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Scooter carb help?

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Apr 23, 2020
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Cedar Park, Texas, USA
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.
 
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.
 
If it has a throttle position sensor, you might have to do a ecu reset.
 
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.
 
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.
 
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.
 
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. :(
 
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.
 
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…
 
Stock Vino carb has an electronic preheat connection on it IIRC. What have the temps been lately in Houston?
 
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.
 
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.
 
OK, so I tried again to fix this.

I had a new size 40 pilot jet to try in place of what was reportedly stock 37.5, but I have no good way to verify that. Eyeball test tells me the replacement pilot jet seems a hair bigger than the one it replaced. Since I was running 4.5 turns out on the pilot needle, I figured a bigger jet was in order. This isn't the stock carb for this bike, so who knows, maybe it had a 32.5 or 35 pilot in it. I put a sized-up main jet in it before I finished putting it all together a year and a half ago.

OK, sorry, on to the current problem.

Same problem really. Scooter won't start. I tried it with the pilot needle 1.5 turns out, which is my best guess as stock basic setting.

While it doesn't start, it also smells of fuel. That makes sense, since it should be putting fuel through the cylinder without burning it, but it got me thinking... Backtracking.

The problem I was trying to fix was poor running, hesitation and low power at low rpms and off idle. This was an issue that appeared gradually. Now that I have had it all apart and goofed with it, I'm thinking the main issue was a very clogged and dirty air filter. I thought the problem might have been a leaf in the fuel tank. I wound up fishing out the leaf, putting in an inline fuel filter and not messing with anything else besides ordering a new air filter which hadn't been installed yet. It started and ran fine, but still had the hesitation I was trying to diagnose, after this repair. Then a whole new problem appeared: all of the fuel leaked out through the carb while it sat parked a few hours.

I thought this was a petcock failure, but it turns out the scooter doesn't have a petcock, it has a vacuum fuel pump. So I replaced that part. But in my dufus wisdom, I also thought maybe the float needle was leaking too, so I replaced it with one that was not guaranteed to be identical but sure looked close enough, and critically, I reset the float level, which seemed to me to be way low. I figured this might be the cause of the hesitation. I used my eyeball test, conventional wisdom, which said that the "seam" in the float should be roughly parallel with the carb bowl mounting surface, since that's the way it is in the other Mikuni carbs I have worked on. The only reference I can find anywhere for proper float level for this type of carb says it should be 21.4mm +/- 1mm, but that's just a forum post for an entirely different kind of bike (a Suzuki GS125, available all over Asia, uses the same model carb).

So now I'm thinking I probably set the float level too high. Can float level too high cause it to NOT START? I haven't heard of that or experienced it before. Float level too low, in my mind, seemed to be something that could cause hesitation that it was having, along with making me set the pilot mixture super rich. But now that I look back, I bet that failing vacuum fuel pump was the entire problem all along, and maybe I should put the original float needle back in and try to get the floats set back where they were.

Thoughts?

b/c if I can't get this working reliably before my end of year commission check arrives, then I'm going to buy mrs72 a new fuel injected scooter whether she wants one or not. Man, I hate carbs.
 
So now I'm thinking I probably set the float level too high. Can float level too high cause it to NOT START?
Absolutely! As float height changes, you are adding/reducing the amount of fuel that will be drawn through the carb at all throttle positions. Did you bench test the carb to make sure the needle valve isn't leaking? Does your oil smell of fuel?
 
Welp, it was float height!

Thanks for the comments. I took it apart this morning and decided to try setting the float height. I lowered it (larger numerical measurement) and then got the scooter to start and run but it was running fuel out of the overflow while running. So the float height was still too high. This time I figured 21.4mm +/-1mm, I should remove the gasket and measure from the gasket surface on the carb and I set it to 22.0mm. Lo and behold, it starts instantly and runs without leaking fuel.

With the 40 pilot jet the idle was very fast so I dialed it in a bit and it still has a weird hesitation or softness, but I could dial out the hesitation off idle. Now it runs like it's supposed to but once the variator kicks in and it starts running under load and WOT, the revs go down and it's like it has a soft spot, eventually catches up. My gut says it's very lean on the main jet now. I need to find that sample pack of jets I got for this, bump it up a couple of notches and see if it clears up this WOT running thing. But at the moment it actually runs better than it did before I started messing with it.

Also I changed the oil, it had over half a quart of gas in the oil. Crazy what happens when the little fuel pump / petcock went south.
 
So, I bumped the main jet up a couple of notches, and knocked the pilot down. Now it's 110/37.5 I think, was 105/40 before. Now it runs much better off idle, doesn't have a stalling behavior when cold like it did (would stall a bit when taking it off idle when cold). But it still has some of that soft spot, and the idle required the pilot screw to be cranked all the way in for it to idle close to right.

Mrs72 is heading out on a cruise this weekend and I'm going to crack the carb open again. I bought jet kits for both pilot and mains, and I figured a way to swap jets in about 10 minutes. So I am going to swap it to 115/32.5 and see if that improves. My guess is it's running rich at idle and lean on main, the softness is from the slide coming up and opening the main jet where it goes lean. I did put a UNI filter in it, so it's not surprising it needs more main jet. I wouldn't be surprised if I have to go up to a 120 main.

The pilot jet is a mystery. I don't have the original that came in this carb, but it isn't the original carb anyway. This is a replacement of the same type of carb, but not identical or OEM stock part. Some reports are that it had a 17.5 pilot jet stock with a 97.5 main. Other reports say it had a 37.5 pilot stock, 100 main. Other reports are the Riva scooter that it replaced had "nearly the same carb" and had a 32.5 pilot stock with a 115 main jet, and miraculously had nearly 2x the power of the Vino with the same displacement. I'm gong to swap to that 32.5/115 in hopes I can get it to run more like a 11hp Riva. My guess is I will need an exhaust swap to get it to behave more like a Riva, but that will be worth it.

While test riding today, I noticed it had some kind of rattly noise which I think is likely valve lash. Needs a valve adjustment, which might wake up the performance a little bit too. I just need it to hang with my ~9hp !Vespa, which is carrying ~60lb more operator than this Vino typically has.
 
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