1988 Mercury 45HP stops accelerating/bogs midrange.

[Idratherbehunting]

Good evening,

I have been attempting to fix a outboard for about 6 weeks now (tried several things from these forums). I have had 2 mechanics take a look and diagnose it with no solutions. The motor idles and runs great until half way. At the same point every time the engine stops increasing speed and stops accelerating and sounds like it is bogging down or flooding out with fuel. I have rebuilt the carbs and fuel pump, and went as far as cutting the fuel line and dropping it directly into fuel to make sure the fuel tank wasn't the issue. Took apart the engine and replaced the reed valves. Replaced the stator. The actual point the motor starts the problem is the moment the spark advance armature stops advancing and the throttle continues to open. I think i may be hearing a miss every once an a while but not sure. Does anyone have any idea or possible solutions? Compression was in the 140s on one set of gauges and 125-130 on another set. I also marked the flywheel at TDC of each cylinder and put a timing light to make sure each cylinder was firing when it was supposed to.

I have a suspicion the previous owner new about this because he also rebuilt the carbs, fuel pump, and replaced the spark plug wires and switch box.

 

[Faztbullet]

Take a timing light hooked up to #1 and see if spark is dropping out when this happens, if so the rectifier may be bad..

 

[Idratherbehunting]

I disconnected the yellow leads From the rectifier to see if that was it, still same problem. Another thing I didn't mention was that I can see fuel being sucked up into the venturi and then into the engine on the lower carb. It doesn't look like it is atomizing or if it's supposed to be like that. I don't see anything in the upper carb.

 

[Texasmark]

Fuel coming up through the venturi tube from the bowl at WOT ought to be visible as a quasi mist. If you are flooding out I'd say your float isn't floating properly in that carb and allowing excessive fuel to enter the bowl pushing it out the venturi tube. Being an '88 model and maybe plastic float that could be your smoking gun. Additionally, if you tilt your engine all the way up with the engine off, dripping of gas out the venturi would be another clue in addition to a needle valve in the carb not seating properly. Over time they can develop a ring where they seat and it doesn't do a good job of sealing once that happens, especially on rubber tipped needles.

 

[racerone]

Remove lower unit.---Take strong flashlight.----Shine up into the exhaust tuner and look for blockage.

 

[Idratherbehunting]

The exhaust looks good, i had the whole power head and lower removed last week. When i rebuilt the carbs I installed new floats (plastic). I set them to the specifications in the Seloc manual. I still had the same problem so i set them so they were level when upside down with the top of where the bowl seats. Both times same problem, i even swapped the main jets between upper and lower and the problem was the same.

 

[racerone]

Have reed valves been checked ?---Lower crankshaft seal ??

 

[Idratherbehunting]

I changed the lower reed valves. I did not replace the seal since I don't want to keep putting unnecessary money into it. Why would a seal create this and how can I check the lower seal?

 

[Idratherbehunting]

I read something about the pressure differences and air leaks in the crankcase but don't fully understand the principle.

 

[Texasmark]

On an '88 year model engine surely you could have some seals drying out if not wearing out. Any seal on the crankshaft would affect the induction cycle if anything. Never having been there but applying some common sense (maybe) the engine could be sucking air through a bad seal (seems highly unlikely considering volume through venturi vs volume through a leaking cc seal but who knows) and this air is diluting the normal 12% fuel air mix from the carb. Again seems highly unlikely. Also if this were the case, why would if affect the midrange and not continue into the top end......swag....butterflies are opened farther and less fuel air is coming through the carb.....course fewer rpms means less volume is going through the air pump (the engine) too.......all proportional to rpms!