Topic: 1956 Johnson RDE 18 30HP w/intermittent misfire on one cylinder

This topic has 9 replies, 6 voices, and was last updated 1 month ago by Don.

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May 7, 2026 at 10:51 am #317821
Jesse Stevens
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Last weekend we used a most unusual spring like day in North Florida to take the boat out on Rainbow River. When I started the motor I immediately noticed it sounded like it was running on one cylinder. I let it idle cold for a few minutes and then I could hear the second cylinder start to fire. I thought “well that’s strange but let’s give this cruise a shot”. The entire river is a no wake zone so idle speed the entire 5.5 miles with a max speed never exceeding 4.5 MPH. The motor seemed ok but after about 20 mins of run time the motor began to intermittently switch from running on both cylinders to one and back again. I adjusted the mixture screws until she seemed to smooth out, but alas… that was not the case. After about two hours of this back and forth misfiring we turned around and headed back to the launch site, putting along on one cylinder for about an hour. Any ideas on where to start?

Full disclosure: About a month ago I replaced the coils, plugs (J6C plugs gapped at 30), plug wires, cleaned & filed the points and set timing with a multimeter. I also took the runaway cutout switch apart to see the condition of the diaphragm and it looked great to me. The motor has also been converted to a single line fuel delivery system and new pump, using Sierra part # 18-7352. Motor runs great at WOT and shows 120psi on both cylinders.

Jesse L. Stevens III
May 7, 2026 at 3:14 pm #317832
JACQUES
Participant
tried cleaning the bad set of point again… just in case

could be condensers…..did you replace them ??? if coils are new and points clean/adjusted condensers are next on the suspect list

just as a test remeasure ohms from boot clip of bad cyl to engine frame ..confirm you have 4-8 koms…pull wiggle the HV wire while you test it

compare results with the good cyl… again testing from boot clip to engine frame.

download the free service bible if you don’t have it yet … print/store locally 🙂

https://www.socalaomci.com/library/johnson_service_manual_10th_edition.pdf

Joining AOMCI has priviledges 🙂
May 7, 2026 at 6:58 pm #317855
Don
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Jesse Stevens wrote:

Last weekend we used a most unusual spring like day in North Florida to take the boat out on Rainbow River. When I started the motor I immediately noticed it sounded like it was running on one cylinder. I let it idle cold for a few minutes and then I could hear the second cylinder start to fire. I thought “well that’s strange but let’s give this cruise a shot”. The entire river is a no wake zone so idle speed the entire 5.5 miles with a max speed never exceeding 4.5 MPH. The motor seemed ok but after about 20 mins of run time the motor began to intermittently switch from running on both cylinders to one and back again. I adjusted the mixture screws until she seemed to smooth out, but alas… that was not the case. After about two hours of this back and forth misfiring we turned around and headed back to the launch site, putting along on one cylinder for about an hour. Any ideas on where to start?

Full disclosure: About a month ago I replaced the coils, plugs (J6C plugs gapped at 30), plug wires, cleaned & filed the points and set timing with a multimeter. I also took the runaway cutout switch apart to see the condition of the diaphragm and it looked great to me. The motor has also been converted to a single line fuel delivery system and new pump, using Sierra part # 18-7352. Motor runs great at WOT and shows 120psi on both cylinders.

Well, could be just about anything, including a funky spark plug. You did the ignition over, did you get all the leads routed correctly so they won’t get chafed by the flywheel? You might want to try disconnecting the ignition from the vacuum switch just in case something is wrong there.

Next, I would probably use a timing light to monitor each cylinder one at a time to see which cylinder is misfiring.
May 7, 2026 at 9:52 pm #317866
labrador-guy
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Look those plug wires over good. The plug wires get stiff and they will crack.

dale
May 7, 2026 at 11:55 pm #317873
JACQUES
Participant
if coils primary wires connected to the points are not tucked under the coil properly …they eventually rub on the crankshaft and get grounded… killing the ignition on the coil .

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May 8, 2026 at 7:59 am #317909
David Bartlett
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US Member
The cut-out switch only affects one cylinder, so that could be your issue. As Don said, maybe unhook that and see if it helps.

