89 Mercury Classic 50 won’t open up

[Two_Stroke_Mike]

Glad to be here with the community ! Admins thanks for the help with the confirmation email. I would like to learn more about outboard motors and how to diagnose them. I have a classic 50 that won’t seem to open up to full throttle. Carbs have been soaked, cleaned and built, both carb butterflies opening up to full throttle with controls. Drop test showed fire on all cylinders. The things I would like to learn how to check/test are: stator, trigger, and max timing advance. I know how to find TDC and check total advance, but I need to verify my timing light is at true zero. I don’t really know how to test the stator and trigger at all. Thanks in advance for any advice to help me figure these simple motors out in the future !

 

[racerone]

Fuel pump diaphragm ruptured ?----Exhaust tuner plugged ?-----Post actual compression values.

 

[Two_Stroke_Mike]

Fuel pump diaphragm ruptured ?----Exhaust tuner plugged ?-----Post actual compression values.

Fuel pump diaphragm is new, inspected. Not sure about the exhaust tuner ? If you are wanting my compression values they are as follows cyl 1-4
125
125
120
120

 

[jimmbo]

Did it run fine, then suddenly stop running Fine?
What "Tinkering" have you done to the Engine?
How long have you had this Motor?

 

[Two_Stroke_Mike]

Did it run fine, then suddenly stop running Fine?
What "Tinkering" have you done to the Engine?
How long have you had this Motor?

I actually purchased this boat as a project to learn more about outboard engines. Previous owner said it would no longer get up on plane and go, he had messed with the carbs and was tired of trying to get it going. I figured a classic 50 would be a good one to learn from. First test drive seemed to me like it had a misfire and was starving for fuel. As far as "tinkering", I removed the carbs, inspected reeds best I could from that pov. First thing I discovered was the sync linkage was not installed correctly so the bottom carb was not synced to the top carb. Also neither carb was opening up to full throttle, butterflies were about 75% open when shift control was at WOT position. I soaked carbs in a vat for 48 hours and cleaned thoroughly, installed new parts from Sierra kits, installed back on engine and synced them, and adjusted throttle stop to achieve actual WOT. Also rebuilt fuel pump. Switch box had been replaced somewhat recently, so I checked for correct placement of color coded wires on the switch box, all seemed to be in the correct firing order, but found several loose connections. 2nd test drive after adjusting idle mixture the misfires seemed to be gone, but still would not get up on plane, no tach but just seems to me like its not turning up enough rpm, wont seem to rev past about half throttle. I have only owned this vessel for about a month now.

 

[racerone]

Perhaps too much pitch on the prop.-----You need to install a new water pump impeller.----Inspect exhaust tuner while lower unit is off.

 

[Two_Stroke_Mike]

Perhaps too much pitch on the prop.-----You need to install a new water pump impeller.----Inspect exhaust tuner while lower unit is off.

The prop is a 15p, numbers on it are as follows: 48 73140 A40L 15P. This is on a small 15 ft Glasstream fishing boat. I did actually replace the impeller, and changed the gear oil, but was not aware of an exhaust tuner. I will research that and figure out what it is. Thanks ! I am noticing a change in sound from about 75% throttle to full throttle, actually runs better at 75%, pushing it past that 75% point seems to make it bog even more. I'm thinking maybe that's how the throttle got to be not opening all the way, PO may have thought that would help it since it does run better at that position.

 

[jimmbo]

Okay, so you(and previous Owner) were messing with the Sync between Spark Advance and Carbs.
Have you verified that the Spark is Advancing as per Spec, and that the Pick Up Timing is Correct?
Racers' Comment about the Prop is Valid. What kind of Boat is the Motor on, and what is the current Prop on it?
To inspect the Exhaust Tuner/Tube, just shine a Flashlight up the Midsection when the Gearcase is off for Impeller replacement

 

[jimmbo]

A 15" sounds a bit on the Hi Side, an 11 would be closer to what you would need on that boat

 

[Two_Stroke_Mike]

Okay, so you(and previous Owner) were messing with the Sync between Spark Advance and Carbs.
Have you verified that the Spark is Advancing as per Spec, and that the Pick Up Timing is Correct?
Racers' Comment about the Prop is Valid. What kind of Boat is the Motor on, and what is the current Prop on it?
To inspect the Exhaust Tuner/Tube, just shine a Flashlight up the Midsection when the Gearcase is off for Impeller replacement

