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Backfire through carb; 1999 Johnson 70hp J70PLSSD

I

Indecentspeeds

New member
Need some assistance on what to try next; hadn’t used the boat in about 6 weeks, it fired up no problem at the ramp but was idling just a touch off, I didn’t think much of it, figured it was just a cold start. Pulled away no problems, ran smoother at 1500 rpm, but it wouldn’t rev past 4000 RPM, felt like a fuel problem, if I bumped the choke while running it woke up and ran normal for a second. With the silencer off I can see a visible back fire through top carb when the throttle blades close. Video included.

Details:
1999 Johnson 70hp
17’ Flats Boat (650lbs loaded with 12 gal fuel)
Boat gets used about once a month
Rec 90 with Stabil additive
Compression checks out 140-142-150
Replaced fuel filter
New spark plugs
Checked timing and it is dead on
Strong spark on each cylinder
Swapped coil one and two the issues did not move
I always purchase OEM parts

Is this a clear sign it’s time for a carb rebuild?
Should a test anything else before I proceed to look for someone to rebuild the carbs? I don’t care to do this myself as I have re-built carbs in the past and can never get them tuned and sync’d correctly.

I live in South Florida but have always done all my own work, I don’t have a 2 stroke mechanic I trust. I have been running this motor for about 10 years with no issues, besides some minor fuel lines, filters, and regular maintenance I haven’t had to do anything crazy to this engine.

If the issue is the carbs is there anything else I should do for added performance, security, etc….? I didnt want to switch to a 4 stroke until this motor becomes a reliability issue, I’m hoping this isnt the start!

Looking for insight and feedback on troubleshooting and any local recommendations for a good, honest mechanic with 2 stroke experience.

Video of issue:

Thank you in advance!
 
Make sure that top carburetor is CLOSED at idle.-----And carburetor cleaning is easy on these.----Also install a new water pump impeller every 5 years.
 
Pulled and soaked the carbs, bowls were pretty clean, tiny bit of residue, I always run REC90 fuel so I wasn’t expecting much. I ran the engine today on a 6 gallon portable with brand new fuel and it ran the same. While I had the carbs off I inspected the reeds and they look perfect, nothing visible.

When running the motor at constant rpm; 2000 rpm, you can feel it mis here and there slightly.
I am running the motor with the lower unit submerged as out of the water it’s hard to detect anything abnormal.

At this point I feel I’ve exhausted my ability to diagnose without anymore feedback.

If anyone knows an old school outboard mechanic in South Florida please share.
 
flyingscott said:
In gear or neutral?
Click to expand...
Both, running the engine in the water. (I have a 150 gallon tank). I found a CDI trouble shooting manual. I started trying to run through testing some of the electronics.

Tested the Coils- all good

Powerpack test good on resistance

Tried testing the stator and timing base.
All checked out on resistance except when testing the white wire against black/white: spec says 400-500ohms but it tests as an open circuit. If any one knows if this is accurate (spec)

Haven’t tested voltage yet

Let me know if this is helpful.
 
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