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Black smoke and hanging idle after a full rebuild

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thomaspigeon:
Hello everyone,

So I’m encountering some issues that starts to be beyond my league. I’m still quite inexperienced and this my very first full project. I’ll try to be as thorough as possible into my set up and problems as I know you’ll ask me a lot of questions if not  ;D ;D

Symptoms
- black smoke when accelerating
- Erratic idle (I have to play with the screw all the time to get it right)I can manage to keep it steady
- Low revs sputter and strong engine brake feeling
- mid/high revs quite ok but struggle to come back down
- engine run rougher when hot

Now about the bike

Model : CB550K3
Intake : Pods K&N (I bought this bike as an unfinished project and the air box was discarded already unfortunately!)
Exhaust : 4 into 2 with a short muffler
Carbs : 069A
Main jet : 110
Idle jet : 42
Needle : aftermarket (I’ve lost one OEM so can’t fit them but if anyone got a spare 273004 I’m happy to buy it)
Needle height : 3rd from top
Float height : 23mm
Mixing screw : 1.1/2 turns was looking like the best one

Top engine rebuilt with new pistons and rings, new valve seal, valves have been lapped

Ignition
I made a DIY LifePo4 battery that seems to run ok (though no BMS on it)
Electronic ignition from David Silver
Spark advanced system with the springs a bit retighened to make sure it comes back down.
Coils : https://www.ebay.co.uk/itm/112789241229
Spark plug caps : NGK 5K ohm
Spark plugs : NGK D7EA

Sliders don’t seem to stick and it looks like they are coming back down ok. I’ve set up the slide heights with a drill bit, the synchroniser was looking ok even though the pressure is very low. Spark plugs are very sooty but dry.

It’s a full rebuild with a lot of mods so I believe it might take me some time to get there but your help would be very appreciated. Let me know what you guys think!

Thanks
Tom

thomaspigeon:
I forgot to mention, I tried to run it with a standard idle jet and the needle a bit higher but none of this helped, it actually was worst. Also the symptoms were quite the same.

What is hard to understand for me is how I have such a soot which means running rich while my idle is hanging which mean lean?

K2-K6:
Think you're quite close with assessment of mixture, black smoke being unburnt fuel in excess of what's needed.

As you say though,  slightly mixed message with hanging idle.

http://www.sohc.co.uk/index.php/topic,17769.0.html have you completed the first stage of method to sync in link? It's that bit that can easily get the throttle mechanism "hanging" slightly if not done accurately and would be my first call to examine.

The pattern needles are giving some very odd results in general site response and would be ideal to get that last one to remove that variable if at all possible.  In the meantime,  you could try setting them at top clip position (needle lowest) to get further assessment.

fogrider:
I suggest you replace the main jet O rings. Even if it's just to eliminate that as a possible problem, it's cheap enough to try. You want 1.2 section by 4.2mm ID. That size is available. The lightest smear of petroleum jelly will help them push in. Don't twist them !!
Make sure they go on the end with the shortest end boss. (it's the deepest location for the O ring.).

thomaspigeon:
Hey guys,

Thanks for you answers. @fogrider, the main jets are new with new o-rings so should be ok on that side.

@k2-k6 I haven’t done this method for syncing the carbs so will get through it probably this week end. Now dropping the needle might be a solution. So far the mixing screw doesn’t seem to do much. I remember with the stock 38 idle jet it was already doing the hanging idle and black smoke.

Also I’m wondering if the electronic ignition can fail at all. I could almost ride that way at some point but it wouldn’t be enjoyable as too erratic. It kind of misfire now as I think the plugs are getting too sooty.

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