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YBR125 "Hanging Idle"


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Hey guys... it's me again! :P

So my bikes gears are fixed - smooth as anything when changing - lovely. Now i'm getting to ride around a bit (aside from the abysmal headlight... that's another story!), i've noticed another little quirk. 

After riding around for an hour or so, I notice that the idle is closer to 2k than 1.5k. Occasionally when coming to a stop at junctions, with the clutch in, it will stay idling at 2.5-3k+. Opening the throttle a bit, then releasing, sometimes makes it idle down. The throttle (on the bars) is fully shut and even forcing it closed at the bar end does not let it idle down. 

I originally set the idle of my bike by letting it warm up for about 10-15 minutes, the mix screw is 1 1/2 turns out, and then idle screw adjusted to 1500rpm. 

The throttle cable does have a small bend + damage to the outer sheath (unsure why, it has been getting pinched between the stop on the bars and the RH stop on the frame) - and so is going to be replaced - but doesn't seem like the cable is sticking at all - the throttle is free to move.

Other things i've read were saying that perhaps a lean condition may be causing it (there is an air injection system fitted to the YBR, which AIUI injects air into the exhaust side to burn unburnt fuel, but relies on vacuum - if a vacuum leak then would cause lean running.

Any other ideas, aside from me taking the carb off and cleaning everything (which I expect it needs, as no service invoices to say this has ever been done!)? Have considered spraying around some of the pipes with an aerosol of some description to see if anything is leaking - I can't hear any hissing.

(Also, I believe normal, when starting the bike with choke on it'll start, sit at 1.5k rpms for ~5-10 sec, then rev up to 3k. Slowly as it warms up it'll increase until I shut the choke off, where it'll sit at 1.5k for a bit where opening the throttle anything other than *really gently* will rev it to 2k then die, but if you open gently it'll rev past 2k and be good. Give it a few more minutes warming up and then this characteristic goes away too.)

Edited by sparxooo
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Could be an air leak somewhere. Try spraying wd40 or some brake cleaner around all the carb/intake joints etc.

 

You'll hear the engine note or idle speed change if any of the spray gets pulled in

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Feeler gauges on the way to measure the valve gaps - spraying aerosol around didn't affect the bike at all (nor did pinching the hoses to the AIS valve).

What did affect the bike is when I removed the air filter the RPM increased slightly and was a little more erratic - air filter back on and it calmed down again. Spraying a tiny amount of brake clean into the air box caused the engine to stumble * a lot * but cleared after opening the throttle.

Idling when warmed up for 10 mins, the bike will fluctuate 1400-1500 with no discernible pattern. After riding around (1 1/2 hours earlier), it was idling back at 1.9k again when I got back, but no hanging idles at junctions.

Wondering if I should just remove the carb and clean it thoroughly too. New throttle cable is here that can be fitted too.

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Took the carb off today - gave it a bit of a clean before disassembling.

The starter cap (that the starter plunger comes through) however, as soon as I touched it, has almost disintegrated. I'm trying to figure out if this has a sealing role, or just to prevent dirt ingress.

Small little bits of brown stringy substance around the bowl gasket, but also some present inside the diaphragm and the small orifice in the cover which I pushed out with a nylon brush bristle.

Having looked up the price for the o ring inside the diaphragm cover, i'm hoping that they're ok!  (£9!)

The starter cap isn't sold separately, but the whole starter set is only £3.40 anyway. 

Edit: Have since looked again to measure the o-rings, wondering if I could source them elsewhere. The emulsion tube o-ring (that the main jet sits on top of) fell apart the moment I even attempted to move it.

Have found a seemingly very assorted o-ring set on Amazon which i've bought, along with ordering the appropriate pilot/mix screw sets too as their o-rings are squashed and hard, and a starter set (for the cap, although the shaft of the starter has black carbon type build up, so will replace it all).

Edited by sparxooo
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Well, that all sounds suspicious...

However, I may not be much of a mechanic myself, but I do know a few things about sealing, and I've gotta say this...
Yes, you might get lucky and find an o-ring that's an apparent perfect match, but not all o-rings are made equal!
It may be a perfect size and profile, but it may not be a match chemically (for example it may not be able to take heat or vibration or whatever like the original).
As much as I object to £9 for a bit of 'rubber', it's a small price to pay for something that will then last another 20 years as opposed to something that may last 2-3 years and not do a proper job for that time anyway!

Long story short, my advice is buy a proper one! lol

 

Oh, and use the specified grease too (be it lithium or red rubber grease (on inspection, as we speak, my tube is gently leaking in a pool on my computer desk, damnit)!

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3 hours ago, Snakebite68 said:

Well, that all sounds suspicious...

However, I may not be much of a mechanic myself, but I do know a few things about sealing, and I've gotta say this...
Yes, you might get lucky and find an o-ring that's an apparent perfect match, but not all o-rings are made equal!
It may be a perfect size and profile, but it may not be a match chemically (for example it may not be able to take heat or vibration or whatever like the original).
As much as I object to £9 for a bit of 'rubber', it's a small price to pay for something that will then last another 20 years as opposed to something that may last 2-3 years and not do a proper job for that time anyway!

Long story short, my advice is buy a proper one! lol

 

Oh, and use the specified grease too (be it lithium or red rubber grease (on inspection, as we speak, my tube is gently leaking in a pool on my computer desk, damnit)!

So, aside from the little o-ring inside the diaphragm cover, the emulsion tube o-ring, and the big honking ones on the carb that connects to the big rubber bit (and the one on the rubber bit), the others come with the pilot screw/throttle screw assemblies. You don't want to know how much they want for the actual diaphragm (which THANKFULLY appears a-ok!)

Taking the mix screw/throttle screws out and checking the o-rings on those, they're relatively hard, and the throttle screw one actually is squished out and flattened where the "seat" (washer) has been pushed into it - not sure the spring would have done this by itself, so wonder if someones rammed it in in a past life!

The ones i've ordered are allegedly fuel resistant rubber - even if it gets me going a bit quicker than waiting for the parts from Yamaha - WeMoto are grand but they're only as quick as the supplier.

A mysterious box of unicorn magic the carb really is - had quite an afternoon exploring all its internals and wondering where on earth tubes lead to (then finding it comes out in a tiny little hole). That, combined with a tuning guide which explains what jet gets used when, really has helped me understand what it's up to!

No grease is specified in the Haynes manual for inside the carb - Lithium salt grease for the engine side of the rubber mount. I've greased the inside of my new throttle cable with some lithium spray grease, after toying with using silicone spray, or the little bit of clean engine oil that's dripped into the bag I put my oil funnel into! More grease ended up on me, than in the cable. Nice smooth action there.

Any harm putting a bit of silicone spray on the throttle slide? Metal surfaces moving on metal surfaces with no lube make me a bit cringey!

Edited by sparxooo
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12 hours ago, sparxooo said:

More grease ended up on me, than in the cable. Nice smooth action there.

 

Yeah, I had the same issue doing my speedo cable!
Back in the days of doing my bicycles we'd spray down the cable outers with no problem, what's the deal with the overpressure doing it on motorbikes??? lol

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Nothing new about my bike! :D

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