1999 Merc 200 efi - tops out at 9mph

I'm new to boats, and took my recently purchased used boat/motor up at a shop in Allen to be repaired as it had trouble starting and idleing. After 2 months they found the ECU needed replaced. So the ECU was replaced and I was told all was good and ready to go. I saw it run in the lot with the rabbit ears and it did sound good. Took it out on the water this past weekend and started right up, idle is nice and sounds great in neutral like it could go 50mph, but once in Forward it tops out at 9mph. Found some articles about a wiring harness/pin issue could be the root cause. With that said I don't have a place to work on this and I'm not that good with electrical anyhow. Please can I get a recommendation on a boat/shop Merc 200 efi friendly? as i'm not excited to go back to the shop I had it in. I am located in The Colony area of DFW, but willing to travel to the best suited shop for repair.

Thank you
Dan

Possibly the prop also, have that checked.

What kind of rpm is it getting at 9 mph? Is the engine rev'd up or does it just have no power? If the engine is reving to redline then you probably have spun the prop hub. Easy to replace depending on which prop.

Apparently I should have paid better attention to the RPM as I have been asked that twice now. I'll have to get it out again to confirm, but my recollection from sound has me stating RPM was not high or close to redlining. As a matter of fact my speedo cable is broken I was using my Hummingbird to report mph. I am not sure if the tac works or not. My apologies for lack of info; I just picked up the boat used and had it in the shop for the initial repairs. This was the first time I had it off the trailer on the water. Thanks for the info and troubleshooting thoughts. And if anyone can recommend an excellent shop that would be appreciated.

Thank you
Dan

Several things need to be checked before you spend any more money. Like the compression.

HI Gary

The compression was checked before I had bought it. I believe 5 were at 120 and 1 was at 115.
But to your point I am looking for a good shop to take it to for proper checkout.
Thank you

bob at fishin' world would be a good one..... guess he's still there?.. gettin' ta be hard ta find a tech that can/will work ona "switch box" motor....

Take to Service Marine in Emory (903-473-3909). You will be glad you did. They will not patch it up or take shortcuts. When they are finished it will be fixed.