Loss of speed-115 mercury

[rkazmo]

I have a 1998-115 hp mercury and lately when I am driving at full throttle for a while I have a loss of speed.

It is like someone pulls the throttle back. After about 10 seconds the speed comes back for a minute or so... then slows down again. Also I noticed that my top speed is approx. 5 mph slower then it was before this problem started.

Hope someone can help?

Thanks

 

[Capt'n Chris]

Re: Loss of speed-115 mercury

What is your tach telling you when this occurs?

 

[rkazmo]

Re: Loss of speed-115 mercury

tach is at 4600.

I also found that the rpms are a bit higher (@100-200) lately at full throttle.

 

[starcraft1982]

Re: Loss of speed-115 mercury

as always,first check compression.No(or very few)power loss issues should ever start any other way.then check spark to all cylinders.7/16 gap for arc.Or,pull plug wires one at a time while idleing.there should be a change in sound of motor when each one is pulled individually(to show theyre all contributing to the running)it will run slightly rougher it its functioning.if one makes no change(or more)report back to help diagnose.If all seem ok,then its fuel time.could be as simple as dirt in tank,fuel pump failing or an air leak somewhere in line.All the steps until fuel should take less than 1/2hr and will tell ALOT.do them or have them done and report back.These great guys will get you going.Its just a process. As for tach reading,it seems a bit low,to me if WOT.but again start w/basics,compression ,spark,and see where you are.And while not very likely,it almost sounded for a second in your description w/rpms slightly higher that a spun hub on prop could be possible.again,just easy things to check to get you going.good luck and get back w/results

 

[rkazmo]

Re: Loss of speed-115 mercury

The sparks I am using are new and do not have the ground. Just a point. Also the problem begins once the motor has ran for a while and has warmed up well. . Could this be the problem.

 

[Faztbullet]

Re: Loss of speed-115 mercury

If you have a spare prop install it just for giggles...... I had a customer years ago with a 60 that did close to what you have posted and it ended up being a hub in prop starting to fail...

 

[rkazmo]

Re: Loss of speed-115 mercury

I did, I installed the 21 pitch pitch prop which is new. It used to never do it with the new prop just the old one. Now it does it with both props!

 

[rkazmo]

Re: Loss of speed-115 mercury

The spark plugs I am using are new that do not have the ground. Just a point. Also the problem begins once the motor has ran for a while and has warmed up well. Could this be the problem.

 

[usskydvr]

Re: Loss of speed-115 mercury

I would focus on fuel delivery, almost sounds like you may have a plugged gas tank vent. ( You can test by loosening/removing your gas tank filler cap and see if the problem goes away)

 

[rkazmo]

Re: Loss of speed-115 mercury

Okay
compression ranges from 101 to 104 on all six cylinders
good spark on all cables
change 8 of 12 ful and air lines
put new prop on (which I had)
took off gas cap-worked the first time, but not the second time
I check my cables in the dark and noticed 3 of the cables were grounding out and arching, also I noticed that at the boot to the plug there is a blue glow, not sure if that is normal or not.
This problem normally only occurs after boating for a while at top speed and engine is really hot. After about 10 seconds it goes back to full speed again?

What should I check for next?

Thanks all

 

[usskydvr]

Re: Loss of speed-115 mercury

If you have 'cables' (I assume spark plug) "that are grounding out and arching" then you should replace them.