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Thread: Carburetor Question

  1. #1

    Default Carburetor Question

    I have a 2000 Yamaha 500 VMAX that starts and runs with no problems whatsoever, except for after the engine warms up. The longer I ride the blacker the exhaust gets. When I stop, the machine leaves a large black spot in the snow where the exhaust is coming out. The engine will also cuts itself back to a maximum of 5500 RPM and I'm having to limp myself back to the truck.

    I took the carbs apart and soaked the parts in cleaner and made sure the pilot screw was set to what the owner's manual recommends (1 3/4 turns back from tight) and I still am ending up with oil coated spark plugs.

    Being completely ignorant on this, can someone recommend my next step of action? I'm guessing I just need to play with the pilot screw setting but I have no idea of how far to adjust it (1/4 turn, 1/2 turn, etc.).

    I've verified that the choke isn't getting stuck open and that the oil pump cable is adjusted per the owner's manual.

    My typical riding area is sea level to less than 1,000 ft.

  2. #2
    Member JOAT's Avatar
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    You're probably well over the pilots as they only work for about the first 1/8 of the throttle movement. How are the rest of your jetting components? What size mains do you have installed? They have the greatest effect over the widest portion of the throttle.

    By the way, the pilot screw is adjusting the signal air to the pilot jets, not the jets themselves.

    Check the inlet valves and float positions. If you're overfilling the bowls, it will run really rich.

    Are you using a lot of oil? Just having the cable adjusted right doesn't mean something else isn't amiss with your oil injection.
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  3. #3

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    Joat -

    I'm not sure what size the mains are. I am the second owner of the machine. I'm assuming everything is stock. The manual says the Standard Main Jet is #151.3. If I pull the carb out again to look, will the jet have the size stamped on it?

    Since I'm a truly novice mechanic (but wanting to understand how it all works), I'm not sure how to check the inlet valves and float position.

    I bought the sled a couple years ago and used it ony a handful of times. Last winter the sled started to kick out the black exhaust and started cutting its RPMs down to 5500 max after riding an hour or so. I ended up buying a Renegade and never got the sled fixed.

    I'd like to get this sled functional so I can invite friends out with us when we ride. I haven't taken it to the dealer primarily because I wanted to learn how to do it.

    I appreciate any advice you have.

  4. #4
    Member JOAT's Avatar
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    As a 2nd owner, I wouldn't assume anything. You'll find the jet sizes are stamped on them.

    Actually the very first thing I would suggest that you do is drop the needles. The needles work the mid-range where you spend most of your operating time. Drop them down a notch and it will lean it out a bit.

    The way all the pieces & parts work together is a little lengthy to pass on in a forum post. I'd suggest you pick up a snowmachine carb tuning book to get a handle on how to work over each section of the carb.
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  5. #5

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    Thank you for the advice!

  6. #6

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    Mikuni carb? Great over view on them @ Affordable gokarts it will learn ya.

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    Quote Originally Posted by Wanna_fish View Post
    Joat -

    I'm not sure what size the mains are. I am the second owner of the machine. I'm assuming everything is stock. The manual says the Standard Main Jet is #151.3. If I pull the carb out again to look, will the jet have the size stamped on it?

    Since I'm a truly novice mechanic (but wanting to understand how it all works), I'm not sure how to check the inlet valves and float position.

    I bought the sled a couple years ago and used it ony a handful of times. Last winter the sled started to kick out the black exhaust and started cutting its RPMs down to 5500 max after riding an hour or so. I ended up buying a Renegade and never got the sled fixed.

    I'd like to get this sled functional so I can invite friends out with us when we ride. I haven't taken it to the dealer primarily because I wanted to learn how to do it.

    I appreciate any advice you have.

    Stop now and take it to a shop before you do something that'll end up with the machine burning down.
    Since this appears after only AFTER the engine has warmed up, it is probably the float needle seat. These are Mikuni carbs yes? How much mileage do you have? So rather than spend $200 for the carbs to be rebuilt, you bought a new machine? Wow, I wish I had that much money.
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  8. #8
    Member JOAT's Avatar
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    Quote Originally Posted by Nitroman View Post
    Stop now and take it to a shop before you do something that'll end up with the machine burning down.
    I disagree. Mikuni carbs are very simple to work on and with the proper manual in hand, a novice mechanic can figure out what to do to them. There is little need for a professional shop to be involved, especially with an older machine that is also a backup sled. Great opportunity to learn how to do it yourself.

