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Old 04-04-2013, 10:28 AM   #61
Twin-shocker
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Originally Posted by joexr View Post
I cant document it either , but 30+ years ago I was removing the oil injection from an RD and was told by a few different mechanics about increasing jet sizes to compensate for the oil increasing the fuel viscosity. Believe they were telling me about 5% increase.
Quite correct, and something that is overlooked by those who still seem to think what was the right way to go 40 years ago is still applicable today!
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Old 04-04-2013, 10:37 AM   #62
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SOrry, but you are talking apples and oranges here. You are referring to the change in oil added to the premix as a %. What we were talking about is the change in ratio affecting the fuel flow through a jet.

That and while it may be a 20-25% change in the volume of oil using your comment, in terms of oil to gas mix in terms of percent, it's less than 1% change. 32:1 premix contains 3.13% oil. A 40:1 premix contains 2.50% oil. So in terms of the amount of oil present in the volume of fuel, it is only a change of 0.63%

Hope that make sense.

Using excessive amounts of oil in a 2T motor wont affect the jetting as much as changing from autolube to pre-mix, and bearing in mind most motors in stock form are set up on the rich side anyway, damage is unlikely to occur due to extra oil weakening the air/fuel mixture.

However excess oil will serve to dull throttle response, reduce power, and mean that servicing of exhaust system parts will be needed much more often. Not really able to understand why anyone feels any of those things are an advantage?
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Old 04-04-2013, 10:37 AM   #63
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That and while it may be a 20-25% change in the volume of oil using your comment, in terms of oil to gas mix in terms of percent, it's less than 1% change. 32:1 premix contains 3.13% oil. A 40:1 premix contains 2.50% oil. So in terms of the amount of oil present in the volume of fuel, it is only a change of 0.63%.
All very good information, and the back-and-forth is enlightening, too. Now, not only am I learning about fuel:oil mixtures, I see I have a lot to learn about jetting a 2-stroke. Makes jetting my 4-strokes seem like child's play. I swear I had that book by Jennings back in the day, may still have it somewhere - have boxes and boxes of books in storage.
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Old 04-04-2013, 10:39 AM   #64
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The Jennings oil test data was applicable 40 years ago............only really of historic interest today though.
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Old 04-04-2013, 11:15 AM   #65
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Originally Posted by Twin-shocker View Post
The Jennings oil test data was applicable 40 years ago...
I am convinced that the new synthetic 2-T oils are the way to go.
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Old 04-04-2013, 02:06 PM   #66
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Originally Posted by CharlieT View Post
SOrry, but you are talking apples and oranges here. You are referring to the change in oil added to the premix as a %. What we were talking about is the change in ratio affecting the fuel flow through a jet.

That and while it may be a 20-25% change in the volume of oil using your comment, in terms of oil to gas mix in terms of percent, it's less than 1% change. 32:1 premix contains 3.13% oil. A 40:1 premix contains 2.50% oil. So in terms of the amount of oil present in the volume of fuel, it is only a change of 0.63%

Hope that make sense.
Still , that small percentage is obviously enough to make a difference. Years ago most people ran 32:1. If that versus none requires approximately a 5% increase in jets , then the difference between 32:1 and 40:1 would be 1 to 1.25%.
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Old 04-04-2013, 03:00 PM   #67
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As a 'mixer for many years, Ive generally used 32:1 in all my bikes except for the bigger bore singles (250cc and up). I run my RT1B om 26:1. Sure it tends to smoke a little more, but a little smoke out the back never hurt anything. My S3A has been running 32:1 for years and it's never had an issue with oiling. I actually went to premix on that bike because 1. the bikes injection system might as well been premix the way it was designed and 2. I lost a cylinder years ago cause the pump quit sending oil to one cylinder. Some people say slotted rods also help with premix keeping the lower bearing oiled. My S3A doesnt have slotted rods nor does the RT1B, so I can't swear thats a required mod, but quite a few people recommend it. FWIW Ive never lost a lower end bearing with premix and non slotted rods. There are some machines out there that the pump directly sends oil to the crank bearing (the H2 Im building) which I'm not sure what the detrimental effects are with running it on premix yet, but I'm going to give it a shot. I may cut off the premixing part of the pump (the bike has lines to the carb bowls and the crank bearings) and just use the crank bearing feed and mix it at 32:1. I havent decided what I'm going to do yet.

That said, There is another issue with premixing that you have to keep an eye on and thats a lean condition. The bikes amount of oil is in direct relation to the amount of gas it recieves. You need to make sure that you have the fuel right before going premix. The pump will continue to pump oil even after it runs out of gas and the engine is still turning, say if you're driving down the road and start running out of gas. On premix that doesnt work cause if youre out of gas, youre out of oil. Also, you have to pay attention and not 'limp' youre machine home (multiple cylinder bikes) if it has a carb issue, say one of the cylinders starts to starve out.

