Dies at mid throttle, runs great at wot

[jschermesser1225]

1988 Merc 9.9 S/N - 0b364319....I bought this boat last year, and knew it had this issue. It would do just fine at idle speed and wot. And would within seconds die in mid throttle. Over the winter I rebuit the carb and the fuel pump. Replaced spark plugs. yadadada. Anyways put the boat on the water today, And I was hoping for the best. It started great like it always does, I idled around the lake going toward my dock. Got pretty close and decided to see if I fixed the mid throttle issue. It fell straight on its face. I can bring the engine out of a stall by shifting to neutral or tapping my choke will make it come alive for a couple of seconds. Took it out one more time to do some tests, and It seems to fall on its face anywhere above 10% throttle in gear, until I get to wot. And it will be just fine. I am so confused, it almost seems slightly intermittent. But I don't know, it seems worse now than it was last year. Im thinking its not enough fuel, because I can put the butterfly valve open and it will still die. It only does this in gear. Sitting in neutral I Can rev up and down no problem. Please help. When I told my wife it wasn't fixed It was as if I had stolen her candy bar. Anyways This is my first post. Im a diesel mechanic, Let me know where I should start. I have read a whole lot of different things about outboards falling on their face at wot, but nothing about mid throttle.

 

[jimmbo]

When it dies at 'mid throttle', is it at a particular rpm? or at the particular throttle setting? When it dies does choking help if it is in gear? if an engine responds to a momentary coke it usually means leanness. Is the float set correctly? Idle mixture screw adjusted properly? Revving in neutral doesn't take much fuel as the engine is not under load. Actually it is way of grenading the engine. When you reinstalled the carb, did you verify the sync between the ignition and carb?

 

[jschermesser1225]

I will say this, I get a puff sometimes when I start it up, the guy that moved the boat for me said that it might be a little lean. So I still am yet to adjust that. As far as the carb, I followed all the steps for the rebuild from the manual. But I have not and do not really know how to verify the sync between the ignition and carb, Its hard to tell what rpm It should be putting out at mid throttle because it just starts chuga-lugin. And yes choking definitely helps when its in gear. It will bring the engine out of the stall, but only momentarily.

 

[CharlieB]

Typical symptoms of a too lean idle mixture.

The 2 stokes do not have an accellerator pump so have to rely on an overly rich idle mixture to provide the additional fuel necessary for acceleration.

It is normal for most people to want to adjust the mixture to 'Best Idle'. In the Marine world this must be done IN gear as the load increases fuel requirements. Now this 'Best Idle' IN gear is still way too lean to accelerate. You may find it needs almost a hair over an additional 1/8 turn out, richer. I like to make adjustments very small, as in just the width of the screwdriverblade slot at a time. Test acceleration by slamming the throttle WOT in gear. Any lean miss, bog, cough or hesitation proves it wants even MORE fuel. make another tiny change to each carb and test again. Repeat to perfection. A heavier boat one day needs more fuel. Another Fisherman, full fuel tank, big full coolers, need more fuel. But you want to adjust for your average load so not to use an excessive amount of fuel the rest of the time.