I wasn't there but Bob the mechanic said the engine started and ran well for a few minutes, then made a "ppttt" sound and died. I went over and we tried a few more times. It would try to start, popping through the carbs and dying. I richened the mix a quarter turn but it didn't didn't seem to help.  Timing is 30 at 3000 rpm. Any ideas?

Engine builder is very well respected. Bob knows his stuff and can probably sort it out.

If he builds engines he will surely figure it out . That could be anything from a large vacuum leak to a wet dizzy cap

 Will it run on starting fluid? My first guess is lack of fuel in the float bowls.

Engine is fine. Check the fuel supply.

Will check the floats.

There was an issue, but Bob called the builder and he sorted  it out. Engine is sounding rather vicious, especially with no muffler and tailpipe, just the headers.