Vintage Dirt Bike Q & A

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PostPosted: Sun Jan 25, 2004 8:49 pm 
This is a loaded question. Maybe it's from a legacy of pre-mix bikes but what are the benefits of running "pre-mix" type oils in an oil injected bike like the MT's? The only reason I can see is less smoke. The oil in an injected bike doesn't have to mix with gas very long so is therefore much more stable and doesn't lose viscosity in the tank. What do you think? Why should I not put the cheapest motor oil in my oil tank?
-Todd


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PostPosted: Sun Jan 25, 2004 8:52 pm 
Hi Todd! I'm going to give this one a shot. Now remember, my field of study isn't petrochemical engineering; it's just my hobby! I think you already answered your own question....because four stroke motor oil and two stroke oil aren't designed to lubricate the same way! Two stroke oil has to:
1. lubricate
2. burn cleanly
3. resist volatility
4. assist in cooling.
1. Lubrication seems like the easy one since that's what you expect from oil. But the other factors complicate that task.
2. Conventional oil doesn't burn cleanly because it contains additives to resist scuffing and add lubricity.
3. Conventional oil also has less resistance to volatility. That is, it evaporates at lower temperatures. When four stroke oil volatizes, it leaves hard carbon deposits. These carbon deposits form a barrier that seals lubrication from the very parts that the oil needs to protect. Two stroke oil has solvents to clean as they lubricate.
4. Two stroke engines run hotter than four strokes....even if water cooled. So here is where the volatility comes into play. Two stroke oil has to stick, not just lubricate and burn away.
So my bottom line is: If you use the cheapest conentional oil you can buy in your injector system, you'll have accelerated wear and when you tear it down, you'll find lots of burnt black residue on the bearings and other surfaces. Long term damage probably won't outweigh the savings on a $5 quart of oil! Hope this helps. Now let's go ride, tomh.


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PostPosted: Sun Jan 25, 2004 8:53 pm 
Thanks Tomh. I always seem to get opinions. When I read around the oil companies literature they just bolster the preconceived notion that pre-mix oils are best for all two strokes and fail to give any scientific reasoning. Do you think the oil burns inside the case? You mentioned my bearings would have carbon deposits in them. Does this suggest that the oil is burning when lubricating these parts? I've never seen carbon inside a two stroke engine (besides the cylinder and ports) and I've been using regular 30 weight oil for years in injected bikes like the manuals say. Thanks. Isn't this fun?
-Todd


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PostPosted: Sun Jan 25, 2004 8:53 pm 
I use Suzuki CCI for my MT-125. I also use it for premix in all of my old race bikes. I have not burned up any more rod bearings since I switched to the Suzuki oil.


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PostPosted: Sun Jan 25, 2004 8:54 pm 
I wouldn't. But at least use a High rated Quality motor oil. $10 or so a week for good oil is a lot better than $150 for a new top end! You dont have to worry about seizing up the piston as much when you run a two-stoke oil.


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PostPosted: Sun Jan 25, 2004 8:54 pm 
It's just that I have an old yamaha 90 twin that has only had 30 weight motor oil in it since new. The manual also says to use the same. Isn't the only difference between motor oil and 2-stroke oil that it has stabilizers to limit the amount of break down it experiences while sloshing around in the gas tank? If motor oils are fine for four stroke bikes (which are much harder on oil and it must last for 1,500 - 3,000 miles between changes) then it should be fine for a single trip around the case before being blown out the exhaust. My point, aren't four stroke oils much better than 2-stroke oils? Thanks for the response.
-Todd


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PostPosted: Sun Jan 25, 2004 8:57 pm 
Its like running 87 octain in a vette. When it calls for 92 octain. ya it runs fine now but it is doing unseen damage. I just dont see skimping on something so cheap. You know, just my opinion. :)


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PostPosted: Sun Jan 25, 2004 9:02 pm 
Oil technology is vastly improved over the past 30+ years. Run the correct oils and your motors will last longer. :)


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PostPosted: Sun Jan 25, 2004 9:03 pm 
So are you all saying two stroke "pre-mix" oil has better lubrication properties than regular oil? I wasn't suggesting using 30+ year old oil in my bikes. It just seems like 4 stroke oils have to be much stronger (last longer, better wear properties, etc...) than two-stroke oils. The only difference is that the pre-mix oils have additives that allow the oil to be broken down easier so that it will burn and won't smoke. 4-stroke oils would be more protective but might cause a little more smoke and carbon build up like in the old days. No one seems to be answering my question with anything other than opinion.
-todd


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