usually around May, they start to put additives into the Gas, so it wont "freeze" in the convertor. then around Sept they take them out
when I had my '89 TB42 Gas/Petty, it was tuned for performance Gas - using Sprint Gas mixer - so it was very very noticeable when they did the winter /summer changes
ended up getting a tune twice a year - late May and early Oct
usually got about 350Kms when tuned, then it would drop to about 300Kms when they f@rted around with it
tuned right, I was getting 96Kw on gas and 98Kw on Petrol
also, with the Gas feed into the Carby, make sure THAT has not been turned open too much - best way to tune them is with a Lambda meter.
first time I got it done for performance, took about 3 hrs to tune and set,
after that, it was a 30 min job to do, as we had the settings
mind you, it also took about 10 different places before I found one that knew what they were doing as well.
hope this helps.....
cheers
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