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.

quote:

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.

You can also put a timing curve in there similar to factory if running the MSD 6014 box.

Pretty sure I have mine set to like:
0 RPM - 16°
500 RPM - 20°
700 RPM - Whatever total timing you are running.

Used this on a couple engines now, one starts up almost perfect hot or cold, the other doesn't like below like 50-60° temps, but normal summer weather it starts great hot or cold engine temps.

Had it once on gas, PV blockoff was leaking past the gasket.

I replaced both the crank and cam sensor which seems to have maybe helped, but I noticed that after sitting for a while it did the same backfire before finally starting. Coolant temp was 140-ish.

I was thinking about what is different from last year to this year and remembered that we added a vacuum pump and obviously sealed up the breather system. I wonder if alcohol vapor is getting trapped and making it unhappy when trying to start.

Anyone seen anything like this before? I don't have this problem on my BBC with a vacuum pump, just a thought and literally the only difference in the setup from last year.

It’s the 6014 box. It’s not seeing the cam signal. Gm sensors help. As much voltage as you can get to the box while cranking helps. Good grounds help. But I don’t think there’s any foolproof way to get a 6014 to start correctly every time.

quote:

Originally posted by BJs Wild Ride:
It’s the 6014 box. It’s not seeing the cam signal. Gm sensors help. As much voltage as you can get to the box while cranking helps. Good grounds help. But I don’t think there’s any foolproof way to get a 6014 to start correctly every time.

Maybe by changing it to this? LOL

quote:

Originally posted by BJs Wild Ride:
It’s the 6014 box. It’s not seeing the cam signal. Gm sensors help. As much voltage as you can get to the box while cranking helps. Good grounds help. But I don’t think there’s any foolproof way to get a 6014 to start correctly every time.

The ignition box is always suspect. This engine has always been push button starting in the last 2 seasons. Again, the only change is the addition of the vacuum pump and sealing the engine up.

I'll go through whatever testing MSD has available and see if I can find anything. Strange that it's only a "warm" engine thing.

If you are on Facebook there are lots of people in this group, >>Carbureted LS Engines Tech Talk<< that might have already dealt with this same problem. There are always threads on there about 6014 ignition problems.