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Discussion Starter #1
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 here are my questions:

1. 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?

2. This carb has 3 mixture adjustments. Low speed mixture, high speed mixture, and idle mixture. At what point does the low speed mixture screw take over from the idle mixture?

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
 

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You should take the carb off and clean it out. Im sure it will be dirty. The fuel pump check valves may need to be replaced too. If you still have the issue, change the low speed jet 2 sizes and let it idle at about 1650rpm and tune the bottom end from there with the mixture. Make sure you blow out all little holes in the carb as they may be partially blocked.
 

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Discussion Starter #3
Thanks GC. I pulled the carb off the other night and went through it. It looked pretty much perfect inside, all the passages I saw were clear and there was no dirt or varnish at all. 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. I deinitely have a problem with the fuel pump for starters. The diaphragm port wont hold vacuum. I'm half tempted to spend the $30 for a new single outlet universal Mikuni pump but I'm also half tempted to just pop on a brand new Super BN with the pump built in for $180. Mikuni makes one that they say is a direct replacement for the one on the Superjet 650 which has the same engine as mine. Do you think the jetting should be pretty close? Close enough that I can get it to run good with some standard adjustments?

John
 

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Discussion Starter #4
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, the needle and seat should hold 10 psi for 30 seconds and if it doesn't the whole needle 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. I'll go through the carb with compressed air and carb cleaner again while its off just to be sure.

John
 

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Discussion Starter #5
I got all the carb parts and fuel pump today. Put the rebuild kit, new needle and seat in the carb. Its still leaking just a bit past the needle but it is popping off at about 10 psi (like I think it should) and it is holding about 7 psi for at least 30 seconds when I stop putting air into it so I guess its OK.

I installed the carb and new fuel pump but it still isn't right. Its definitely lean and although I can occasionally get it to idle for a little bit its definitely not right. When I pulled the fuel output hose off the fuel pump there doesn't seem to be a whole lot of fuel coming out but I don't know how much this thing is supposed to be pumping.

The pulse into the fuel pump seems OK to me but I don't really know how its supposed to feel. Is it possible that the reeds are bad and not giving a good enough pulse to the pump to get it pumping?

I'm at a loss with this thing. The fuel filter is new, both the pickups are OK, and all the lines are in excellent condition inside and out (I've had them off enough times by now to know :mad:) but its still seems to be starving for fuel. Is there some sort of spec for what the pulse should be?

It will idle with the choke on but as soon as I turn the choke off it starts to lean rev. To get it to idle with the choke off I have to back the low speed and idle screws way out (4 turns)and even then it idles like crap. I looked through the site that hosts the Yamaha manuals online but I don't see anything there for this thing. I've spent more time fixing this thing than riding it in the year that I've had it! I definitely got f'ed by the guy that sold it to me.

John
 
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