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  1. #1

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    1990 650LX Carb (BN) Troubles

    I am having an issue with it running lean at idle/low rpm and falling on its face if I open it up from anything below 1/4 throttle. The low and high speed screws are supposed to be 1 1/8 turn out +/- 1/4 turn. I left the high speed alone since it runs OK up top and found the low speed screw 2 turns out. I set it back to where it should be as a baseline but it runs worse now. However, it runs pretty well with the choke on. I already ordered a genuine Mikuni rebuild kit, and the fuel filter is brand new so I have two questions:

    1. I see the low speed adjustment on the throttle cable side, and the high speed adjustment on the opposite side near the bottom. I found the book for the Super BN on Mikuni's site but nothing on the standard BN. What is the second adjuster screw on the side with the high speed mixture for?

    2. Based on the fact that the thing runs well with the choke on is there anything in particular I should be looking for or at when I tear the carb down?

    We're going away in 2 weeks on vacation with this ski and I'd like to have it running pretty well before we go.

    Thanks.

    John


  2. #2

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    OK, I found some info by looking at Mikuni's catalog on their site. The mystery screw is an idle mixture screw. I have the carb off so I'm going to go through it anyway. Any input would still be very much appreciated.

    John

  3. #3

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    I pulled the carb off tonight and went through it. It looked pretty much perfect inside. All the gaskets came off in one piece so I put it back together for now. The fuel pump on this model is not built into the carb but attached to it on a bracket. The pump looked kind of rough, I might try dropping one of those Mikuni universal ones in it. I'm going to put the carb back on tomorrow and run it in the driveway for a little while to try to get some kind of base tune on it. What kind of fuel pressure should I be seeing going into the carb from the pump?

    John

  4. #4
    Moderator OsideBill's Avatar
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    Post up a picture of the carb and fuel pump.

  5. #5

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    It looks like this carb is a "one year only" type deal. The part number supersedes to the 1991 and 1992 WR650LX and those both have the fuel pump built in. I'll post some pics after I put the kids to bed.

    I made a pop off pressure tester today but the local hardware store didn't have a gauge below 160 psi and the only thing I had lying around was a 300 psi one that's as old as dirt. I tried to use it but I can't get a decent reading on such a large scale gauge. I'll stop and pick up a lower pressure gauge tomorrow so I can get a decent reading but it looks like the needle is leaking anyway.

    What kind of pressure should I be looking for? About 25 psi?

    The fuel pump is done too, I can blow in the pulse port and I get some leakage out one of the fuel ports.

    I have a brand new triple outlet pump sitting here, can I block off the two unused outlets and give it a try?

    A brand new SBN44 that is jetted from the factory for a Super Jet 650 is $222. I'd imagine that jetting should be damn close for the 650 I've got. I'm tired of playing games with this thing and would like to just get it together this week so I can test run it this weekend before we leave for vacation the following weekend. Putting the SBN on would solve the old carb problem and the fuel pump problem.

    John

  6. #6
    Moderator OsideBill's Avatar
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    Pop off pressure is determined by the size of the needle and seat "N&S" and spring that are installed. Fuel pressure and pop-off pressure are two different things. I have attached a mikuni manual that explains pop-off pressure and other factors.
    Attached Files Attached Files

  7. #7

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    Thanks Bill, I've read that before but I went through it again just to see if I was missing something. I think I had the two of them down pat all along. Fuel pressure is the pressure the pump is putting out into the fuel inlet of the carb, pop off pressure is the pressure at which the needle valve is pushed open by the fuel. Here are pics of what I've got:











    After I popped it back on tonight I wasn't getting any fuel to the carb. I checked for a pulse from the crankcase and that felt very strong, so I cranked it over with the fuel outlet from the pump aimed into a soda can and still no fuel even after 3 or 4 15 second bursts of cranking with the starter. This ski did have a broken coupler in the exhaust that was allowing lake water into the bottom end when it was shut off and causing the engine to hyrdo-lock. Maybe the pump was damaged as you had suggested in a different post. The diaphragm is definitely leaking when I blow into the pulse port on the pump. I might be able to just get away with an inexpensive universal pump but I'm nervous that the pump won't be the only problem and then I'll be out of time to fix it before vacation.

    Can you give me your thoughts on my running the BN444043 that is listed in that link you posted? How about the BN44408052? I can get either one brand new for $189 shipped. The 043 is supposedly an OEM replacement in the 650 Superjet so I'm thinking it should work in my 650 LX. I'm at the point where if it's $189 standing between me and this ski running good on vacation then I'm willing to cough up the money for it. If that's not going to work for me then what about one of the other SBN44's that are listed in that manual?

    Thanks!

    John

  8. #8
    Moderator OsideBill's Avatar
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    Not sure if the other models listed will fit, these are pretty simple units, I would go with a fuel pump they are cheap around $30 on ebay new. Look in the snow mobile stuff they use them also. or just search on Mikuni Fuel pump

  9. #9

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    Yeah, they seem pretty similar to the ones we use on the 2 stroke ATV's and dirtbikes with the exception of the diaphragm set up in place of the floats/bowl.

    What is your opinion of my just putting on the one that says its a direct replacement for the 650 motor in the Superjet?

    I already have a brand new triple outlet pump sitting here. Can I block off the two outlets that I'm not going to use just to see if it will run properly like that? If it does then I'd order up a single outlet Mikuni pump to permanently replace it.

    What pop off pressure should I be shooting for with this? I got the proper gauge today and will be going outside to have a go at it again in a little bit but I don't know what I should be looking for.

    John

  10. #10

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    Thanks for taking the time to help me with this Bill.

    I was able to do a decent test tonight since I now have the correct gauge. First of all the fuel pump is definitely toast. As soon as I put any air into the pulse port it immediately starts to leak out the other two ports. The needle pops off the seat at about 10 psi. The seat is marked 2.5 and the spring appears to be shiny silver so if the spring is in fact the shiny silver one, that spring/seat combo should be giving me 12psi. I'm guessing that for a 19 year old machine a 2 psi loss of pop off pressure isn't that bad. However, I am getting seepage around the needle as soon as I apply pressure and from what I am reading that isn't supposed to happen. According to the "How To Make A Pop Off Tester" program I downloaded from here the needle and seat should hold 10 psi for 30 seconds and if it doesn't the whole need and seat assembly should be replaced. I'm going to order the following from the local Parts Unlimited dealer:

    1. Mikuni BN Diaphragm Repair Kit - MKBN3844 (I can't use any of the fuel pump parts but the rest of the stuff looks like its just what I need)



    2. Needle/Seat Assembly - VM26/26-2.5



    3. 65g Needle Valve Spring - 730-03030 (It looks like this comes with the diaphragm kit but for $2.00 I'm not taking any chances)

    4. Mikuni Single Fuel Pump - DF-44

    I have to hunt down a carb gasket or get a sheet of gasket material and make a new one. The old one is in one piece so worst case scenario I can Hi-Tack it and re-use it until the new gasket arrives. I think that has to be everything. If they can get it to me for Friday or Saturday I'll have a chance to have a go at it this weekend. If there's anything I'm missing please let me know.

    John

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