SOHC.co.uk Forums > CB500/550

Starting from cold issues

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Bryanj:
Biggest problem today is the fuel, its simply crap compared to the 70's mix. The ethanol separates out and the "volatiles" evaporate away.
Even a perfect engine does not like starting after more than a week of sitting, it can make a difference if you drain the carbs but i have on ocassion had to drain the tank as wall(usually if stood over a month though)

paulbaker1954:

--- Quote from: K2-K6 on December 12, 2020, 09:58:46 AM ---You've ordinarily got most conditions (as stated) going against you to initially fire petrol into combustion.

Full choke and "priming" may not help in this instance. With the plug and whole combustion chamber metals cold,  along with ingesting damp cool air,  then the mix is difficult to propagate a flame front fully so it snuffs itself rather than advances.  As soon as you get even a modicum of heating and normalising combustion conditions the problem will reduce.

You could try initially cranking it with no choke and listen for any firing,  then without stopping turning it over, start to bring the choke in slowly to see if that catches it.
This is just trying not to overwhelm the air with too heavy a fuel load initially so that the cold mixture is moved more toward "dryness" to favour getting it to light up. A bit like holding a newspaper across an old fire place to get it to draw  :)

--- End quote ---

I like the idea of cranking with progressive choke actuation. I agree with your combustion theory as the engine catches with full choke on no problem but just doesn’t seem to pick up, it’s almost feels like a flooding scenario

shifter:
Hi , I had the same problem with the 550 f1, on 69a carbs in the cold weather , the only way I over come this problem was to warm the motor / Carb by waving the hot air gun around the engine area , I don’t have these problems with the two CB200’s , S90 and Cbr , they all start with a nearly flat battery .don’t miss the 500/4   ,

K2-K6:
"I like the idea of cranking with progressive choke actuation. I agree with your combustion theory as the engine catches with full choke on no problem but just doesn’t seem to pick up, it’s almost feels like a flooding scenario"

It's almost exactly that Paul, the initial fire as it starts to draw fuel through into combustion lights off as it's only just beginning to suck the fuel,  subsequently with choke full closed the atomisation and quantity break down into too large a droplet size to be easily encouraged to burn,  hence the flooding feel.

If you can think of how to coax it over those initial few seconds of running then the localized temperatures start to rise and off it goes.

It's one of those fascinating areas of combustion,  that the spark plug only lights the mixture within it's immediate vicinity,  subsequent mixture is effectively daisy chained on one fuel molecule to another to reach the outermost perimeter of chamber. If the droplet size is not within range (everything cold etc) then as it goes away from the plug it just can't handover the torch, as it were. 
Often it'll give a few chuffs as the ignition advance is trying to get it going before TDC,  with failure before that point producing no crankshaft impetus.

So although it's cold and logic suggests max choke is needed, this can work against you. If you get it back into range by reducing the restriction then the conditions become more favorable to develop full combustion happening around TDC.

peterengland:
Always use the bike in winter if conditions are ok, pain to start when left for a week or so but never fails, usually start it twice a week to keep everything up and running. If its freezing cold and I've been away then a hairdryer on the intake manifolds works fine

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