91 Mercury 100 won't run past about 1800 rpm

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bikerider

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The motor is a 91 Mercury 100hp 2 stroke outboard. I took the boat out this morning and it was running well. It would start and go through the whole rpm range no problem. I brought it to idle for a minute then when I throttled up it would get to about 1800 RPM or so and stumble, run rough and sputter. I could not get the boat to plane. I changed the spark plugs on it (which didn't make a difference), checked all wire connections from the throttle control box back to the motor, looked in the fuel tank which looked very clean, and checked the coils and wires. The motor was not running excessively hot and the water pump was functioning perfectly. I can get the boat to start up easily and idle no problem. Any idea what could be wrong?
 
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Chris1956

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Did you try pumping the fuel line primer bulb as you accelerate? That will remove fuel starvation (to the carbs) from the causes list. If that fixes it, suspect the fuel pump or fuel filters.

Mercury Carbs transition from the idle jet to the main jet at about 1200RPM, however, this can vary from motor to motor. You might check the main jets for dirt.

Alternatively, Mercury's have a low speed stator winding and a high speed stator winding. Maybe your high speed winding failed.
 
CaptnKingfisher

CaptnKingfisher

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bikerider said:
The motor is a 91 Mercury 100hp 2 stroke outboard. I took the boat out this morning and it was running well. It would start and go through the whole rpm range no problem. I brought it to idle for a minute then when I throttled up it would get to about 1800 RPM or so and stumble, run rough and sputter. I could not get the boat to plane. I changed the spark plugs on it (which didn't make a difference), checked all wire connections from the throttle control box back to the motor, looked in the fuel tank which looked very clean, and checked the coils and wires. The motor was not running excessively hot and the water pump was functioning perfectly. I can get the boat to start up easily and idle no problem. Any idea what could be wrong?
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Sounds like your not running on all your cylinders. If you have an open air spark plug tester set it for 7/16" verify spark on all cylinders. Otherwise search YouTube for cylinder drop test. Either way let us know the results then we can help you start to isolate which ignition component has failed
 
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bikerider

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Chris1956 said:
Did you try pumping the fuel line primer bulb as you accelerate? That will remove fuel starvation (to the carbs) from the causes list. If that fixes it, suspect the fuel pump or fuel filters.

Mercury Carbs transition from the idle jet to the main jet at about 1200RPM, however, this can vary from motor to motor. You might check the main jets for dirt.

Alternatively, Mercury's have a low speed stator winding and a high speed stator winding. Maybe your high speed winding failed.
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I did not try pumping the primer bulb but I will when I can get the boat in the water again to rule out a fuel issue. It almost felt like the motor was getting too much fuel and wasn't able to burn it all. Is there a way to test each stator?
 
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bikerider

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CaptnKingfisher said:
Sounds like your not running on all your cylinders. If you have an open air spark plug tester set it for 7/16" verify spark on all cylinders. Otherwise search YouTube for cylinder drop test. Either way let us know the results then we can help you start to isolate which ignition component has failed
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That's what it felt like too. Is this what I would use?
 
flyingscott

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You need to be looking at the bottom 2 cylinders. Yours is a 2+2 motor it runs on the top 2 cylinders until 1800 rpm then the bottom 2 kick in. All 4 cylinders should have spark all the time. The 2+2 function is controlled by the carbs. The bottom 2 carbs only have a tiny idle circuit to provide enough fuel mix to lube but not run. Get a factory manual for your motor as they can be tricky to service. Also only buy the open air spark tester nothing with a light in it.
 
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bikerider

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flyingscott said:
You need to be looking at the bottom 2 cylinders. Yours is a 2+2 motor it runs on the top 2 cylinders until 1800 rpm then the bottom 2 kick in. All 4 cylinders should have spark all the time. The 2+2 function is controlled by the carbs. The bottom 2 carbs only have a tiny idle circuit to provide enough fuel mix to lube but not run. Get a factory manual for your motor as they can be tricky to service. Also only buy the open air spark tester nothing with a light in it.
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I will pick up the correct spark tester and test all cylinders on the motor and report my findings here.
 
flyingscott

flyingscott

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Also look at the carb linkages and make sure all 4 carbs are opening. Also maje sure the carbs are on the correct cylinders.
 
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bikerider

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flyingscott said:
Also look at the carb linkages and make sure all 4 carbs are opening. Also maje sure the carbs are on the correct cylinders.
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I will take a look and make sure that they are all opening. Could the carburetors really be on the wrong cylinders if the motor has run well for several years up until this point?
 
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bikerider

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Can someone clarify if any of these spark plug testers match what was mentioned earlier in this thread? I am going to start working on diagnosing the problems with my boat motor soon. Thanks.



www.wholesalemarine.com

CDI 511-9766T Spark Tester Sealed Threaded

CDI 511-9766T Spark Tester Sealed Threaded available at Wholesale Marine. We offer CDI Electronics products at lowest prices with superior service.
www.wholesalemarine.com www.wholesalemarine.com
 
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bikerider

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Checking over this thread again I realized I forgot to add information about running the boat from the dock it was at to the trailer. Maybe it will change what approach I need to diagnose it, maybe not, but it can't hurt to post it. I drove the boat from the dock it was at to the ramp to trailer it back on August 8th. Up until the 8th I could get it to fire up no problem. On the morning of the 8th it took high idle to keep the motor running in neutral and when I put it in forward and gave it throttle it sputtered and backfired slightly. I could get it up to about 1800rpm but no more than that. It would run that way for about 15-20 seconds then act as if it was out of fuel. I had to put it in neutral, put the high idle on and go back to the primer ball and pump it, then put it in gear again and go for another 15-20 seconds. I had to repeat this process all the way to the ramp. The boat sat for a few days in between the time it would fire up no problem and need the high idle on to staying running in neutral. It wasn't worked on or looked at between those days.

I got the correct style spark tester as suggested above and will report my findings.
 
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Dave1027

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If you had to keep pumping the primer bulb to keep it going that probably means the fuel pump is not working. It's an easy fix and as old as the motor is, if the fuel pump has never been rebuilt it's due any way. It's just new gaskets and new small diaphragms.
 
Faztbullet

Faztbullet

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If its got a grey fuel line the liner may have come aprart internally also
 
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bikerider

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I tested the spark on all four cylinders just turning the motor over and got spark on the top cylinder and second from the top cylinder. The bottom two cylinders have no spark. Is that because the bottom two cylinders do not kick in until 1800 rpm or does this indicate a problem?
 
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bikerider

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Dave1027 said:
If you had to keep pumping the primer bulb to keep it going that probably means the fuel pump is not working. It's an easy fix and as old as the motor is, if the fuel pump has never been rebuilt it's due any way. It's just new gaskets and new small diaphragms.
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I just tested the spark. Testing the fuel pump will be next.
 
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