Prince George Vintage Motorcycle Club
Technical Section => Engine => Topic started by: Dennis on August 15, 2012, 04:37:03 PM
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This is really a part of my Drag Bike thread, but thought for future or related reference, it would be easier to find here.
My old (1976) IT400 manual says mix fuel/oil at 20:1 ratio. I have heard and read a lot of stuff about using different ratios with modern oil. Basically that old oil was crappy compared to modern stuff, and that even in old engines I should use 40:1, 50:1 or even 100:1 (high tech synthetic).
Here is a link to an article that I think is particularly good. It seems to make sense to me.
http://www.off-road.com/dirtbike/tech/two-stroke-gasoil-ratios-20502.html
I ran my IT400 on 40:1 with good quality snowmobile oil (rated for 50:1 snowmobiles). However, I ran it like a trail bike ... mostly slow. In its drag configuration, I hope to be doing just short full out blasts.
Any pearls of wisdom from the bretheren?
Peace & two-stroke oil, Dennis
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My pearl of ????? is I run 30:1 in everything. Outboard, chainsaws. weedeater, motorcycles. My belief is that it is a good compromise. Reading the link that you left puts me right in the same park as that guy. On the bike you will only be running wide open for 12-18 sec ;D which really isn't that long. A saw or weedeater is eun wide open all of the time. In fact when you are setting the saw up, you hold it wide open for the high speed setting untill you get the right sound. It kind of scares me, but my brother who was a sthil mechanic for years told me to do it that way. I guess the long and short of it is that I have never had one blow up on me yet. With that being said, the next one I start probably will.
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I've never raced a 2 stroke bike (other than the 66cc gas powered bicycles), but I use Opti 2 at a ratio of about 80:1 or 100:1 in all my two stroke motors (chainsaws, weedeaters, gasbikes, trolling motors etc.) and I love it.
Here is a (very redundant!) thread about the mixing of Opti and other oils@ 100:1 - http://motorbicycling.com/showthread.php?t=9850
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The less oil you run in the mix the richer the final fuel mix will be. more oil equals a leaner mixture ass you can only get so much mix through the main jet. Too much oil could actually hole the piston, too little and it goes squeak. I'd use what it calls for.
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Hans is correct about less oil CAN mean the final air:fuel ratio may be richer, as more fuel of a thinner viscosity can get through the jet in a given amount of time. This can be adjusted by fine tuning the needle, or in an extream case - going to a smaller jet. Always check your plug colour to determine the air:fuel mix is not too lean after messing with the needle or jet...
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I mix 50:1 Amsoil Dominator mixing oil and 94 octane in my Banshee, Indy Storm, chainsaw, weed whacker and any other 2 stroke that I get my hands on. They usually need to be re jetted and or tuned to use the leaner mix to it's full potential but in my experience alwyas run better in the end. My brother runs the same oil and octane in his kx 250 at a 100:1 mix and has been since he bought the bike 2 years ago. No problems yet.