1973 Johnson 9.5hp Motor Troubleshooting

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xcelegacy

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Hello all. I have a 1973 Johnson 9.5hp motor that I have been using for some time. The motor idles fine and starts easily however only gets about half power, and only reaches about 10kmh in a 14ft john. I have figured out that the motor has been running on only the top cylinder, however at higher rpms the bottom cylinder begins to work in short stutters.
I have replaced the ignition coils and aligned them and checked the point gaps and condensers and all seems to be working fine. At first the new coil for the bottom cylinder rubbed the flywheel how ever I adjusted it and it no longer does. The pressure for the bottom cylinder is about 65psi and the top cylinder is about the same. The carburetor has been cleaned out well. Before I open the engine itself, is there anything else that could possibly cause the bottom cylinder to function like this?
 
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Nordin

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Check the ignition system once more, it sound like an ignition issue.
Check for spark with a spark tester or use a steel rod to connect in the HT wire rubber boot and hold it about 5/16 or more from good ground.
Pull the recoil starter and you should see a spark jumping that gap.
Replace points if you can not clean them to a good no pitted surface.
replace condensers cause they can dry out by time and they are hard to test without special instruments. A good micro Farad value by testing with a multimeter with capacitor is not confident.
BTW 65 PSI in compression is low but as they are pretty equal it could be the compression gauge.
A 9,5Hp should at least have 90-100 PSI in compression.
 
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xcelegacy

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The spark plug wires were replaced. However I believe that the spark plug threads are worn down. Could this cause any issues with the bottom cylinder not lighting?
 
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racerone

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No it can not be the issue.----Does spark jump a gap of 1/4" or more on both leads , yes or no ?
 
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Crosbyman

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check resistance on the lower plug boot to engine frame ground while wiggling twisting the plug wire and turning the mag plate with the throttle resistance should be steady 4-8 kohms if properly connected the the coil HV pin.
 
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xcelegacy

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The resistance is steady 9.3 kohms on the top, while the bottom one is not steady and jumps all over the place when moving the mag plate and twisting the plug wire. Have not tested the gap yet. What does this mean?
 
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xcelegacy

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xcelegacy said:
The resistance is steady 9.3 kohms on the top, while the bottom one is not steady and jumps all over the place when moving the mag plate and twisting the plug wire. Have not tested the gap yet. What does this mean?
Click to expand...
Bottom will not jump 1/4" top will jump it easily.
 
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racerone

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The 1/4" or more gap jump puts the magneto and components to a full test.-----Most are unaware that it takes more voltage to fire a plug when air is compressed to 100 PSI----So a plug that sparks touching the block will not fire in the cylinder.----That makes you think you have spark and keep pulling the recoil till it breaks or your arms fall off.-----If there is a crack in the plug wire , spark may jump to ground before jumping the gap in the cylinder.
 
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racerone

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Remove bottom coil , it is possible that insulation failed.----Look for " burn through " between coil and magneto plate.----Voltage would rather take the easy path to ground rather than jump gap at the plug.-----Do not have enough fingers and toes to count how many times I have seen that !
 
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xcelegacy

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racerone said:
Remove bottom coil , it is possible that insulation failed.----Look for " burn through " between coil and magneto plate.----Voltage would rather take the easy path to ground rather than jump gap at the plug.-----Do not have enough fingers and toes to count how many times I have seen that !
Click to expand...
Thanks for the help. The wires on the coil were resistive wires before but I replaced them recently with copper core. I will recheck the wire for any cracks, and anywhere it may jump to ground.
 
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xcelegacy

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Crosbyman said:
check resistance on the lower plug boot to engine frame ground while wiggling twisting the plug wire and turning the mag plate with the throttle resistance should be steady 4-8 kohms if properly connected the the coil HV pin.
Click to expand...
Bottom is not steady.
 
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Nordin

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From earlier in the thread you mention you have change both ignition coils.
That´s right?
From my experience ignition coils for OMC engines from 50-70.es suffers from dry out and cracks. Other brands like Merc and Chrysler did not have that problem. I have not have these issue with these brands but just OMC.
Mercs from early 70.es with Thunderbolt ignition have had these issue (7,5, 9,8 and 20Hp).
The insulation between prime and secondary side of the coil fails and the spark will jump internally in the coil because of bad insulation.
As the resistance for the bottom is not steady you have an issue with the HT wire for that cylinder.
 
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xcelegacy

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Nordin said:
From earlier in the thread you mention you have change both ignition coils.
That´s right?
From my experience ignition coils for OMC engines from 50-70.es suffers from dry out and cracks. Other brands like Merc and Chrysler did not have that problem. I have not have these issue with these brands but just OMC.
Mercs from early 70.es with Thunderbolt ignition have had these issue (7,5, 9,8 and 20Hp).
The insulation between prime and secondary side of the coil fails and the spark will jump internally in the coil because of bad insulation.
As the resistance for the bottom is not steady you have an issue with the HT wire for that cylinder.
Click to expand...
I replaced the coils last week, does this mean they are faulty or can this result from the condensers? I cleaned the point gaps which did not fix the issue.
 
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xcelegacy

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Here is an update on my situation. The coils are replaced, both points are cleaned and have no pitts in them. They are also properly gapped. However when I switched the condensers, the motor does not start. This means that one condenser doesn't work. But I am confused as to why it doesn't start at all now. If one condenser worked than it should start no?
 
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Crosbyman

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condensers being what they are can be bad ones off the shelves where they may have sat several years . or simply failed and not checked proerly at the manuf.

the only way to be certain is to test for internal leakage with a high voltage test using old but good test eqpt. or build your own to generate over 250 volts DC and check for leakage.( a simple idiot light design will tell you if good or bad .

the alternative is to buy several and test them in real life. $$$

don't give up.. your motor is a 9.5 and it's good... cyl with it's low 65PSI needs all the help it can get . so fix the bad one .

here are a few articles on capacitors, testing, repairing and building a test box (like mine) if your contact the author. mr. Mohat

his test box #1 is easy to build, works great and if you plan working on oldies as a hobby you need one :)

send me a pm with your e-mail



 
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xcelegacy

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Crosbyman said:
condensers being what they are can be bad ones off the shelves where they may have sat several years . or simply failed and not checked proerly at the manuf.

the only way to be certain is to test for internal leakage with a high voltage test using old but good test eqpt. or build your own to generate over 250 volts DC and check for leakage.( a simple idiot light design will tell you if good or bad .

the alternative is to buy several and test them in real life. $$$

don't give up.. your motor is a 9.5 and it's good... cyl with it's low 65PSI needs all the help it can get . so fix the bad one .

here are a few articles on capacitors, testing, repairing and building a test box (like mine) if your contact the author. mr. Mohat

his test box #1 is easy to build, works great and if you plan working on oldies as a hobby you need one :)

send me a pm with your e-mail



Click to expand...
Thanks for the help, I really appreciate it. I think I will just replace the condenser and maybe even the points as well. I will update once they come in. Thanks guys 👍
 
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