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Thread: Carburetors

  1. #1
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    Default Carburetors

    I am looking for a couple of things. First, does anybody know where to find good schematics, diagrams, and tuning guides on the Net for Mikuni Carbs? And, does anybody have advise on the following?
    I have a 250cc Elan. It has the Mikuni VM carb (round slide). Last year it ran strong. Ok guys, I can hear you muscle sled heads laughing. Careful now! This year, when I pinch off the throttle, and go just as absolutely frightening fast as an Elan will go, I lose power and it backfires.
    I took the carb apart twice. I have soaked it in aerosol carb cleaner overnight and run wires through all the jets and orifices I can.
    I have yet to check the tank and fuel line into the tank to be sure I don't have a vacuum or blockage. Pretty sure the vent line is clear, need to pull the fuel line and check it.
    Any great ideas? Thanks.

  2. #2
    Member JOAT's Avatar
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    Default

    Lots of online info out there...

    http://www.google.com/search?q=mikun...ient=firefox-a

    Losing power at full throttle with backfiring is indicative of a lean condition with detonation. A quick check is to see if giving the engine a little bit of choke will solve the problem. If it gets better, it's lean. If it gets worse, it's rich. If your lean at only full throttle, that's the main jets. Put in the next size bigger and see how it does.

    If in doubt, you can try to read the plugs. Get the engine to operating temperature, do your full throttle run, and kill the engine while at full throttle. Pull the plugs and check them for color.

    This page was at the top of the list for reading plugs. At the bottom are a few photos. I've seen better out there, just Google it and look around.

    http://www.bcchapel.org/pages/0003/pg11.htm
    Winter is Coming...

    Go GeocacheAlaska!

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    Default same carb?

    Hey JOAT, how you doing. Thanks for the help. I googled a bunch of stuff but did not find what I thought I was looking for. I did find a PPT that was pretty cool, and had a section on plugs too.
    Running lean? Probably. It loses power, backfires, and I have to feather the throttle and reduce the throttle setting to keep it from backfiring.
    Main jet problem? Everything in that carb is the same as the day it was bought. If it did not back fire before, why would it now?
    It has a primer (which I like) so hard to make the fuel/air ratio richer in a manner that it stays running.
    It takes off and runs good for a minute or so. Then it starts to drop rpm and then backfires. At lower rpm (up to 1/2 throttle) it runs good.
    The first time I tore it apart there was a little gunk. I blew it out w/ compressed air and flushed it w/ carb cleaner.
    The second time, I blew it out again, and sprayed cleaner over all the parts and let them soak 24 hours, then ran the wires through the orifices, and finished with compressed air.
    What would make a carb change? Especially on the top end?

  4. #4

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    Is it possible that your carb boot is cracked? This would definately cause problems. Maybe a leaking head gasket. Food for thought! Good luck

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    Member SusitnaAk's Avatar
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    Fuel flow? If not the carb. Keep going back ,fuel pump? lines, filter, Maybe water in the tank could also do this.

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    Member AKDoug's Avatar
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    Definately look for a carb boot crack. A crack between the carb and the cylinder will lead to more air being drawn into the cylinder resulting in a lean condition. Elans have chronic problems with carb boots.
    Bunny Boots and Bearcats: Utility Sled Mayhem

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    Default boots and heads

    I did pull the boot off. The outside is cracked, but even with a good light and pulling and prodding, I could not see any cracks on the inside. I can go back and look again.
    *Does it make sense to clean it off (degrrease) and coat it with GOOP or something just to be sure?
    *Should I use silicone where the boot bolts on to the block?
    *Would a head gasket leak show up with oil on the head/cylinder? I think it looks pretty clean, but again, easy to go back and look. Especially on an Elan.
    Fuel pump was taken apart. It was clean. I also looked at the vacuum line to the block, and fuel lines all the way to the tank. I did not see any leaks. I can go back and double check where they slip over fittings, and safety wire them or clamp them on just to eliminate that possibility.
    *How would you know if a fuel pump is bad?

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    Member mod elan's Avatar
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    Default Also try....

