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Thread: Cold Air

  1. #1
    Senior Member Maxpower's Avatar
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    Cold Air

    More than ever I see I need to richen my bike. It runs better with the choke on. I'm guessing the cooler dense air is leaning the mixture out even more. There is so much info on tuning my 87 carb that I get confused reading about it. What is the stock main and pilot jets? Also what carb is on my 87?
    I've been needing it for transportation. I smile everytime I go out to the drive at 4am to go to work and it lights by the 2nd kick.(I still haven't installed my starter clutch ) I wish my wife would deal with my mental abuse as well as my TW can deal with my physical abuse
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    87 TW

  2. #2
    Senior Member GaryL's Avatar
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    Quote Originally Posted by Maxpower View Post
    More than ever I see I need to richen my bike. It runs better with the choke on. I'm guessing the cooler dense air is leaning the mixture out even more. There is so much info on tuning my 87 carb that I get confused reading about it. What is the stock main and pilot jets? Also what carb is on my 87?
    I've been needing it for transportation. I smile everytime I go out to the drive at 4am to go to work and it lights by the 2nd kick.(I still haven't installed my starter clutch ) I wish my wife would deal with my mental abuse as well as my TW can deal with my physical abuse
    I am not sure you have this correct. All of my bikes and other small engines always seem to run better with colder air. I am not sure if cold air has more density or less??? Your 1987 TW was the first year and as such the carb should be the "Old Style", TK carb. These tend to run best with the pilot screw turned out between 2-2 1/2 turns from the bottom. Basically the pilot screw is the air to fuel ratio adjustment and it should only be adjusted when the engine is at operating temp. We tend to burn our knuckles adjusting it so be careful and a small straight slot screw driver works best but, but, the carb comes with a brass plug over this adjustment screw that must be removed if it has not been already. As far as the jetting goes, it is my opinion no change is necessary unless you ride in very high altitudes and NJ just does not have them. I am in the NY Catskills and even the highest peaks up north don't require any jet changes.

    GaryL
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  3. #3
    Senior Member RockyTFS's Avatar
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    Quote Originally Posted by GaryL View Post
    .....All of my bikes and other small engines always seem to run better with colder air. I am not sure if cold air has more density or less??? ....GaryL
    Colder air is denser than warm air, so the bike will make more power.....IF it has the right mixture. If a bike is running with the correct mixture at 80 degrees and it is run somewhere at the same altitude at 20 degrees it will be leaner....same amount of fuel, but more air (denser). So Max's bike running lean below 30 degrees makes sense. That doesn't make it right, however, and one should not have to run with the choke on all the time.

    It is possible that the jet has been changed by a prior owner and should be changed back to stock, as well as doing the recommended 2.5 turns out on the pilot jet. You can turn the carb after loosening and take the float bowl off, then with a bright light you should be able to read the jet number. I'm not sure of the stock number for the '87, but someone else surely will. (114, maybe?)
    Rocky
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  5. #4
    Senior Member RaZed1's Avatar
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    Cold air is denser than warm air given the same elevation (lower elevation=denser).

    With correct jetting, an engine will make marginally more power with denser air vs thinner. However unlike EFI carbs do not compensate for this automatically, so as temp/elevation changes, fueling does not.

    If it runs notably better with the choke partially on, you're now too lean. Turn the pilot screw in until it lightly seats and count the number of rotations. Now back it out that same number, plus 1/2 turn more to compensate for cooler air. If that's more than 3 turns total, you probably should swap the pilot jet for the next larger size (effective adjustment range is from about 1-3 turns- less than 1 turn, go down a size on the pilot- more than 3 turns, go up a size).

    To "set" the pilot screw, have the bike fully warmed up and idling. Turn the idle speed up a bit faster than normal (this exaggerates changes and makes it easier to detect). Set the pilot screw to 1.5 turns as a baseline, then adjust it in 1/8 turn increments giving a couple seconds after each adjustment. Just listen to the engine speed. You're looking to find the spot engine RPM is fastest. There may be a small range where there seems to be little or no effect, that's normal. For an air cooled engine I shoot for the richer end of that range. Then turn the idle speed back down to normal.

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