David Bartlett
Pine Tree Boating Club Chapter

"I don't fully understand everything I know!"
May 8, 2026 at 1:01 pm #317943
Johnson RD
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That’s a lot of idling for an old 2 stroke. Needs to be opened up to clear it out.
May 8, 2026 at 2:19 pm #317951
JACQUES
Participant
especially… with fuel oil puddling inside if the drain holes got plugged up by the mod to pump… purge system may also be blocked .

Joining AOMCI has priviledges 🙂
May 22, 2026 at 8:52 am #319578
Jesse Stevens
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US Member
Don wrote:

Jesse Stevens wrote:

Last weekend we used a most unusual spring like day in North Florida to take the boat out on Rainbow River. When I started the motor I immediately noticed it sounded like it was running on one cylinder. I let it idle cold for a few minutes and then I could hear the second cylinder start to fire. I thought “well that’s strange but let’s give this cruise a shot”. The entire river is a no wake zone so idle speed the entire 5.5 miles with a max speed never exceeding 4.5 MPH. The motor seemed ok but after about 20 mins of run time the motor began to intermittently switch from running on both cylinders to one and back again. I adjusted the mixture screws until she seemed to smooth out, but alas… that was not the case. After about two hours of this back and forth misfiring we turned around and headed back to the launch site, putting along on one cylinder for about an hour. Any ideas on where to start?

Full disclosure: About a month ago I replaced the coils, plugs (J6C plugs gapped at 30), plug wires, cleaned & filed the points and set timing with a multimeter. I also took the runaway cutout switch apart to see the condition of the diaphragm and it looked great to me. The motor has also been converted to a single line fuel delivery system and new pump, using Sierra part # 18-7352. Motor runs great at WOT and shows 120psi on both cylinders.

Well, could be just about anything, including a funky spark plug. You did the ignition over, did you get all the leads routed correctly so they won’t get chafed by the flywheel? You might want to try disconnecting the ignition from the vacuum switch just in case something is wrong there.

Next, I would probably use a timing light to monitor each cylinder one at a time to see which cylinder is misfiring.

Yes the leads were all tucked away with no harm done. Where I went wrong with my ignition rebuild was pure speculation on my part. Meaning the points and condensers looked ok so I left them alone, only replacing the coils. I’ve replaced everything now and she seems to be running great.

Jesse L. Stevens III

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David Bartlett
May 23, 2026 at 7:41 am #319688
Don
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US Member
Jesse Stevens wrote:

Don wrote:

Jesse Stevens wrote:

Last weekend we used a most unusual spring like day in North Florida to take the boat out on Rainbow River. When I started the motor I immediately noticed it sounded like it was running on one cylinder. I let it idle cold for a few minutes and then I could hear the second cylinder start to fire. I thought “well that’s strange but let’s give this cruise a shot”. The entire river is a no wake zone so idle speed the entire 5.5 miles with a max speed never exceeding 4.5 MPH. The motor seemed ok but after about 20 mins of run time the motor began to intermittently switch from running on both cylinders to one and back again. I adjusted the mixture screws until she seemed to smooth out, but alas… that was not the case. After about two hours of this back and forth misfiring we turned around and headed back to the launch site, putting along on one cylinder for about an hour. Any ideas on where to start?

Full disclosure: About a month ago I replaced the coils, plugs (J6C plugs gapped at 30), plug wires, cleaned & filed the points and set timing with a multimeter. I also took the runaway cutout switch apart to see the condition of the diaphragm and it looked great to me. The motor has also been converted to a single line fuel delivery system and new pump, using Sierra part # 18-7352. Motor runs great at WOT and shows 120psi on both cylinders.

Well, could be just about anything, including a funky spark plug. You did the ignition over, did you get all the leads routed correctly so they won’t get chafed by the flywheel? You might want to try disconnecting the ignition from the vacuum switch just in case something is wrong there.

Next, I would probably use a timing light to monitor each cylinder one at a time to see which cylinder is misfiring.

Yes the leads were all tucked away with no harm done. Where I went wrong with my ignition rebuild was pure speculation on my part. Meaning the points and condensers looked ok so I left them alone, only replacing the coils. I’ve replaced everything now and she seems to be running great.

I’m glad it worked out OK. Keep in mind that extended idling like this is severe service for most any two stroke. You really need to clean them out every hour or so with a good high speed run. It sounds like you don’t have any choice due to no wake zones, so I would carry extra plugs…
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