    A bad crank seal could also suck air and lean it down. Take a can of starting fluid or WD-40 and give it a spray behind the primary clutch while the motor is running. Listen for any change in the RPM.

    Pull the plug on the float bowl again and check for garbage. Sometimes a piece of crap will sit in the bottom until the suction of full throttle will pull it to the main jet and cut down fuel flow. I know you've cleaned a few times but it may be continuing to suck stuff from the tank.

    What year is it?

  9. #9
    Member JOAT's Avatar
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    Lightbulb

    Main jet problem? Everything in that carb is the same as the day it was bought. If it did not back fire before, why would it now?
    Because something is different now . We just have to figure out what changed. First, the jetting in a carb is fixed and optimum for a group of altitude and temperature ratings. For the same jet, colder air and lower altitudes make the mixture lean as there is more oxygen per volume of "air". Warmer air and higher altitudes make the mixture rich as there is less oxygen per unit. For example, if you're jetted for +20 at 4000' ASL where it runs good and you go down to your buddy's place on the coast and go for a race at -40, you may have just blown your engine. Anyhow, if the jetting was running good before, you didn't change it, and it's not working now at similar altitude and temperatures, then the jet size probably isn't the issue (though the main jet can still be corroded or plugged).

    So if you're lean on the same jetting, that means you are either getting more air than you used to or your float bowl isn't filling up with fuel, therefore your fuel getting through the jet and up into the airstream is less. The air problem can usually be sorted out pretty easily and others have given great tips on that. For your carb boot cracks, if you can't see through them, they are fine. A little duct tape will fix any holes that you find. Don't bother with trying to clean and goop the outside as that probably won't help anything and sounds like way too much work for a $20 part. Just replace them when they get holy and completely covered with duct tape. But for now we need to find your real problem.


    It has a primer (which I like) so hard to make the fuel/air ratio richer in a manner that it stays running.
    Yeah, that won't help. A mechanical choke is the only way to do that test. Even a partial primer squirt is so rich that it won't tell you a dang thing.


    It takes off and runs good for a minute or so. Then it starts to drop rpm and then backfires. At lower rpm (up to 1/2 throttle) it runs good.
    This is good info. Runs good cold and then loses power... now this set of symptoms actually says "rich" mixture. So we haven't really nailed down your exact problem yet. Time to read some plugs and it's best to start with a brand new set. Run the engine in the problem rpm range for a few minutes, kill the engine with the ignition cut off while holding the throttle in the problem band, then pull the plugs out and let's see what you have. Are they black, orange, white? Are they wet or dry? Take your handy little flashlight and look down the holes at the tops of the pistons (slowly roll the engine over to inspect all areas that you can). Are they covered with black? Can you see shiny aluminum? Is there a "wash" pattern from excess fuel? Is there pitting? The answers should get us closer to knowing if we are talking rich or lean. But don't forget that it is entirely possible to be too rich at idle and too lean at full throttle due to the way a Mikuni VM uses different circuits for the different throttle positions.

    The first time I tore it apart there was a little gunk. I blew it out w/ compressed air and flushed it w/ carb cleaner.
    The second time, I blew it out again, and sprayed cleaner over all the parts and let them soak 24 hours, then ran the wires through the orifices, and finished with compressed air.
    What would make a carb change? Especially on the top end?
    So we had some "gunk". When you cleaned the components, did you actually take everything apart? Or did you leave all the jets and valves in the carb body and just take the fuel bowl off? Without taking all the pieces out, there is no way to clean the passages behind them. You can remove the main jet, needle jet, pilot jet, and inlet valve easy enough. Clear out everything behind them, but I wouldn't recommend sticking anything through the holes as you can easily enlarge the hole, thereby making the jet size bigger than it should be (rich). Of course you could have corrosion build up inside the passages that makes the hole smaller (lean). These are brass parts, so you'll usually be able to see the green corrosion if that were the problem. Jets are $3, so if there is any doubt, just replace them.

    The inlet valve (needle & seat) can wear out and not completely shut off the fuel flow into the bowl. Hard thing to check with the eye, so if this is an old carb and/or high mileage, consider just replacing the inlet valve while you're in there. It's another pretty cheap part.

