Mercury 175 Black Max no low end power

[mooman333]

I recently bought a 1979 Baja 15SS with a 1978 175 Mercury Black Max attached. It had not run for 4 years, sat in a barn.

I replaced the water pump, rebuilt the fuel pump, re did all the carbs (two floats could probably use replacing but havent found parts yet), checked all spark, all cylinders 120-125 psi on compression test, and replaced shift cables.

I have tried a 17", 21", 23", and high five 25" pitch prop on this boat last weekend. All of them preformed pretty similar supprisingly on the low end. I chose the 21" three blade SS prop because it ran at about 6k RPM's WOT, so it is about perfect (this motor should run 5800 RPM's WOT)

My problem is the motor is a dog out of the hole. It struggles to get the boat out of the water. Once it gets up on plane it takes off and is fine. When you open the throttle up from a stop, the engine acts like it just boggs down and doesn't have the horse power there. After about 10 seconds it will struggle and get it up on plane then it takes off like normal. If you put it in nuteral, it has no problems reving up like normal.

What is going on here?

I had a Johnson 115 that would yank the boat up on plane and pull a skier, so i don't understand what the issue is with this 175 HP motor.

 

[j_martin]

Re: Mercury 175 Black Max no low end power

Link-n-sync.

 

[mooman333]

Re: Mercury 175 Black Max no low end power

I am assuming that link-n-sync means to sycn the carbs and adjust the timing advances. I did that tonight. Synced the carbs according to the manual and adjust the timing accordingly. 14 degrees ATDC for idle throttle position and 20 degrees BTDC for WOT. Adjustments made at cranking speeds. All throttle stops adjusted according to manual as well.

Still doing the same thing if not even worse. I'm at a loss for what is going on. I made a recording and put it on youtube so you can see for yourself. During the video, the engine is at WOT the whole time, but struggles to get on plane, once it is up, it takes off like it should. Runs fine through the RPM ranges as long as it is on plane. It did come up on plane like it should one or two times out of 30 attempts, I am just lost. Here is the link for the video:

http://www.youtube.com/watch?v=CRWB8On9t_s
http://www.youtube.com/watch?v=CRWB8On9t_s

 

[j_martin]

Re: Mercury 175 Black Max no low end power

Assuming it's not just recording artifacts, you have a serious miss right at max torque.

Start by checking the stator connections to the switchboxes with a DVA. You might be idling and running on 3 till you hit the crossover point at about 3000 rpm.

You could also pull wires at idle. You'd find one whole bank dead at idle.

If you hook up an inductive timing light to each spark plug wire (in turn, of course) you should be able to find which one is mis-firing. Then you look for the obvious fault (insulation failure for instance) or move the fault by swapping parts around.

I don't know what's wrong with the 2 floats in question, but it could be flooding in a couple of cylinders. Usually that will be the worst at idle.

Maybe others have another take on this.

just my 02
John

 

[CharlieB]

Re: Mercury 175 Black Max no low end power

Take another real close look inside the carbs.

A lean idle circuit will kill acceleration until the air velocity is enough to begin pulling the majority of the fuel thru the main circuit.

Low float levels can contribute to the problem.

Test this by 'bumping' the choke/enrichener while attempting a WOT holeshot. If the additional fuel makes any improvement you can be sure the problem is a lack of fuel.