My kid's car has a 6.0L LS engine with a 950 carb on alcohol. This year out of nowhere it's been really hard to start when it's warm. backfiring through the carb or shooting flames out the headers. Runs fine once it finally starts. Starts great when it's cold. We've been in the ignition box to make sure there isn't anything strange going on there, which there isn't. This is coil near plug ignition and there is no damage to spark plug wires and the plugs themselves are new.

Initially I thought it needed a little more fuel to start but that hasn't been helping. he's tried holding the throttle open some when cranking with mixed results.

I've had the carb apart and made sure floats are set right and no leaks anywhere.

What am I missing?

If you are using the crankshaft position sensor and camshaft position sensor as part of the ignition box controls, one of those could be at fault, or the connectors. What you are describing sounds like ignition firing at the incorrect time.

Is the starter spinning the engine the same RPM hot as cold.

How old are the cam and crank sensors. I know there are some bad quality LS cam and crank sensor. Make sure you are using high quality from GM dealer.

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Originally posted by 1leg:
Is the starter spinning the engine the same RPM hot as cold.

How old are the cam and crank sensors. I know there are some bad quality LS cam and crank sensor. Make sure you are using high quality from GM dealer.

Cranking rpm is the same. Sensors are old but OEM GM. Might be worth replacing to your point.

Check your voltage while cranking. Don't forget your grounds and connections.
A weak battery might produce plenty of amperage to crank but at reduced voltage. Anything less than 9.5 volts while cranking will be a failure.
A good battery should maintain 10.2 volts or more.

Do your reseach on the crank and cam sensors. Back when I ran the LS the quality was a big deal I ran parts directly from GM dealer.

Also make sure you have ground straps on each head to the chassis. and that the coil packs are ground to the heads. For some reason I remember that being important.

My guess you have bad sensor or dirty ground.