Hyundal Accent explosion

Hello friends, I have a 1998 Hyundai Accent. I had a natural gas system installed, but when I start it and it needs power, it backfires towards the valve cover and intake manifold. I've already changed the spark plugs and high-tension leads. Could someone tell me what this could be due to? Just to clarify, the ignition advance mechanism wasn't installed on this system. Thanks a million

That's a lack of fuel; return to the workshop to have the LPG system recalibrated

They took me a similar problem but from an isuzu trooper and the problem was the EGR but it is best to see you who installs it because it must know why it does that.