    Since this appears after only AFTER the engine has warmed up, it is probably the float needle seat.
    No. All engines need to be rich during the warm up phase, hence the reason for a choke. Since the machine runs fine during warm up and transitions to rich after it's warm, then clearly the carb is making it rich.

    The first question you must ask is, are both cylinders rich or just one? If you're getting both plugs fuel fouled, then one should check jetting. Most of the time you'll have both carbs jetted identical, but a failure of an inlet valve will be only with 1 carb, not both at the same time.

    I'd also be very suspicious of the choke cable and first ensure that there is slop in the cable when it is off.

    Float position is a very simple adjustment. Remove carb, remove bowl, hold carb upside down an ensure the tab on the float that closes the inlet valve is perfectly level when compared to the carb body. Not sure why you'd want to pay a shop $200 to do that.

    Learn to read the plugs. Learn to read the pistons (looking down through the plug hole at the top of the piston will reveal a fuel wash pattern if it is rich.
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  9. #9
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    Because he said he didn't know anything about carbs, and if done by the shop, it'd be under warrantee, if the shop warranties their work.

    The jets, being the "main jets", don't really come in until 3/8" throttle and up. Since he said the engine ran fine until "except for after the engine warms up. The longer I ride the blacker the exhaust gets." This is why I think the needle seat is worn, it is allowing too high a float level gradually overloading the motor as all rpm ranges. You are completely correct though, the carbs need to be pulled and the needles raised to check the seat. Not sure if the Mikuni's had the Viton seats that early.

    If you do this and see even the slightest ring around the seat, it needs to be replaced.
    Great spirits have always found violent opposition from mediocre minds. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence. Albert Einstein

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  10. #10

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    The machine has Mikuni TM-36 carbs and has 3,023 miles on it. It starts very easily and I can let it sit in the back yard for 30+ minutes and it will idle away with no problems. But, when I take it out riding after awhile the exhaust gets blacker (I've got a black cloud around me), the machine will not idle - I have to hold the throttle down some to keep the motor running. When trying to start it after it starts doing this, I typically need one person on the throttle while I sit and crank it over for a long period of time. Once riding, the machine loses power and will not allow its RPMs to go above 5500.

    So, I lose ability to idle (pilot jets), I lose all upper-end RPMs (Main jets), and my exhaust is very dark (too rich). Both spark plugs look identical...completely oiled.

    My Main Jet is stamped 151.3 (at least that's what my teenager said, since I couldn't get my eyes to focus on the small numbers).

    At this point, I'm tempted to take it to a dealership and have them resolve the issue. Joat is right, the carburetor does look like it would be something very simple to work on - there's not all that many parts to it. I just need to understand what all the parts do. Because I'm clueless and don't know what more to do than simply make sure the parts aren't fouled up with crap, broken, loose, or some other basic issue - I will probably turn it over to a professional.

    As for your comment on the Choke cable, it is not very loose. To start it, when it's cold, I use the choke and when I turn the choke switch off, I often have to push the switch to make the cable go all the way in. On the carbs, I cleaned up the rod and spring that the choke cable is controlling. When everything is connected, the choke lever on the dash easily opens the choke assembly, but the lever on the dash is very slow to closing the choke assembly when I turn the choke off.

  11. #11
    Member Vince's Avatar
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    what your all missing is the V MAX part..


    Yamaha is one of the hardest sleds to work on

    they have a self compensating altitude and temp system for a large part .. lots of vacuum operated parts and almost an IM system seems like.. i nearly gave up trying to keep current on the Ha-ha"s after 99. and only took in the ones i had already been servicing..

    to be blunt...take it to Yamahahahah.. and get it tuned... have the tech explain the issue and get some BOOKS.

    sorry. we can all build Mic carbs in our sleep. this is not a carb issue..

    the oil injections is done at the intake level if your getting too much oil... your pump timing is off.. you may be able to adjust it with the cable.. but due to Yammes, technical issues i would start with an education from a tech familiar with them.
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  12. #12

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    Do you have heated carbs? I know the 700 did that year. If you do make sure, your peck cock for the coolant bypass is in the off position. Otherwise, when your coolant warms up you will cause a rich running condition.
    Just a thought

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