about the RPMs fluctuating. I'd also look for a cracked or loose vacuume bleed line between the block and carbs. BillS
jimh	posted 07-06-2006 10:51 PM ET (US) I think you were very smart to get a tow into the dock. WIth your engine running at 3,000-RPM it would be very difficult to maneuver. It would also be potentially harmful to the engine to shift in and out of gear at that speed. You did the right thing to shut it off. A two-stroke outboard motor like your classic Mercury 90-HP is really a very simple engine, and it operates on relatively primitive technology. Even though it is somewhat primitave, it should be quite reliable. It is unfortunate that your first 11 months of use of this Mercury outboard have been so filled with problems. At this point you should rely on the dealer to make proper repairs. Working in your favor is that just about every Mercury dealer in the world is familiar with these simple engines and should be able to work on them. If the dealer you are using for support is not able to quickly locate and remedy this problem, he is probably not much of a dealer. Are you using the Boston Whaler dealer to support the engine? In some situations the Whaler dealer may not be the best choice for support of Mercury outboards, as these two brands have not been married for very long.
jmontani	posted 07-10-2006 12:21 PM ET (US) Thanks for the replies. It turned out that a screw came out of the middle intake on the back side of the engine and was prohibiting proper throttle. I am still trying to get the full story since the person that worked on the engine was not there this weekend. The boat is fixed and appears to be workng well. (I ran it for a few hours on Saturday and 2 more on Sunday.) As for the repair, I did not use a Boston Whaler dealer but and authorized service center on Texoma. Jack