You never told us what type of gas it's on. But here are the common problems.
Dirty or blocked flue/firetubes. Check down through the chimney with a flashlight for dropped material or carbon build-up. Also pull out the burners and look at the diffusers inside the fire tubes. If the gas pressure has changed, the mixture off, or never been properly calibrated you may have a carbon build-up. Check the edges around the front of the fire tubes to make sure they are not leaking from a crack in the tub. Lastly, make sure that the burner area has all the sides and top properly installed and the burners are installed correctly. They have to line up with the center of the fire tubes.
I suspect that you may have had a point where the mixture was off and made carbon/soot. That would build up on the diffusers inside the fire tubes. You can clean it, but you need to recalibrate the carburetors for proper flame. That means you should check the supply and burner pressure with a manometer.
Your model is the same as a SM-40 Frymaster without the frills. Manual here:FM-ESG35T_spm.pdf (static-pt.com)