I am considering blocking off the oil pump on a Yamaha 1999 GP1200x and pre-mixing the oil and gas. Does anyone know the recommended ratio if this is done and any recommended carb adjustment changes if any?
Bmezo, 40/1 would be where I'd start and if you have plug fouling issues work you way toward 50/1 till the fouling problem is gone. No further than 50/1.
Actually I don't recommend doing a pump block off and here's why. Most will tell you to do so because the pump fails but that's not true. The systems are quite bulletproof if maintained properly. Meaning replacing the oil lines on a regular basis. As it's the lines that actually fail. They shrink and harden and eventually pull off or break.
More importantly on your ski the pump is a variable ratio pump. At idle your around 200/1 then at full throttle your around 32/1. This gives you proper lubrication at full throttle and doesn't foul plugs at idle. This not only saves quite a bit of oil. It makes a marina fill up much easier. No messy oil to be carried. A tank of oil will last about 5 or 6 tanks of fuel.
If it were mine I'd replace the oil lines and attach them well with wire ties and a tie gun and let her eat. It's an easy winter repair. 3 or 4 hrs on a Saturday afternoon. Cost about $40/50 with tie gun from Lowe's. And a LG suringe also from Lowe's small engine section to bleed the oil lines as you put them on. Change the lines bleed the system and go.