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Jeff G.
Jeff G., Marine Mechanic
Category: Boat
Satisfied Customers: 4725
Experience:  42 years experience, Certified Master Technician, All Major Outboards, Sterndrives, Hi-Performance
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I have a 1984 Mercury Black Max 150. From idle when you go

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I have a 1984 Mercury Black Max 150. From idle when you go to accelerate and come up on plane, the engine stubbles and studders. But when it breaks over and planes off it is a rocket ship and runs great. I have been told that it is the low speed jets and the carbs probably need cleaned. Does anyone have a pdf for the proper procedure and adjustments. Diagrams would be nice also. Or if there could be something else causing it ?
Welcome,this question may need more than a single answer. Please post back with additional questions on this at any time.

Before we start tearing into the carbs, how is the idle itself?
Also can I get the engine serial number as that's how Mercury identifies the engine.
Customer: replied 5 years ago.
hey jeff, i was hoping you would respond. You had said the last time for me to put your name in the title box and it would come to you, but for the life of me I cant find anything titled Title Box. We worked with the boat this weekend and ended up raisng the engine over 3 inchs. Its performing much better on top end and the " walking" has decreased about 60 percent. but we lost our water pressure gauge, it just quit. We have blowed the tube and everything. But its stuck. So for now we arent raising it anymore till we get that fixed. As per your question, it idles decent. Its a little rough, but it doesnt die out or anything. It will probably be tomm evening before i can get the serial number, its 15 miles away from my home at my brothers , but I can get it tomm.
It will help, in the mean time lets check something.
Lets take a look at the synchronization of the carbs and to the timing.

Take the air silencer off the front of the carbs.
Look into the the carb throats and manually open the carbs. With the carbs wide open the brass throttle plates should be parallel to the throat. If they are not then adjust the linkage between the carbs with the linkage #17/18 in the first drawing.
Now close the throttle and make sure all of the throttle plates are closed.
Next we want to check the synchronization. Look at the throttle arm. At the top it attaches to the timer base. At the bottom is a roller that pushes against the carb linkage.
Disconnect the throttle cable from the throttle arm. Push the throttle arm slowly to open. Observe when the carb linkage just begins to move. The timer base should be advanced all the way before the carb begins to open. There is a mark on the cam.
If the carb begins to open before the timing advances enough then the acceleration will stumble. You can adjust this with the screw on the throttle arm.

Check these and let me know what you find.

Customer: replied 5 years ago.
Hey Jeff, im at work, as soon as i get off i will go check those things and get back with you latter this evening. Meanwhile, the engine serial number is.(NNN) NNN-NNNN N Thanks John
See you then.
Customer: replied 5 years ago.
Hey Jeff, finally got a chance to perform the carb linkage test and the timing advance test. All of the carbs are in sync and at wide open all the plates are horizontal. After disconnecting the throttle I checked the advance. The advance is well over half way of its travel before the cam starts to move the carb linkages. By the time the carbs are 1/3 open the spark advance is against its stop. After doing those checks last night we put the boat in the water to try a different prop. The idle seems to be getting rougher and the stumbling seems to be getting worse also. Its getting harder to get it to break over and go to plane. We have a drive on trailer and when you go to load the boat you can open it up all the way and it wont even push the boat up on the trailer a foot. Ive tried all different throttle positions to try to get it to spool up enough to push it up on the trailer. I did notice last night when taking off. From idle to about 1600 to 1800 rpm the engine just doesnt respond at all. But after about 20 seconds it picks up and once it gets past about 1500 to 1600 rpm it comes alive. Once on plane, the engine has great power and throttle response and is a rocket on top end. Awaiting your next advise. Do I need to go ahead and except your previous answer ? Im not sure how that works, so just let me know. Thanks John
OK lets check a couple of other things.
First remove the drain plugs from the bottom sides of each carb. Drain into some paper towels. If the drain plugs are discolored or have gum on them the carbs will need a through cleaning and new gaskets.

