I had a share in a C150 for several years and flew it a lot, including over to France at least twice.
The C150's Continental engine is VERY prone to carb icing. I had obvious symptoms of icing (rpm getting gradually lower etc) at some time in every phase of flight except with full power for the climb. I used to apply carb heat for 30 seconds every five minutes, regularly. With high humidity, rain, or over water, I kept the carb heat on all the time. I checked the fuel consumption quite frequently and it never seemed to make any difference.
When I applied carb heat the rpm did drop quite a bit, though I can't quite remember how much. That seems to be normal. However, the one flight when it did that but also ran rough, we got it checked out and there was indeed something wrong...sorry, but I can't remember exactly what! So it might be worth getting the engineers to look at it if that's happening all the time.
When and how often to use carb heat is something that pilots will rarely agree on....ten pilots, elevent opinions on carb heat.
But too much will rarely do any harm, and too little easily could...especially in a C150.
But please don't be put off flying it. It's a great little aircraft. We took ours everywhere, and in and out of some short fields that you'd never dare attempt in a PA28...and for a lot less cost.
