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not running all 4 but no dead cylinders

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Chaserman3

New member
1985 aeto v4 yamaha 115hp, hard to start and backfires when i rev a little bit.
I unplug bottom plug wires when running and it doesnt change anything, top wires kill it.
compression is even (100-105) and bores look the same in all 4 (worn but nothing too bad) with endoscope.
swapped top and bottom coils, no change
carbs went through. when i ran it i disconnected the linkage so i can throttle individual carbs so i know its not a problem with the carbs
inspected reed valves and they look fine.
Bottom cylinders are not dead, I started the engine with the top plug wires off gave it throttle and it started on just bottom cylinders, retest starting all 4, unplug wires and nothing...
im ordering a spark gap tester to see if it might be bad quality spark
 
I think you need to confirm if the bottom plugs are sparking or not.

Very strange what you are reporting.
 
Are the bottom plugs wet or dry? Could be leaking bottom crankshaft seal or sealing rings between crankshaft throws. Fuel charge loss through the bottom seal ( not enough fuel to fire the bottom cylinders) or disrupted crankcase pressures from worn sealing rings. As Boscoe stated, validate good hot spark for all four cylinders
 
fitz73222 said:
Are the bottom plugs wet or dry? Could be leaking bottom crankshaft seal or sealing rings between crankshaft throws. Fuel charge loss through the bottom seal ( not enough fuel to fire the bottom cylinders) or disrupted crankcase pressures from worn sealing rings. As Boscoe stated, validate good hot spark for all four cylinders
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yea it ran on just the bottom cylinders but i had to give a bunch of throttle, plus i think i remember zapping myself on the bottom plugs. and yes there was oil coming from the plugs so there was fuel mix there.
 
Start motor.----Let it idle.-----Remove 1 plug wire at a time.----Report EXACTLY what you observe.---Checked the flywheel key?----How did it run for you last season?
 
racerone said:
Start motor.----Let it idle.-----Remove 1 plug wire at a time.----Report EXACTLY what you observe.---Checked the flywheel key?----How did it run for you last season?
Click to expand...
i bought the engine as is to tinker with and the previous owner hadn't ran the engine in prob over 5 years, not sure when ill have time to mess with it though because im going on vacation soon
 
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