Author Topic: Starting  (Read 1159 times)

Offline nick7

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Re: Starting
« Reply #15 on: August 09, 2017, 08:41 »
Lots of knowledge on here! Anyhow my instinct which may be completely wrong is that with an engine that runs superbly, no smoke no oil burn idles  hits rev limiter before you know it starts on the button..albeit needing warming up before good idle per Mr Nairns tuning ...does not need fiddling with nor do its carbs simply because it needs a brief choke ..like a second..to help starting when hot. Thank you!

Offline SimonR501

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Re: Starting
« Reply #16 on: August 09, 2017, 09:48 »
Don't worry Nick! My Jota 120 is the same - needs just a whiff of choke if the engine is hot to start.
It is a Keith Nairn tuned one as well.

I would imagine that residual heat evaporates fuel off! Hence the brief period of choke.
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Offline CLEMTOG

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Re: Starting
« Reply #17 on: August 09, 2017, 10:03 »
ha, so,   Nicko's bike wont start when hot without a wiff of choke and nor does Simo's  !!

your carbs will fill themselves up with fuel, should any evaporate, due to residual heat, as long as the pet cocks are turned on and fuel is available!

argument busted
CLEM

ps TOG restarted instantly without choke when I managed to stall her following PAL up a very tight and twisty Dolomite road after at least an hour plus of total thrashing, in 36c ambient, because I got the cogs wrong, stab button, instant restart, while still hard on brakes, it may have even been enrichment due to fuel surge/road bumps/braking, but she still restarted no choke.

Offline SimonR501

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Re: Starting
« Reply #18 on: August 09, 2017, 11:05 »
Just stating it how it is Clem!
Keith has got the exhaust analyser when he sets them up so can detect rich or lean shifts.

I suspect that our engines have exactly the right amount of fuel/air in normal circumstances, but when shut down it takes a second or two for the flow of cool air to reset normal conditions inside the inlet and re-establish normality. Engines don't like idling when lean, so the brief period of choke will get the engine up to speed.

In summary, I think your engine is running richer at low rev's.
 
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Offline nick7

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Re: Starting
« Reply #19 on: August 09, 2017, 11:20 »
Yeah Tog is rubber ducked Clem!😂have you fitted the new ignition yet are are more dyno runs scheduled? Hey could have used your help at tge w/e when i managed to  screw up taking thecwheels out of xt for tyre change..yup, both ends on deck sans wheels..lovely. Sorted now..tyre shops wont fit to those unless to loose wheels...sorry thread hijack!

Online sfcpiet

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Re: Starting
« Reply #20 on: August 09, 2017, 19:28 »
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
180s feel quick, 120s are...      If it ain't broke, fix it till it is.

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Offline MotoJouni

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Re: Starting
« Reply #21 on: August 12, 2017, 23:46 »
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.
« Last Edit: August 13, 2017, 00:03 by MotoJouni »
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Offline Laverdalothar

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Re: Starting
« Reply #22 on: August 13, 2017, 10:09 »
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...

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Offline MotoJouni

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Re: Starting
« Reply #23 on: August 13, 2017, 18:30 »
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.
SFC 1000 Classic 1989
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