Got Spark, Got Compression, Got Fuel! No go!

[Berlin]

I admit defeat! I've got a little Yamaha Bwizz 50 two stroke that is just steadfastly refusing to start.

We've got a good spark (with jumpers, the battery is knackered)
We've got plenty of fuel.
We've got plenty of compression (new head, cylinder and piston, correctly installed, no snapped rings etc).

Its had a new reg/rec

It turns over on the starter and kick start but just won't fire. We have the brake switch triggered so that it will start.

Any ideas?
Cheers, C

[merlin427]

I admit defeat! I've got a little Yamaha Bwizz 50 two stroke that is just steadfastly refusing to start.

We've got a good spark (with jumpers, the battery is knackered)
We've got plenty of fuel.
We've got plenty of compression (new head, cylinder and piston, correctly installed, no snapped rings etc).

Its had a new reg/rec

It turns over on the starter and kick start but just won't fire. We have the brake switch triggered so that it will start.

Any ideas?
Cheers, C

Is the 'good' spark being monitored with the plug you are using to run the bike not a spare (saving the bother of removing the plug) and is it 'really' good? Having a spark on a plug when it is out in the fresh air is not the same as having one when it is under compression and covered in petrol and oil. Is the spark appearing at the correct time?

You may have plenty of fuel in the tank but is this getting into the cylinder?

Apart from having new head, cylinder, piston and rings what evidence do you have to indicate plenty of compression?

First thing I'd try is a new plug.

[Dave20046]

plug, fuel & blocked carbs are likely candidates. bit of easy start might help.

[Speedy]

Is it the RIGHT plug?
Is the plug being "wet"?

[Berlin]

Plug is wet (indicating fuel getting to cylinder) but not overly wet. lets call it "Moist" :) The plug is brand spanking and correctly gapped.

No idea if spark is timed right but as it's CDI and it was running fine before top end rebuild, then I'd presume it is.

Carb freshly cleaned out and all jets checked. New gasket in top of carb. no air leak between carb and engine.

Compression tested by hand cranking the kick start. plenty there compared to other two strokes I've tuned. Pilot air screw set to correct turns out. Cholk not sticking.

Its not even trying, that's the perplexing thing. I'd expect it to at least have had a gowhen being cranked. I even tried heating the plug up in the oven, turning the engine over with the plug slackened and changing the fuel to fresh stuff.

C