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Tan
Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 10806
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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1977 25 hp johnson runs fine at low rpms but dies at high rpms.

Customer Question

1977 25 hp johnson runs fine at low rpms but dies at high rpms. compression is good, spark is good, carb is fine. any suggestions
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.
Hello. My name is ***** ***** are running the engine squeeze the primer bulb when it is acting up. Does the engine run better? When you are running the engine choke it when it is acting up. Does it run better?
Customer: replied 1 year ago.
Squeezing the bulb doesn't make any difference. I don't think it matters if the choke is on or not.
Expert:  Tan replied 1 year ago.
Ok Well we won't know that till you have tried it. We don't want it on we just need it on momentarily to give the engine a little extra fuel. If no change then go to the next step.We need a timing light to check the spark at full throttle to be sure both cylinders are firing. Connect the light to the top cylinder run the boat watch the flash of the timing light. The flash should be steady with noi dead spots. Then do the same with the other cylinder. If you find a misfire on one cylinder swap the couils does the misfire move?
Customer: replied 1 year ago.
I Don't have a timing light, but I used aN ignition spark checker. The top seemed really consistent and the bottom did too but seemed to miss a little when the engine was dying. I switched the coils and the bottom didn't seem to change, but maybe the top was worse, it was a little hard to tell
Expert:  Tan replied 1 year ago.
Ok since it is so intermittent this will require the use of a DVA adapter or a peak reading volt meter to determine which is the problem. An ohm meter won't do any good with an issue like this. We must do a voltage out put test of the timer base. The possible causes are power pack or timer base. The timer base is what tell the power pack to fire. The most likely failure is the power pack. But that is just an educated guess from experience. The timer base is about $28.00 and the power pack is about $74.00.Do you have either or can you get one? What would you like to do at this point.
Customer: replied 1 year ago.
I can't even see a part called a timer base, nor could the parts store. And I have no access to a peak volt meter.
Expert:  Tan replied 1 year ago.
I ask what would you like to do and never heard back from you. Here are the links to the parts and the meter. The parts guy should have known what a timer base is. The smaller motors they call it a sensor assy. # ***** in this diagram.http://www.boats.net/parts/search/BRP/JOHNSON/1977/25R77H/MAGNETO/parts.html Here's a link to the peak reading volt meter. http://www.maxrules.com/fixtools530.html Here is a link to the DVA adapter. http://www.maxrules.com/fixtools640.html
Customer: replied 1 year ago.
I was checking with my father in law who is a diesel mech, he didn't have one. I don't want to buy either too. How likely is it to be one of those two parts?
Customer: replied 1 year ago.
Should say either tool
Expert:  Tan replied 1 year ago.
Those are the only 2 parts it can be. Most likely the power pack.
Customer: replied 1 year ago.
If I had the right tool, what would I do to test the timer base/sensor assy?
Expert:  Tan replied 1 year ago.
Connect test leads to the 2 leads from the sensor and crank engine. The voltage output should be .6V+
Customer: replied 1 year ago.
A local parts place has the sensor assy, I might just try that first. How do I get to it?
Expert:  Tan replied 1 year ago.
You remove the flywheel then you can access the sensor.
Expert:  Tan replied 1 year ago.
To remove the flywheel you will need a puller. A steering wheel puller will work. That is the purpose of the 3 small holes in the flywheel to attach the puller and press against the top of the crank shaft. Remove the nut then install the puller pop flywheel off.
Customer: replied 1 year ago.
I changed the sensor assembly...no change. Well maybe it dies out at a little higher rpm. It will take about two weeks to get the power pack. Hopefully that will fix it. I will order it on Mon. Unless you think I should try something else.
Expert:  Tan replied 1 year ago.
Nope as I suspected the power pack is the issue. Now your sure that squeezing the primer bulb at higher rpms doesn't help?
Customer: replied 1 year ago.
I am sure squeezing the bulb doesn't help, but I still double checked.
Expert:  Tan replied 1 year ago.
Ok then the power pack is the issue.
Customer: replied 1 year ago.
The power pack came, I installed it and now it won't start
Expert:  Tan replied 1 year ago.
So you went from bad spark to no spark by replacing the power pack and the sensor. I don't see how you can remove old parts and replace with new and the problem got worse. Disconnect the black/yellow wire from the power pack do you have spark now?
Customer: replied 1 year ago.
It still won't start, but seemed easier to pull. I never had a hard time starting before
Expert:  Tan replied 1 year ago.
Ok at this point we need a peak reading volt meter to do some testing. Or the adapter to plug into your volt meter. https://www.google.com/shopping/product/6839087715725588451?rls=aso&client=gmail&q=DVA+adapter&authuser=0&sa=X&ved=0CH0QuSRqFQoTCKr-nrfRyscCFcnVgAodDSUEwA Do you have spark?
Expert:  Tan replied 1 year ago.
Did you have to remove the pins from any of the plugs in the power pack? If yes are you sure the pins are installed all the way in the plugs. If you don't have the pins in al;l the way you won't make contact.
Expert:  Tan replied 1 year ago.
If you install the old pack will the engine run?
Customer: replied 1 year ago.
Ok, so I did some looking and found one of the pins pushed out enough while connecting the plugs that it wasn't working. Fixed it and a different one did the same thing. I made sure all were connected and it fires up now! It will wrap up a lot higher now and will maintain a higher rpm. However, it still dies if the throttle is turned all the way up. What do you think is causing that?
Expert:  Tan replied 1 year ago.
Are you in the water? The engine will not turn rpms up till it blows there is a rev limiter in the pack. If you are revving the engine with no load out of the water you will blow the motor apart. They are not to be run out of the water at high roms. That's like having you car in neutral and pushing the accelerator pedal to the floor. How does it run in the water in gear pushing the boat?
Customer: replied 1 year ago.
I wondered that. I have only used muffs I will see if I can get it on the water today
Expert:  Tan replied 1 year ago.
Ok if it does act up squeeze the primer bulb when it is acting up. If no change pull the choke on momentarily. Do either change the issue?
Customer: replied 1 year ago.
In the lake it runs a little Stronger but still chokes out at full throttle. Squeezing the bulb does nothing and pulling the choke kills it faster. Someone asked me if it isn't venting correctly at the gas tank. Any thoughts?
Expert:  Tan replied 1 year ago.
If the tank vent were the issue the engine would quit running. Squeezing the primer bulb and choking the engine eliminates a fuel issue. Leaving spark as the only thing left. This will require the use of a timing light to see if you are loosing spark on one cylinder. Connect the timing light to the top cylinder run the boat watching the flash of the timing light. The flash should be steady with no dead spots. If no misfire check the other cylinder. If you find a misfire swap the coils does the misfire move? If yes then the coil is bad, if it stays we have another ignition component failing.This will require the use of a DVA adapter for your volt meter or a peak reading volt meter. These are the final tests.

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