[Leighton 1979]

I bought an 89 js550a a while ago and have rebuilt it from scratch. New everything:

Pistons
conrods
crankshaft rebalanced
spark plugs
cylinder head
carb kit
full gasket kit
flywheel
stator plate
starter motor

Have try'd starting but it turns over but will not fire. Have checked he spark plugs and they are sparking, there is fuel getting into the cylinders but can not work out why it want start. Tryed using easy start straight down the carb but still want fire. Can anyone help have run out of ideas.

 

[Moonracer324]

Fuel-Compression-Spark
If you used Ether that would give you fuel (for a second).
Check to make sure you have compression.
Check your spark, you said you have it, but is it at the right time. Check timing and stator coil alignment. Is there a gap spec for your plugs?


3-89 550's

 

[Leighton 1979]

Have checked compression and both cylinders are good 120psi each. swapped the ignition leads around to make sure the timing wasn't 180 degrees out and still would not fire not even a fart. Could it be possible that the spark is not good enough under compresion if so what woul cause it to brake down.

 

[Moonracer324]

It may not be actual break down, but a spark plug requires more voltage to jump a gap under compression than it does under normal air pressure. That's why spark plug gap is critical.
Staight from the CLYMER SHOP MANUAL:

ENGINE CRANKS, WON'T START, BUT FUEL AND SPARK ARE GOOD.

Check:
Defective or dirty spark plug
Spark plug gap set too wide
Incorrect ignition timing
Shorted "kill" or stop switch
Broken piston ring(s)
Cylinder, crankcase or cylinder sealing faulty
Worn or damaged crankcase seal(s)
Damaged CDI unit
Damaged magneto coil(s)
Damaged ignition coil

Good luck, i hope its something simple.
3-89 JS550's

 

[JUST-IN-TIME]

you static timed it?