can anyone advise me on the montesa pre mix ratio,i have been running on 100ml of oil per 5 litres of petrol but bike does not respond well to full throttle.having just obtained a manual of ebay for my 315 i read that i should be running a ratio of 80 to 1 which i understand to be in the region of 75ml per 5 litres of petrol,would be grateful for any advise as i do not want to seize the engine.

80:1 works out to 1.6 oz per gallon. The best thing to do is get a premix mixing cup, and then you don't have to do the math.

There are a variety available such as the Ratio-Rite:

http://www.speed-sports.com/shop/product_i...?products_id=62

i run my 02 montesa at 80:1,thats 56.25ml oil to 4.5 liters of petrol(1 gallon) i use optimax petrol and runs with no problems i also use a syringe to measure out the oil (get from local bike shop or vets) its a more accurate measurement