25hp mercury runs high rpm

[Johnmerc98]

I have a 1996 mercury 25hp sea pro it starts right up and idles rough but if you give it the tiniest bit of throttle I revs real high any ideas on what this could be from????

 

[Johnmerc98]

I have a 1996 mercury 25hp sea pro it starts right up and idles rough but if you give it the tiniest bit of throttle I revs real high any ideas on what this could be from???? How can I fix this issue????

 

[jimmbo]

Welcome to iboats

Does it do this when on the Boat and in the water, or when it running on Muffs, out of the water?

 

[Johnmerc98]

I just got the motor it's only been on muffs on the stand it starts up idles decently but if I touch the throttle and give it any gas at all it revs really high

 

[jimmbo]

There is no Load on the engine in Neutral in the water, just some Backpressure. On the Muffs, in gear or not, there is not even Backpressure, so it doesn't take much to rev them up. Even with a Dead Clylinder, they can still rev up fast. Revving the Engine with no load, can and has grenaded many Outboards. Don't exceed 1600rpm in Neutral

 

[Johnmerc98]

There is no Load on the engine in Neutral in the water, just some Backpressure. On the Muffs, in gear or not, there is not even Backpressure, so it doesn't take much to rev them up. Even with a Dead Clylinder, they can still rev up fast. Revving the Engine with no load, can and has grenaded many Outboards. Don't exceed 1600rpm in Neutral

 

[Johnmerc98]

Ok so is that why it's revving and holding that Rev then backing down? If I start it it will idle if I touch throttle and give it any gas it revs real high then I shut it off start it back up and it's back normal until I give it any kind of gas?? I can't get the mixture screw right either? I have a mercury 18xd also I run it on muffs and it idles great throttle is great and perfect revs perfect so it has me confused??

 

[jimmbo]

You are experiencing a Runaway Engine, Do Not rev it on the Muffs or in Neutral in the Water. I can't see your engine, and if anyone has messed with the Throttle Linkages and/or the sync between the Carbs and Spark Advance.
Running rough often occurs when someone has messed with the Carb Butterfly, thinking they are adjusting the Idle Speed, like is done on a Car/Truck/Motorbike. Do Not try to tune these engine like you would tune a Car/truck, even a Motorbike. A complete check of the Throttle Linkage, and its Sync with the Carbs needs to happen, and adjusted as per spec, Do not try to set the Timing at Idle, there is no Timing Spec for Idle, as the Timing is what controls the Idle Speed, and will vary due to a few factors.
As for the Mixture, it needs to be adjusted when the motor is in gear, and under load.

 

[Johnmerc98]

You are experiencing a Runaway Engine, Do Not rev it on the Muffs or in Neutral in the Water. I can't see your engine, and if anyone has messed with the Throttle Linkages and/or the sync between the Carbs and Spark Advance.
Running rough often occurs when someone has messed with the Carb Butterfly, thinking they are adjusting the Idle Speed, like is done on a Car/Truck/Motorbike. Do Not try to tune these engine like you would tune a Car/truck, even a Motorbike. A complete check of the Throttle Linkage, and its Sync with the Carbs needs to happen, and adjusted as per spec, Do not try to set the Timing at Idle, there is no Timing Spec for Idle, as the Timing is what controls the Idle Speed, and will vary due to a few factors.
As for the Mixture, it needs to be adjusted when the motor is in gear, and under load.

 

[Johnmerc98]

Ok how does this look? (Pictures below) and how can I fix it?

 

[Johnmerc98]

If I have alot of carbon build up or unburnt fuel what should I do?

 

[jimmbo]

There are Carbon Removers like BRP Engine Tuner, or Mercury/Quicksilver Power Tune, that will soften and help remove Carbon from behind the Rings