How do know it is very lean on petrol? With your CO trim set at 1.5v it is unlikely to be very lean. The other thing to bear in mind is that as far as I am aware there are some holes in the standard fuel mapping which does cause lean spots - these are intentional to improve fuel economy and also to meet certain emission standards at the time the system was introduced.
If your concern about running lean is based on a single measurement of exhaust gasses I would respectfully suggest you need to get more data. The lamda reading is th eone that typically indicates rich or lean but it is of little use on a non-cat system, if I really try hard I can get a lamda reading of between 0.97 - 1.0 but the air flow meter is turned all the way down I mean CO trim 0v. Currently, I have good CO and HC with acceptable lamda at a CO trim of about 1.2v but remember also that any change to exhaust system or free flow air filter will affect how the system responds, mine has a stainless exhaust with 4 branch headers and I also have a K&N air filter fed through a snorkel.
As you said, you need to get it on an exhaust gas analyser for a bit of time to tune it up, also remember that the ignition timing will have been altered to run on gas, this will affect the way your engine burns petrol and hence the emissions.