From the teacher's perspective
Yeah, $0.02 here but something to consider.
Great idea for getting bonding and learning for both of you.
Depending on the judges and the level of the science fair, I'll say up front that unless a hypothesis is being "tested", judges won't be impressed regardless of how well done the project or how much he knows about the function. BTDT
By testing, I mean how has a question about carburetors been thought thorugh and a variable introduced that can be analyzed, documented and reported. I took a Sr to MN State Sci Fair one year with an engine project, mostly related to fuels in a small engine (my old lawnmower eventually became a donor). While he did well in his category, I was stunned by many of the entries. Stunned at the "corporate sponsorship" involved, too.
Do the "process" while having a good time and you will both get great rewards. The process is what wows judges.
BTW, have you considered the similarities between carburetoors and hearts? This can get mom into the process, too.
Illegitemi non-corundum est.