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Scooter carb help?

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mrs72's scooter won't start now that I "fixed" it. it's a '06 Yamaha Vino 125 with a Mikuni BS26 (stock-ish) carb that was running fine before I messed with it. I replaced the float needle and reset the float height to 17mm, also removed the pilot jet and ran a wire through it, put it back. I also replaced the air filter, stock paper filter was mega clogged, I put in a UNI drop-in foam filter.

Here's what it's doing:

It cranks and won't start when fully assembled.

If I pull the airbox off, I can get it to fire if I put my hand over the carb inlet for like 1/2 second while it's cranking, which BTW splashes fuel out the carb. It will fire for a second and I can even keep it running for a few seconds if I get it to fire then take my hand away.

There are no specs for the pilot mixture setting. I set it by trial and error and feel, riding it on the road with it fully warmed up. I had it 4.5 turns out, which is where it is now. I did try tinkering with this thinking maybe it was too lean.

Anyway, can someone with more real world experience with small singles carbs offer some help here? I'm sure I did something wrong when I had it apart but I don't know where to look.
 

DFW_Warrior

Hmmmm.....
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My first thing to check would be to make double and triple sure that the air passageways are 100% free and clear. Those small little carbs are notorious for having those get clogged in no time at all. 4.5 turns seems like a LOT to me, but having never messed with that particular bike, it might be what it needs.
 

Rydah

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If it has a throttle position sensor, you might have to do a ecu reset.
 
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Paper towel in the carb inlet that you forgot to remove? I've done it.

Otherwise has to be the float needle or the float not moving right to control fuel to the float bowl. I deal with single small engine carbs on dirt bikes all the time. Not a fan of wire cleaning any jet. Pilot jets are soft and can be altered easily, but that is not your issue on a no-start deal. I clean jets with air and solvent and pilot jets just replace if possible.
 
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The thing may just be too lean to run right without the paper air filter. I recall being instructed the carbureted bikes are set up very lean to the point where they don't like to run without the air filter in place. I have to wonder if the drop-in UNI foam filter is too free-flowing.

Doesn't it have a manual choke? Maybe it's new enough to have an auto choke.
 
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Yeah it is auto choke. I'll try the old air filter. I did actually think of that, just didn't want it to be the problem. Seems it shouldn't affect it at idle. I have a bunch more main jets for this carb if it is running lean with the uni.
 

DFW_Warrior

Hmmmm.....
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Not a fan of wire cleaning any jet. Pilot jets are soft and can be altered easily, but that is not your issue on a no-start deal. I clean jets with air and solvent and pilot jets just replace if possible.
Same here, but if I have to do it I always reach for some strands of really thin and soft copper wire I have laying around. Usually solvent and air does the trick, but sometimes you do just have to get a little more forceful. Heck, it seemed that pilot jet on the XR200 would clog if you left it overnight with ethanol fuel in it. :(
 

bwdmax

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4 strokes generally have a fuel screw. If it is on the bottom and on the engine side of the carb it is a fuel screw.

Usually but not always a fuel screw that is more than 2 to 2 1/2 turns out means the pilot jet is plugged or too small. The other thing is an air passage is plugged not allowing air flow for the pilot jet or fuel screw passage. One more thing is if the idle screw is in too far holding the slide up you are allowing air through the main part of the carb weakening the air signal to the pilot circuit.
 

bwdmax

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An intake valve not sealing will also keep the carb from functioning correct. You mentioned it blowing back through the intake.

With a carb all fuel in delivered by venturi effect. The fuel is sucked in by the air velocity. The way the air enters the intake is known as inlet signature and there are lots of factors that play into it. Cylinder ring seal, intake valve seal efficiency, cam shaft profile.

Wish you were closer…
 
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4 strokes generally have a fuel screw. If it is on the bottom and on the engine side of the carb it is a fuel screw.

Well this is on the top of the carb and certainly on the "engine side". I can find no manual or instructions on this carb. Doesn't really matter though, I tried starting it from 0 turns out all the way to 4.5 turns out in 1/2 turn increments and same result.

Usually but not always a fuel screw that is more than 2 to 2 1/2 turns out means the pilot jet is plugged or too small.

Yeah, that's the conventional wisdom I assumed, and of course I already cleaned the pilot jet.


The other thing is an air passage is plugged not allowing air flow for the pilot jet or fuel screw passage.

That's a definite thing to check considering how filthy the original air filter was and I might have over-oiled the UNI filter.

An intake valve not sealing will also keep the carb from functioning correct. You mentioned it blowing back through the intake.

That's just when I put my hand completely over the carb inlet. It's like the vacuum goes so high it sucks in way too much fuel and overflows some of it out of the intake.

Wish you were closer…

Yeah, no kidding.
 
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FYI, this is the carb that's on the scooter:

1669047614593.png


The pilot mixture is adjusted via part #23 which in all appearance is a pilot needle metering fuel to the #30 pilot jet.

Oddly enough, it looks like the pilot jet is the same as the ones used in the BST33s in my GS500. So I have a bunch of them around here, but I think the stock jet size is same as Suzuki stock, 37.5, and I need a 40. I will order some parts, because 4.5 turns out is way over the limit. It could be that it's just too lean at idle with the foam air filter.
 
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