I inspected piston and ring,replaced fuel with 89 octane 25:1 mixture ,tested ignition system per zenoah manual,adjusted spark plug gap.Still wont idle when low speed screw
is set to factory spec.Thinking about buying a new carb...

What color is the spark? Should be blue and "snappy"
Inspect the adjustment screw and seat. If they were over tightened at some point, it can put a step in the screw or seat, then you will never be able to get it adjusted right until they are replaced. Will it idle if you back the screw out giving it more fuel or turning it in for less fuel. Adjustments should be small and incremental. No more than an 1/8 of a turn at a time. Do you have good compression?

spark is blue.I tried opening the gap too far on an old plug and the spark still jumps the gap.I inspected the needle with a magnifier -it looks undamaged.
it idles when turning screw in.

To clarify you are not using a clutch and are trying to idle a direct drive setup?
One- does it idle out of water?
Two- does it idle in the water?
Three- if idles out of water and dies in the water is your idle to slow under load?
Would not try to idle a direct drive setup.Would recommend a clutch setup. IMO.

Hello.I have a direct drive. It doesnt idle correctly in or out of the water(See original post).Using a wt644 carb it wont idle unless I run it lean 3/4 out on low.With a Wt257 it wont idle unless I run it lean 1.5 turns out. Raising the idle rpm doesnt seem to help.

Hi
Could you upload a photo of your engine & engine bay? It might show something not apparent from words alone?

Just a couple of other thoughts, on the fuel tank and supply tube. I am throwing this into the pot because the only time I ever had running issues with a Zen was with my first gas boat a ready to run one. Two things caused bad running:

1 The fuel tank pick up tube had a clunk filter (filled with like a cotton material). Though not at all obvious when looking at the filter it had in fact become almost blocked solid. Never again will I use such a filter, far better to have one in the external tube that has a fine cleanable filter that is easy to get to.

2 The fuel tank filler cap had a tiny vent in it. It too became blocked after a few runs causing fuel starvation. The cap was badly designed.

Personally I doubt that either of these two points are causing your problem however I thought it worth mentioning them.

Oh and so obvious to say this, but if you have a choke do make sure it is off when starting the engine (forgive me for such an obvious point).

Cheers
Craig

If it's a different carb, the base settings would be different. The carb will need to be adjusted with the engine under load. Get it to where it will stay running. Blip the throttle and see if there is any hesitation in the engine winding up. Keep richening until a slight hesitation is noticed then tighten a 1/16 - 1/8 of a turn. For the high speed, keep going rich till you hear the engine "cycling" then back off a 1/16 - 1/8 of a turn. By "cycling" I mean when you hear the engine's tone going up and down. Get it set to where it runs properly, and keep an eye on the spark plug. The electrode should be cardboard color. Keep in mind that with a 2 stroke, it's always better to run it on the rich side.

Here is a pic with 257 carb

Do you suppose you could just have the throttle trim on your transmitter set too low.

I tried taking idle screw out and using the trim.Still reacts the same.I'm going to replace all the fuel lines next.