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Discussion Starter #1
1982 Suzuki GS300 cruiser (GS 300 LZ)

Long story short, where can I find a definitive spec for this bike's carb float height? This little 300 was only available in the US from 1982-1984. I'm about to get into trial and error with the floats, but I'd rather set them correctly on purpose.

Many thanks. More details below.
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As far as I can tell it uses a Mikuni BS30SS twin carb. It has been garaged and unused for a long time, so it's been a bit of work to wake it from its slumber, but I think I'm down to just carb tuning now.

Checking with a clear tube the fuel level is 1-2mm below the float bowl gasket line, which seems reasonable. The floats are basically parallel to the gasket surface, which also seems reasonable. I was not able to find a service manual for the 300 so I bought a Clymer manual for the 400-450 model. As per that manual I checked for 22mm carb float height and it was almost precisely there already, so I didn't alter it, but that seems to be too high for this bike and it's running rich.

I'm pretty sure the richness is only (or mostly) in the pilot. Once I coax the engine up to higher RPMs it behaves more normally, presumably because the over-rich pilot has a smaller effect.

I'm unable to tune the idle. The idle air/fuel screws are all the way in and it still runs. Turning them out does not change the RPM at all. It still needs the enrichment circuit engaged a bit to start, which normally indicates a lean condition, but in this case I think it's because it gets air through that circuit also. It stinks of fuel, it bogs down or stalls easily, soot builds up on the plugs, throttle reacts slowly, engine speed decreases slowly, and when the engine gets really hot it will die and not start again. So... it's rich.
  • Carbs are clean and bench balanced
  • Needle valves seem to work fine. Carbs used to overflow when petcock was set to prime, but after cleaning they no longer do.
  • Fresh fuel, fuel line, fuel filter
  • Petcock responds as expected to vacuum
  • Air filter is clean and clear
  • No intake or exhaust leaks that I can find
  • Valve clearances and compression are to spec
  • Timing is perfect
  • Plugs gapped correctly, spark is healthy blue
  • Coil resistances measure to spec
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According to the list on this PDF, 2nd to last page, the Mikuni BS30 should have a float height of 17.5mm, not 22mm. I'm not convinced it's the same carb they're talking about though, because that spec is for the Yamaha XJ650 and my float could not move that much before bumping into the top of the float chamber. Also, that would make it significantly more rich and I think I need to move the other direction.
 

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Discussion Starter #3 (Edited)
Wonderful! Many thanks. I don't know why I wasn't able to find that service manual when searching myself, but I guess the site isn't search engine friendly.

For anybody else who finds this in the future, here is the service manual for the 1982 GS250T-GS300L:

And here is the owner's manual, which is much more readily available elsewhere:

As a specific conclusion to my post:
Fuel level should be 4mm below the mating surface of the float bowl. (without gasket)
To accomplish this, float height should be 21.4mm from the mating surface. (without gasket) Measure to the top of the rounded portion of the float, not the flat pivot arm part which is slightly taller.

Solved.
294066

294064
 

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EDIT: Disregard you you posted while I was typing went all rockstar on us and found manuals and other bits of unobtainium. Good job Sir.

Hi Jezza. The float hieght is bike specific. The manufacturers engineer for them to work up to certain degrees from vertical, I have read 20 degrees either way on some. That being said the specific angle that your carbs mount to the engine will necessitate different float levels from bike to bike for the same carb.

A service manual for that bike would have it but likely hard to find. Try Carl Salter, they have manuals free but sometime hit an miss with years or models. The clear tube method you used is good and may be the way you have to go. It would be worth it to pull them and measure the height once you get them dialed in so you know what worked in the future.
 
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