    The needle jet is a long tube that is under (above when the carb is installed) the main jet and runs into the air path of the bore. This item is nearly fail-proof, so as long as it is clean don't worry about it. The needle that runs down into this from the throttle slide is going to be one of our points of adjustment. It's held in place with a small E-clip on top of the slide. There are 5 rings around the top of the needle to adjust its height in the needle jet. Raising the needle richens the mixture in the mid-range.

    Also, we need to look at the floats. After everything is cleaned up and reassembled, hold the carb upside down (with the bowl off) and the levers that the floats lift to shut off the inlet valve should be resting on the now gravity closed valve and be perfectly horizontal. Gently bend things to make sure all is well. You can also check to make sure your floats aren't "waterlogged" by filling the bowl with gas and make sure the floats move freely up and down and feel buoyant enough to close the inlet.

    Fuel pump output... while you have the carb off (and the ignition turned off), pull the engine over a few times to verify good fuel flow coming from the pump. A plugged pickup in the tank or minuscule valve leak inside the fuel pump can cause lean problems by not delivering enough fuel. These can be difficult to diagnose as well. For the moment we just want to see fuel pulsing out of the carb end of the line as you pull over the engine.

    There is an idle mixture screw on the side of the carb. It is a very small slotted screw that sits nearly flush with the carb body. This is the first mixture adjustment to play with. At idle, the main jet does nothing. So fire up the engine and very slowly tweak the idle mixture screw. They usually start about 1 turn out from fully seated. As you turn it, you should get lower rpm and bogging in one direction, that's rich. Then go back the other direction until the rpms increase a little bit and the engine smooths out. Don't go farther than that or you'll end up with a lean idle and you don't want that. But try to get the idle cleaned up first so we can eliminate that end of the spectrum. If you can't clean up the idle with the mixture screw, our problem lies beyond the carb.

    With idle set, start to slowly give it throttle up to about 1/2. This is the area where the pilot jet, needle jet, and needle have the most influence. We'll assume the pilot and needle jet to be good as they rarely have issues. If the engine runs well at the bottom half of the throttle and doesn't start to give you issues until you're in the top half, then we are pointing more towards main jet and/or fuel delivery problems.

    Since you don't have a choke to do the rich test, we can try the needle test. Pull the slide out of the carb and take out the needle. Note which notch the e-clip is in! Now raise the needle all the way up (i.e. put the e-clip in the lowest notch). Put it back in and run it again. Did the problem get better or worse (top half of the throttle range)? If it got better, you're running lean. If it got worse, you're running rich. (TIP- get a set of locking hemostats for moving the e-clip on the needle.)

    If it was lean, install the next bigger main jet and put the needle back in the middle and then start over with the tuning process. If it was rich, start dropping the needle down, starting 1 notch above where it was originally installed. Test at 1 notch increments when going leaner.

    There is a lot to this, which makes it hard to do from a distance. My biggest suggestion would be to invite over a buddy who knows a bit more about carb tuning to actually have a look-see at the engine. Easier to clue in on problems when you can see, touch, hear, and smell it running.

    But for the moment, try a bit of this tinkering to see what you get, we might be able to narrow down the problems a bit more.

    I'll check back tomorrow and see what happened...
    Winter is Coming...

    Go GeocacheAlaska!

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    Default Whew

    Hey JOAT, buddy, what are you doing this weekend? I have brats and beer. Come on up for a good time!
    I am scratching my head. You said I should start with a new set of plugs? I find the one, but for the life of me can't find the other?
    Carb was as best I could tell, 100% taken apart, including seats and needle jet. I saw no green inside on the brass. I was pretty careful about what I pushed through holes. I have several different sizes of safety wire, so just kept finding the right one.
    My understanding on VM Mikunis is that you adjust air, not fuel on the idle mixture. Turning out is leaner, in is richer. Sound right to you? Both the Elan and my 440 polaris had the idle mix screws set at 1.5 turns. It doesn't seem to want to idle great (neither does the 440), so at least I know where I am either way I go.
    If I get the idle better, I will change the plug to new and go for a run. I will let her warm up a bit at about 1/2 throttle. Then I'll pinch it off and see if I can hang on. Scary dude, just scary. Once it backfires, hold the throttle, and hit the kill switch. Hey, that's 2 things at one time. Once stopped I pull the "plugs". That will or should tell me whether it is rich or lean.
    If lean, I can use your advice and Mod Elan's (leaky seal/starting fluid) to start finding the problem.
    Hey Mod, you are a buddy too. Same offer I gave JOAT, beer and brats. Northway knows the way to the house.
    Thanks guys. I'll try and hit this early before I have to go wood cutting.

