5.0 mercruiser wot problems

[Rickey1126]

First time posting any thing long time reader
I have a 1990 5.0 mercruiser with an alpha 1
It runs great at any thing below wot. I can be on plane at 3500 rpm and then apply the last 1/4 of the throttle and the engine will rev to about 4100 rpm for about 4 seconds then it will drop to about 3200 to 3500 and surge really bad if I pull back on the throttle it will pick back up and run fine
Replaced
Cap and rotor
Coil
Plugs
Wires
Distributor sensor
New fuel pump
All filterers
Anit syphone valve
Cleaned out the tank and pick up tube
Rebuilt the carb double checked the float
Checked the timming set at 8
Any help would be greatly appreciated

 

[Fun Times]

First time posting any thing long time reader

Hi there Rickey1126, welcome to iboats. Being a long term reader, You must of liked the guys that tend to hangout in the Mercury outboard sub-forum section better than the guys that hangout in the Mercruiser stern drive sub-forum as you posted in the outboard section first. Just kidding, I moved you on over to the Merc stern drive sub-forum.:tea:

4100 rpm

Even 4100 sounds to low. Most likely part of the engine being down on power but just in case, what size/brand prop are you running? Do you know your gear ratio too?

Checked the timming set at 8.

Now you need to ensure the timing is advancing up to somewhere around 25ish degrees as you advance the RPM.

Also you'll want to check fuel pressure to see where it's at all RPM's.

 

[Badfish95]

Hi Ricky,

Your problem sounds very familiar. Once you've ruled out a fuel issue and your compression checks out, replace your TB ignition system with an aftermarket marine ignition. It'll save you a ton of gas money and headaches. I had similar issues and that's how I finally fixed mine.

http://forums.iboats.com/forum/engin...rm-and-sitting

Many on here will swear by the Mercruiser Thunderbolt ignition system. Your ignition system is 26 years old and they no longer make the modules for them. Take my advice and do not not waste your money trying to repair the TB ignition. The ignition module is what is responsible for advancing your timing and it doesn't sound like it's doing the job.