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Thread: Oil compensating for poor jetting-250r

  1. #1
    Join Date
    Jan 2007

    Oil compensating for poor jetting-250r

    There has been tons and tons of threads about jetting, and tons and tons about what oil to use at what ratio. Then there has been a few debates that more oil means less gas and vice-versa. However, I have read the theories that adding more oil to the gas will cause less gas to be deliverd to the engine. And on the other hand the more oil you add, the oil will burn also and you will get better ring seal and that one will need to jet richer or leaner. I know what I believe, and that is not the answer I am looking for. I want to take this one step further. What I am wondering is thisit might take a minute to explain) These numbers adn measurements I am using are for an example only and may not be correct.

    ALL NUMBERS in the following are made up for EXAMPLE only and are APPROX and ROUNDED!
    Say I am running 50:1 gas to oil ratio, and I am running a 150 main jet. Say that means that (Just for my simplicity sake here) that it will allow 150cc of gas/oil to enter the engine per minute at constant WOT. That means APPROX. 150cc of gas and 3cc of oil will enter the engine in one minute. Now if I change the main jet to 100, that means that APPROX. 100cc of gas and 2cc of oil will enter the engine in a one minute period. For every 50parts gas that enters, one part oil will enter.
    Now if I change my mix ratio to 20:1, that means at 150 main jet, that means that I will get 150cc of gas and 7.5cc of oil. and for 100jet that it would be 100cc gas and 5cc oil.
    Are you still with me?? Ok, now that we have established that, here comes the question.
    If I am jetted at 150 at 50:1, and I decided to jet to 100, but I still want to retain the same amount of oil entering the engine, then I could change my mix ratio to 33:1. Both would give me 3cc of oil per minute. So when jetting from winter to summer riding, you have to go with a smaller main jet, to compensate for less air. However, that means you are also allowing less oil to enter the engine. So to compensate I could mix my ratio a tiny bit richer.
    I guess basically what I am getting at is honda recommends 20:1 at 142 main jet. According to my innacurate examples that would mean 142cc of gas to 7.1 cc oil. Now when rejetting richer to say 150, it is really necessary to have that added oil entering the system, or could I change my mix ratio so still only 7cc would enter no matter how high I jetted. Also same thing with going down. If I jet down to 100, should I increase my gas to oil mix ratio to 14:1 so I still get 7cc of oil a minute?
    Normally, one would not compensate by adding more oil when leaning out the main jet, so if you were to jet lean, you would be short on gas and oil and seize your engine. So that brings me to my title question. If I am running the said 100main jet but still maintaining the 7cc of oil per minute, could that that much oil be compensating for my not -enough gas and be giving me false reading on my plug chops?
    What a confusing book. Good luck with the replys.

    edit-So that means that all you guys out there that are running 170 at 50:1, it is the same as running 140 at 40:1. So that means when you tell someone your mix ratio, there is a lot more to it than just the oil mix, you have jetting and altitude that plays into it as well. So maybe the title shoud be, "Jetting compensating for low oil ratio"? However, I am aware that your engine would probably overheat and seize if you normally took a 170 and tried a 140.

    One last question then. Everyone tells me that since I live at a high altitude (5280ft), that it is necessary to lean out my gas/oil mix ratio and that stock is way too rich for this altitude. However, according to my litte jetting blog above, shouldn't I be running just a slightly richer gas/oil ratio then to maintain enough oil in the engine since I have to jet down for lack of air?
    Last edited by Rustytinhorn; 01-25-2008 at 07:11 PM.
    A few wheelers, + an
    '73 Honda 49cc Mini Trail
    '85 200 Big WHEEL

    "Despite the cost of living, its still popular."

  2. #2
    Join Date
    Nov 2006
    southern oregon
    Jets are cheap... mix your oil the same every time...
    2-stroke lover

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