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    Member mod elan's Avatar
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    LOL!!!!!!! I just remembered this sucker has POINTS! When's the last time anyone here has dealt with those? In some climates it is quite common for a set of points to corrode over periods of storage. They could also wear poorly as they are run. If they have corroded and/or worn the timing would be affected.

    Get me the year of the machine and I'll get the factory carb settings and air gap for the points. If you don't find any junk in the carb and the carb is at factory settings or close you could check the points. If needed they could be touched up with a very small file and readjust the gap.

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    Default 1983 super elan muscle 250

    Built May 1982, so assume it is an 83. It is the 250 single. Wish it was the 300.
    Of course I am 2 hours behind schedule already today. Have not had a chance to run it yet.
    Got to go cut wood to keep the house warm. This looks like a night time project for this evening.

  13. #13
    Member JOAT's Avatar
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    HA! That figures... last night as I was typing I kept thinking "this sounds more like a timing problem than a mixture problem, but you just don't have timing problems on a snowmachine..." Well, points changes everything. So I'd spend less time laboring over the intricacies of your carb and working over the mixture until we're sure that the ignition is all good. Hopefully Elan can get you hooked up with the settings there.

    And yes, the idle mixture screw adjusts the air signal.

    Would love to do the beer and brats, but I'm at work this week. Gotta keep bread on the table and beer in the fridge!
    Winter is Coming...

    Go GeocacheAlaska!

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    Member mod elan's Avatar
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    Yup '83.

    Main Jet: 160
    Needle Jet: O-8
    Pilot Jet: 30
    Needle: 6DP1 e-clip on 3
    Air Screw: 1.5 turns out

    Point Gap: .015" (.37mm)

    Weight: 285! Those are good machines. I really need to get my Olympique running. Kinda miss that old tillotsen spraying gas in my lap

    The page out of the shop manual refuses to cooperate here. Is it possible to fax it to you?

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    Default thanks folks

    this is cool. Now i get to fool with points. Tell me, where are they? On the muffler somewhere?
    I may try to get photos up of my new machine. After sitting in my garage for 8 years I got an old ski-doo 340 running. It is an early 80 model w/electric start. And the electric start works!
    My wife brought it out today to haul firewood. It is quite a bit faster than the Elan and except for the baloney skin got around pretty well.
    I cut wood all weekend, so hope to work on my old faithful Elan over the next few nights.

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    Default Hey hey hey

    It runs. The Elan is back to its old self. MOD ELAN was 100% correct about the points. I pulled everything apart, and used some emery cloth to shine them up. Pretty hard to tell what they looked like prior and post shining because the fan is in the way, but figured 10 pulls or so ought to do it. Gap was right on at .015".
    Back together it went, didn't even lose a bolt or washer. And off we flew. I pinched that throttle as hard as I could for as long as I could. We got a good 2 to 3 minute ride, and she ran fine! My hair was flying and I was getting bugs in my teeth. Ok, so I'm bald, no hair flew, but I'm sure there were bugs I was going so fast.
    I took her out on the Tanana, and across to the flats. Promptly got stuck in fresh deep snow. Oh well, that is what is nice about an Elan. A few tugs and we were off and running. I drove up into the trees, and wound around the big spruce. I pity the guy on the super long track that tries to follow that snake trail. I was cutting back and forth like a slalom run.
    Thanks guys for the pointers, faxes, and advise. Now on to the next mx issue, clutches on a Polaris 440.
    ARR

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