787 has spark but won’t fire

[Davet6740]

just installed 2 MSD 4255 ignition modules due to a MPEM failure. During my troubleshooting I also replaced the pickup and rebuilt carbs (while waiting for parts). After wrapping everything up I gave her a shot...
She fired right up, for a minute, then stopped abruptly while making an unpleasant sound. The motor still turns over fine and I still have good spark at the plugs, but it won’t fire?

Any ideas? Possibly some sort of mechanical failure internally?

Any help would be greatly appreciated

 

[Dr Honda]

See if the rotary valve is spinning.

 

[Davet6740]

good news... bad news
It is spinning.... still haven’t come up with anything.

I tried a quick shot of starting fluid, still doesn’t fire?

 

[Davet6740]

Going to check compression... how low can it get before it won’t start? Any idea

 

[Davet6740]

Also wondering if maybe my carbs aren’t set correctly. Would that prevent starting?

 

[Davet6740]

Update...
Got one cylinder under 30psi
Time for a rebuild

 

[Dr Honda]

Well... looks like you answered your own question. A fresh build on an 800 with be 150 psi. Generally they run down to 130 psi. But once they get that low, they quickly eat themselves since hot combustion gasses are getting past the rings, and are burning/displacing the oil that keeps them safe.