From what I can tell the spark advance has an adjustable stop on it as well. I have not adjusted it at all, so it's still in the same position it was when I got the boat. The spark advance lands on its same stop regardless of how far the throttle stop is adjusted, unless I'm not understanding it correctly. All I have done is inspect it to see that it is advancing when throttle is applied. I have not checked to see how much it is actually advancing, but will be one evening this week. I want to make sure my timing light is accurate somehow by checking it at true zero, as the knob turns backwards and goes past zero. Just want to make sure I'm reading it correctly when I do check the advance. Not sure how to go about checking the pick up timing, I'm assuming that is similar to idle timing, or base timing setting. There is a hard, non adjustable stop that controls that part, so the whole lever would need to be adjusted to change that, I think ! Thanks for the help gentlemen, I am loving this learning experience !

 

[Two_Stroke_Mike]

A 15" sounds a bit on the Hi Side, an 11 would be closer to what you would need on that boat

I will see if I can find a different prop and give that a try also.

 

[jimmbo]

There is No Idle Timing spec on your Motor. Idle speed is set by changing the Timing, which will vary upon the Load the Motor has on it. When you adjust the Idle Speed, you will also have to adjust the Length of the Throttle Cable
Do you have the Factory Service Manual for that Motor? I suggest you get one. Forget about 3rd Party Manuals, they are next to Useless.

 

[Two_Stroke_Mike]

There is No Idle Timing spec on your Motor. Idle speed is set by changing the Timing, which will vary upon the Load the Motor has on it. When you adjust the Idle Speed, you will also have to adjust the Length of the Throttle Cable
Do you have the Factory Service Manual for that Motor? I suggest you get one. Forget about 3rd Party Manuals, they are next to Useless.

That is what I was thinking, since my throttle butterflies are fully closed, and I have a good cold and warm idle, idles perfect in gear, in the water, that should be about right. So when you said pickup timing, that's the point where the timing starts advancing correct ? I don't have a factory manual, but I will get one if its necessary.

 

[racerone]

Pick-up timing is where throttle plates start to open.

 

[Two_Stroke_Mike]

Pick-up timing is where throttle plates start to open.

I think I am following, there is a spec for advance for soon as the throttle butterflies start to move/open

 

[racerone]

Pick up timing is where throttle plates start to open !

 

[jimmbo]

These engines keep the Throttle mostly closed until after the Timing is Fully Advanced. The Pick Up timing is where the Plates are permitted to crack open a tiny bit, then more Throttle Lever movement advances the Timing further with the Carks open a wee bit along the way till it hits Max Advance, then the Carbs open fully. A linkage like that permits, after the Engine has revved up, the Throttle Plates to close, but retains Full Spark Advance, improving High Speed Part Throttle Economy

 

[Two_Stroke_Mike]

These engines keep the Throttle mostly closed until after the Timing is Fully Advanced. The Pick Up timing is where the Plates are permitted to crack open a tiny bit, then more Throttle Lever movement advances the Timing further with the Carks open a wee bit along the way till it hits Max Advance, then the Carbs open fully. A linkage like that permits, after the Engine has revved up, the Throttle Plates to close, but retains Full Spark Advance, improving High Speed Part Throttle Economy

Thanks for clarifying that, much appreciated. I will do my best to check these things this afternoon and report back some results in the morning hopefully !

 

[Two_Stroke_Mike]

Just a quick update, didn't get around to checking that max timing. Last night I double checked my firing with my timing light and revved on the engine this time, number 3 cylinder spark is cutting out briefly and coming back in. Looks like I may actually have an electrical problem after all. I will keep posting updates till I figure this out in case anyone has similar issues in the future.

 

[Two_Stroke_Mike]

@racerone, I have discovered another problem that may be at least partially contributing to this. Just wanted to give you credit for mentioning it. I was reading another forum thread where a guy had a similar problem with his classic 50 and you mentioned the boost venturi in the carbs. These are missing from my carbs, they were initially, but I also failed to install them during rebuild not knowing what they were. Not sure if this would cause such a dramatic throttle problem but I will be finding out.