Weired starting

My Xj600 starts when cold but there is definitely a knack to it.

Full choke - wont fire
1/4-1/2 choke - wont fire
3/4 choke - fires right up (no throttle)

Any other combination of choke/throttle when cold and it will turn over without firing however long I try it using prime or not. Carbs have been thoroughly cleaned a few times now and I'm running a fuel filter. I should also add that even after cleaning all earths and connections my spark is not bright blue but the battery is fully charged and I have even used a car battery.

I just don't understand it. Can someone please put me out of my misery by either shooting me or explaining why

 

Well I'll give you a choice. I have a .22, a 270 savage and a 1903 Parker Brothers 12 ga. shotgun......just kidding!!! Cold starts seem to be a real problem with these bikes. I have trouble with mine and I ride all winter. The only things I know of that really helps are clean carbs, fully charged battery, iridium plugs and dyna coils. Last two are the only ways to get a better spark that I know of.

 

Cheers for that BM and its nice to know that the gun is available if things get tough although if it means a trip to the states to go get it I might feel better by then

 

Ritz, what color is the spark you are seeing? Weak coils can be picky.

 

I find that mine is a little picky about doing it right the first time, by which I mean that if I estimate the amount of choke I need based on the temperature, set it, then try to start, and I'm right, it fires up really pretty. Otherwise, it gets cranky and takes a few cranks before it'll start. At this point, I usually set the choke halfway, and ease the throttle on and off until it fires, then I let it warm up under choke.

Short answer would have been to leave the throttle closed and set the choke higher than you think you need, in my experience.

EDIT: Re-read the question--

The problem is that fuel and oxidizer need to be gaseous to react properly. All liquids vaporize (evaporate) at a temperature dependent rate. Imagine boiling water vs. room temperature water--boiling water is vaporizing at a much faster rate. Same is true with gasoline. When the cylinders and the air inside them are cold, it doesn't vaporize very well, so you need to put more of it in to get enough gasoline vapor (gasoline gas) in the air for it to react with the oxygen. Reactions can only reliably take place at certain ratios of fuel to air, so you're hunting for a choke setting that gives favorable cylinder conditions.

 

http://xjbikes.com/Forums/viewtopic/t=8918/
that might help ya...I know you can get it to start when cold but this helps

 

Robert, spark isnt blue, more yellow really. Just cleaned all earths and connections again and actually it has started first time every time the last 3 days, choke on and no throttle so I think I'm kinda OK at the moment.

Is it usual for both sets of coils to go weak?

Thanks for the pointers all.