Autolube Oil Mix Ratio Questions
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Q: I recently purchased a 1966 Yamaha 305 Big Bear. I had one of these when I was a teen and found it to be almost indestructible. I am wanting to reduce the oil mix ratio on the Autolube oil system. I have looked at several shop manuals and the factory only shows one setting. There is a pin that aligns with a mark on the pump and is adjusted with the throttle cable. I think that by using a synthetic 2-stroke oil (Amsoil) that I can reduce the oil mixture ratio, thereby reducing exhaust smoke and increase spark plug life, as well as improve overall performance. What are your thoughts on this, and what procedure would you recommend? — John Botts/Ponca City, Oklahoma
A: My usual solution to oil injection problems has been to bypass the pump if possible, and pre-mix to your desired ratio. The Yamaha manuals I have mention removal of the oil injection system as an option for competition and suggest a 40:1 gas/oil ratio. I have little experience here in changing the pump output, so I thought I’d get some expert advice from the folks at HVCcycle, 2-stroke specialists in Nebraska. Brad Obidowski from HVCcycle says: “Keep in mind the oil in the mix has to lubricate the crankcase main bearings too, so be careful you don’t cause yourself engine problems chasing less smoke. Modern low ash oils burn better, leave less residue, smoke less, and protect better anyway. If you do decide to change the autolube pump, 0.012-0.015 inch is the standard shim gap. You need to reduce this to reduce the amount of oil. I’s mostly guesswork once you deviate from the factory settings.” He also mentions that trying to adjust the flow by modifying the cable pull could result in too little oil at higher rpms, causing problems. Reducing the amount the cable pulls will leave the output of the pump at an idle state longer, thus reducing the needed extra oil at cruising speed. Adjusting the shim stack keeps the oil delivery constant with the required throttle position and rpm range. My final advice would be to set it up as stock. Too many problems arise from getting the mix wrong.
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