I have three Weston .25 size engines. I have had problems with all of them. They all either run too lean or too rich where fuel spits out the size and the engine loses power. Before you chalk it up to my incompetency I'll have everyone know that all the old school RC combat flyers I battle with attempted to tune these engines as well. Most of them are in their sixty's and have been flying nitro combat for over 25 years and none of them could get these engines to tune or work properly. The O.S. on the other hand tunes up and runs great with no problems. I've had better luck with the ASP's as opposed to the Westons.