New 454 carb engine quits at idle in gear

I have a new 454 - 330 install

I have a new 454 - 330 installed in a Donzi 22 classic in July this year. Seems to run fine except at idle (~850 rpm) in gear after about 10 minutes. At that time the engine dies and then periodically thereafter while at idle in gear. Engine always starts right up and idles along fine for ~ 5 to 10 min and then the same drill. If the engine rpm is bumped up to 900 in gear the situation does not occur. The 850 rpm idle situation is the same with engine hatch open or shut. Outside temperature in the range of low 90s. Gas is half ethanol less than 10% of unknown age and half new 87 oct non ethanol. Engine has ~ 2 hours total operation. I am located in Lake county Florida. What are your thoughts please. Dwight

idle seems a bit high.

Look

idle seems a bit high.

Looks at the plugs - see if it is burning rich at idle - could be due to late timing or a bunch of other issues.

Check the neutral saftey switc

Check the neutral saftey switch located at the throttle and gear cables on the motor