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At wits end on 70ESL75E

R

ronward

Member
I have a 1975 70 hp Johnson. Yesterday it ran, but only on 2 cylinders. Cylinder drop test shows number 1 cylinder dead. Swapped coils, same result. Replaced all 3 coils, power pack, and rectifier. Have 7/16 spark on all 3 but no start, just shortly after it ran for cylinder drop test. Removed and cleaned all 3 carbs and still nothing. Pumping fuel bulb makes no difference. Sprayed pre-mix directly into cylinders and still nothing. Have verified wiring several times to be correct. Plugs are wet with fuel. Plugs are new Champion L77jc4 gapped at .030. Nothing else has been changed. Flywheel key has been checked and is not sheared. Any suggestions would be greatly appreciated.
 
motor must spin 250/300 RPMS to produce strong spark; could be weak starter or weak battery; have battery load tested. Others will have more thoughts. Good luck
 
Battery has fresh charge and is spinning motor at fast pace. Spark is not an issue, I have strong blue 7/16 open air gap spark on all 3.
 
I see no mention of compression. What are the compression readings of all three cylinders.

If you have normal compression and spark, spraying premix into that top (#1) carburetor should have that cylinder firing.

An exception would be a slight imperfection of the head or exhaust baffle gasket area that might allow a fine spray of water enter that cylinder.
 
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Or possibly low crankcase compression, not allowing the fuel charge to enter the cylinder properly.
Leaking reeds, blown crank top seal, crankcase half seals, things like that.
 
when you do the spark test, make sure the plugs are installed, as that changes your cranking speed. My early days in the shop we had some problems with reeds on the early 70hp motors
 
Sorry for delay, work has been hectic and I haven't had time to mess with the motor. Compression is 120 on all three, a bit low but it works. Thanks for all replies but motor is now running. I decided to clean carbs again and bought 3 more new plugs. I can't explain it but it fires right up now with the slightest turn of the key. Thanks again.
 
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