Rough Starts

If you run the engine for 30s after a cold start and then stop it, is it still rough if you start it again immediately?

I am not sure how you can tie it down to fuel, ignition or mechanical. You could try spraying some carb cleaner into each carb venturi and see if that results in a smoother start. If it does then you have a fuel problem.

Failing that, you could look at the mounting of the trigger coils and make sure that the gaps are correct. It might also be worth measuring the plug cap resistances, you need to do this in pairs where you will measure 2 in series giving you about 15kohm through the coils.

Could there be a fault with the soft start circuit? You could temporarily disable it, especially since most 912UL engines don't use it.

I had a similar situation that was solved by replacing the float needle. It was not sealing completely and when I would pre-start I ran the electric fuel pump for a minute or so to fill the bowls. The right bowl would over fill because of this leaking needle. The over fill was slow enough that it went into the intake instead of on the ground. The now rich mixture on one side would cause a very rough start for about 30 seconds. The leaky needle would not cause excessive fuel once the engine was running probably because the consumption was more than the leak or the vibration helped keep the needle sealed.

Thank you Walt I will look into your suggestions.

Thank you Kevin.  I will look into that.

Walt wrote:

I had a similar situation that was solved by replacing the float needle. It was not sealing completely and when I would pre-start I ran the electric fuel pump for a minute or so to fill the bowls. The right bowl would over fill because of this leaking needle. The over fill was slow enough that it went into the intake instead of on the ground. The now rich mixture on one side would cause a very rough start for about 30 seconds. The leaky needle would not cause excessive fuel once the engine was running probably because the consumption was more than the leak or the vibration helped keep the needle sealed.

Maybe also check the weight of the floats while you’re in there, one or two just beginning to sink could give a similar result.