Carb tuning - Classic symptoms, but what of?

[tanto]

Got the NC30 back together on Sat with the 112/116 jets (original 116/118 were running rich with Chiang Mai's altitude & temp. Dr Rick O. consulted the charts, and prescribed the 112's). Beautiful 32C day, and the bike felt fabulous for twenty miles up and down a nearby dual carriageway with the engine pulling strongly throughout it's range. One symptom, slowing to a stop tended to result in the revs dying away and the engine stalling. Idle at 1400 was fine.

It was a different story today. Heavy rain left high humidity and 27C, not to mention a debris strewn road. Once warmed up to operating temperature, the engine was unhappy with very poor pull from stationary. Had to really gun it to catch enough beans to move off without stalling. The symptoms were consistent, with the engine anaemic at low revs. Once the revs picked up beyond 5k it was a different story. Plenty of power 5k to 12k. Nonetheless, the pre-existing problem of the engine revs dying away on coming to stop still manifested itself. Frankly, it got a bit tricky navigating lights & junctions to get home.

Two different symptoms I guess means two different problems. My stumbling block is, at this stage I'm not sure what the correct adjustments should be. The revs dying away on slowing, I'm going to guess needs some alteration to the low speed screw (it's at 1.8 turns out), or an adjustment to the float height (standard setting).

As for poor running in cooler weather, does more available oxygen equate to leaner running? If so, what's the fix for a lean bottom end of the rev range with the NC30's carbs? I think the advice of a carb guru is called for!

[Neosophist]

Put the pilot screws on 2.5 turns out.. this will only affect idle mainly.

You can also lower the float height (only by 1mm) this will let more fuel into the bowl and richen up the idle - 3000rpm range a little

[80sman]

Could be a whole host of things giving you these problems,

What condition is your fuel tap and vacuum line in?

Just a thought that it could be fuel starvation to the carbs.

I'd certainly look at that first as it's easiest. Then setup the carbs as per the guide at the top of the page.

Make sure the carbs are all spotless inside.

Envy you in Chiang Mai, there's some beautiful countryside around there. I hired a cb400 while I was there and rode over to Pai, fantastic run although got hit with heavy rain on the way back.

[tanto]

Put the pilot screws on 2.5 turns out.. this will only affect idle mainly.

You can also lower the float height (only by 1mm) this will let more fuel into the bowl and richen up the idle - 3000rpm range a little

The starting point was everything (air filter, carbs, etc) reset to standard. Now the jets have changed, I guess it'll need a bit of fine tuning. I'll try backing out the pilot screws a bit further. The static idle is smooth, and adjustable via the idle screw from 900 rpm to 1500+. Will the pilot screws have an affect on the engine dying when it comes off the throttle? Or do they affect the static idle only?

Now the engine is running leaner, lowering the floats sounds like it makes sense. Does the float height only affect the low rev range, or do they influence the entire range? I've been reading around on this, but found contradictory views.

The fuel line and vacuum lines are both new, from R.O. The tank is completely clean, as are the tap filters. I'll double check to see the vacuum line isn't pinched.