Starting

[sfcpiet]

I have found that triples with correctly set idle mixture always require a couple of revolutions with choke upon hot starting.  It's usually those that belch out petrol fumes at idle that will start without choke...  :

I also often use an exhaust gas analyser, especially with problematic motors, helps find the root cause of many troubles in no time!

I'd say the problem are the relatively large carbs and low vacuum while cranking, the engine simply doesn't suck enough to draw fuel from the pilot jets alone.  All is well as soon as intake velocity picks up.

piet

 

[MotoJouni]

I have here LB125, GT750 and SFC1000. All of these cold start very well when taps opened a littlebit before start (say 30 seconds or more to get the gasoline levels to settle well, first when you come to the bike), turning the throttle two times wide open, then choke to maximum,  and then start. Same method allways no matter are there accelerator pumps or not. The needle will pick up some gasoline and the float level goes a littlebit higher when the needle is not taking some volume from the float chamber.

Not sure about warm starting yet, different methods for different vehicles. Laverdas often need choke even when hot, immeadiately set if off when the engine starts (reasons which Piet said).

If an engine having a carburetor is not starting my universal method is to first give it more air and at the second more fuel, if not known which one it is needing. If it got too much gasoline it will start first because it could not be badly flooded yet. Giving more gasoline at first will flood it badly, if it was a littlebit flooded. Mostly with Lavs it was too less gasoline.

 

[Laverdalothar]

I'd say the problem are the relatively large carbs and low vacuum while cranking, the engine simply doesn't suck enough to draw fuel from the pilot jets alone.  All is well as soon as intake velocity picks up.

piet

doesn't cold starting typically mean the sliders are almost shut? Size of carbs should not matter to much there, right?

Besides that, what makes really a difference though is how fast the starter can spin the engine and which carbs you are using and in which shape they are. With the Keihins, cold or warm starting is no issue at all. Had far more issues with the 32mm Dell'Orto than with the 40mm Keihins... That said, the Keihins shut the intake completely I think for cold starting, they use bypass air-jets to define the right amount of air for the starting circuit IIRC...

 

[MotoJouni]

doesn't cold starting typically mean the sliders are almost shut? Size of carbs should not matter to much there, right?

Besides that, what makes really a difference though is how fast the starter can spin the engine and which carbs you are using and in which shape they are. With the Keihins, cold or warm starting is no issue at all. Had far more issues with the 32mm Dell'Orto than with the 40mm Keihins... That said, the Keihins shut the intake completely I think for cold starting, they use bypass air-jets to define the right amount of air for the starting circuit IIRC...

When the channels and carbs are of big diameter, they have more volume that reduces the pumping effect.