Next you will need an inductive timing light.
You won't have to take the boat off the trailer.
First make sure the boat is securely tied to the trailer.
Back it in until the lower unit is just covered.
Take the engine cover off.
Start and warm up the engine. Shift to forward.
Advance the throttle to wide open, if the engine stumbles push in the choke briefly to see if the engine picks up rpm. If so rebuild the carbs.
If that makes no difference then on to the ignition.
Put the timing light on the #1 cylinder. Run the engine at a fast idle in gear, and look at the flashes from the light. Make sure they are steady and even. Repeat on the other 5 cylinders. See if the flashes stumble up until the time the engine runs well.
Next repeat the same test but at a higher rpm about 3000 and see how the engine runs.
Turn the engine off when moving the timing light lead.

Try this and post back what you find. Then we can narrow down the problem and look for a fix.
As for the accept, you only accept when you are satisfied with my answers.
Jeff G. and other Boat Specialists are ready to help you
Customer: replied 5 years ago.
ok i will do those checks sat morning, although getting the engine to 3000 rpm while sitting still is prob going to be next to impossible. As for the accepts, I have been well pleased with your answers and guidence on these boat matters. I didnt know if you could still follow through if I clicked on the accept. But if it dont break the chain and we can still keep digging until we find the problem I will gladly accept each one of them. So just let me know if we can continue the follow ups, Thanks John
You can always ask follow up questions even after accept. If you accept each time, you'd be charged each time unless you are a subscription customer.

If you can't get to 3000 no big deal I'm trying to narrow down where the problem is and we may find it quickly and not have to do all the tests.
Customer: replied 5 years ago.
Hey jeff, The problem might have diagnosed itself. They went to take it out last night and it was really hard to start. Once it started it wouldnt idle. If you gave it fuel it would just shake real bad and wouldnt rev up. So this morning I went to check the spark as you had said. I found that the whole right side (if looking from rear) cylinder back, all 3 cylinders, has no spark whatsoever. The left side cylinders have spark. Im thinking the power pack for that side has went bad. My question now is this. 1.Is there anyway to check the power pack ? 2. Could the power pack going bad been what has been causing the low rpm take off problems ? Im here with the boat right now if there is something else to check. Thanks John
Hey John. your switchbox, power pack could be going out. In order to properly diagnose the problem we use a peak reading voltmeter. If you don't have a few hundred dollar meter, then diagnosing a switchbox is a process of elimination.
What we would do is to do resistance and voltage tests of the stator and trigger assembly, the coils under the flywheel.
However if you are missing spark on one bank only then the chances are 99% that it is the switchbox.
But before replacing the switchboxes remove any modules, idle modules etc and retest.
Are you running oil injection or do you premix your fuel and oil?
If you premix remove any modules on the engine.
If still the same replace both switchboxes. It is always a good idea to replace them in pairs. In addition the bias circuit in one that is still firing can cause the other one to not fire, yet is bad.
I would recommend replacing them with switchboxes from CDI Electronics.
Customer: replied 5 years ago.
Ok Jeff, I dont have a really good meter. I will have to do the stator and trigger voltage and resistance checks. We pre-mix the fuel. Can you send me a diagram that shows the location of the modules ? Im not sure what your calling a module or where they are at. Also can you include the test procedure for the stator and trigger ? I will go to CDI electronics and check the prices on the switch boxes. We have found a local guy that has a 1986 blackmax 150 that has a bad piston. The very lower part of the lower unit has been sold. But other than that the engine is complete including the trim. He just wants 200 for the whole thing. So I figured it was well worth that for parts, or to rebuild and have a spare engine. Whats your oppinion ? He said it ran really good before it lost compression on that one cylinder. Thanks John
For $200 if you have room for storage it is a good deal.

You won't be able to do a voltage test only a resistance test.
If you buy the other motor try those switchboxes.

On the stator check blue to ground and blue/white to ground 5800 to 7000 ohms
red to ground and red/white to ground 135 to 165 ohms

IF you have any modules, they changed all the time they will be black boxes about 4" X 5".
Just disconnect them if there are any and take them off the engine. You may not have any on your engine.
Customer: replied 5 years ago.
Testing those wires, does that check the trigger also jeff ?
No it tests the low speed and high speed coils on the stator. Usually when you lose one bank it is the switchboxes. It is possible, not common to also be the stator. Rare for the trigger as each coil supplies two cylinders.
Jeff G. and other Boat Specialists are ready to help you
Customer: replied 5 years ago.
Ok Thanks Im going to borrow a good meter from work and do the test late tomm night. Thanks
Let me know.

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