Motor running fine, lost 10mph off of top speed.

[carpking]

Hey everyone. I have a 22 ft deck boat with a 98 model Mercury 150 XR6 2 stroke on it. The boat has a top end speed of about 40mph. A couple of weeks ago, I noticed the boat felt noticeably slower, 10 mph to be exact. The motor seemed to be running fine, other than what sounded like a very occasional miss. I assumed I had a fouled plug so I put a new set of plugs in and it made zero difference. Next, I cleaned the carbs and still made no difference. I am now at a loss. The drop in speed is consistent everytime except with the occasional miss or or "burp" in the motor. Someone said I could have a prop hub that is not totally spun but Ive never heard of one only partially spun. Any ideas on where to look next would be greatly appreciated.

 

[ondarvr]

It's not the prop hub.

Check compression and spark on all cylinders.

 

[jimmbo]

What condition were the old spark plugs when you took them out, Was one or more of them fouled, or wet

 

[Teamster]

My merc 150 ran like that,...Was dropping a cylinder at higher rpms,...

Had the power packs replaced and it's been good again,...

 

[Dukedog]

switch box...........

 

[Coxwain]

What is the power pack?

 

[jimmbo]

What is the power pack?

Switchbox(s). Usually should be replaced in pairs, @ $300+ each

 

[dsiekman]

Switchbox(s). Usually should be replaced in pairs, @ $300+ each

Is there a good way to test them if it only drops out at higher rpm?

 

[jimmbo]

Is there a good way to test them if it only drops out at higher rpm?

Not really. Loose or corroded terminal connections along with broken/pinched wires have lead to many good switchboxes being unnecessarily replaced.

The trigger is usually suspected when a miss occurs at higher rpms but engine runs fine at lower speeds, and vice versa