Hope this helps
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Old 04-04-2013, 04:54 PM   #68
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Still , that small percentage is obviously enough to make a difference. Years ago most people ran 32:1. If that versus none requires approximately a 5% increase in jets , then the difference between 32:1 and 40:1 would be 1 to 1.25%.
But you are making those alterations in jetting because of the change in the air/gas ratio, not because of the change in flow thru the jet.
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Old 04-04-2013, 05:01 PM   #69
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But you are making those alterations in jetting because of the change in the air/gas ratio, not because of the change in flow thru the jet.
That does make sense. Unrelated off the wall question. Do you think the oil would increase octane?
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Old 04-04-2013, 06:36 PM   #70
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That does make sense. Unrelated off the wall question. Do you think the oil would increase octane?
Increase the octane rating? Nope it actually does the opposite. I beleive the old rule of thumb was that the oil lowered the octane rating by two points. 93 octane would drop to 91 octane.
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Old 04-05-2013, 05:40 AM   #71
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But you are making those alterations in jetting because of the change in the air/gas ratio, not because of the change in flow thru the jet.
It does sound like a minor change in percent of gas:oil ratio will not effect the "viscosity" of the PREMIX all that much, i.e., jetting should be MAINLY be based on atmospheric conditions. I remember reading my 2-stroke tuning info (many years ago, likely the Jennings book) about humidity/temp/barometric pressure, etc., and jetting changes on race day. It was too much to get in to my small brain. So, let's say my YZ400 has stock jetting - considering my atmospheric conditions (desert air, 4500 ft. elevation, etc.) - what do I look for, how do I determine if my jets are correct? Also, considering my riding "style" which is mainly desert enduro riding - NO all out MX racing. Yes, I will study the Jennings 2-stroke tuning info, just looking for some quick "rules of thumb". I do know how to jet my 4-strokes, have even used the "Dynojet" kits with success, major improvement in my DRZ400, for example, when I lived in Cloudcroft, NM at 8800 ft. elevation. Can we change this thread to a JETTING/TUNING THREAD?
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Old 04-05-2013, 03:35 PM   #72
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For maximum performance jetting does need to be changed to suit atmospheric conditions. However there seems to be little understanding of how to jet a bike properly, and this has perhaps been made even more difficult as modern fuels mean getting an accurate idea of mixture strength from plug readings is now almost impossible, its maybe not a good idea to try and improve on factory settings which are generally far too rich?
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Old 04-05-2013, 03:47 PM   #73
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But you are making those alterations in jetting because of the change in the air/gas ratio, not because of the change in flow thru the jet.
If you choose to run 25:1 mix rather than 70:1, the viscosity of the fuel will be far higher, which will result in reduced fuel flow through jets. To compensate for this reduced flow rate, the jet size needs to be increased to maintain the same air/fuel mix, that would have been the case had you been running lower viscosity 70:1 mix, able to flow through the jets more easily.

The compensation factor is difficult to determine, as there are many variables, but essentially you would be trying to jet a bike so it would run as well on 25:1 as it would on 70:1. In some cases this might mean bigger main and pilot jets, in others simply raising the needle a little.

However that would only apply to bikes jetted somewhere near right in the first place, and as most stockers are set up very rich anyway, the 25:1 mix is only likely to make a dead feeling bike, a little worse.
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Old 04-05-2013, 07:11 PM   #74
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Well, since you insist on jetting changes due to change in viscosity, yet either won't or can't provide any supporting documentation, I decided to seek the opinion of a third party.

A friend of mine has a fair amount of tuning experience. Ever hear of a a guy named Trevor Nation and Norton? How about a Ducati rider by the name of Carl Fogerty? He was a race engineer/tuner for both those gents. H-e then moved on to cars and has been the chief engineer for anything from Porsche GT cars to LeMans prototypes.

So I asked him if you increase the premix ratio. Did you change jetting primarily because of the change in the air/fuel ratio or the change in viscosity.

This was his reply: " you are actually burning less fuel and more oil , the viscosity doesn't have much to do with it."

So unless you can come up with some supporting evidence back-up your peristent claim of the change in viscoaity requiring a change in jet, no offense, but I'm afraid I'd stick with the opinion of my friend over yours
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Old 04-05-2013, 07:24 PM   #75
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and as most stockers are set up very rich anyway...
So true. Generally speaking, I think the stock jetting from the factory is set for sea level. What about us desert rats at higher elevations? But, I have always said I would rather be running slightly rich than to running too lean. For me, that would also apply to premix fuel:oil ratios, too. Now some of the new factory MX 4-stroke bikes have FUEL INJECTION, all computer controlled stuff. I like my carbs, I can work on 'em, don't know a thang about fuel injection. But the beauty of computer controlled fuel injection is... one does not have to know anything about it, it's all automatic, works GREAT, very reliable and LOW MAINTENANCE. Okay, back to 2-stroke jetting...
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