Starting Mystery

[Otis]

I have just done a search of previous topics relating to starting issues but cannot find anything that helps to explain my strange situation.

I have already mentioned that my starter clutch needs replacing/overhauling but notwithstanding that, the bike should still kick start easily.

This is what seems to now be consistent. Ignition on, fuel on, choke full on, do not touch the throttle. Float bowl on Carb 1 overflows quite quickly. After wearing my poor old leg out kicking and kicking I eventually get a lumpy start and the bowl overflow stops. Cylinders 1 & 2 are firing as expected and the downpipes get hot very quickly. Even when downpipes 1 & 2 are too hot to touch cylinders 3 & 4 are ice cold. All of a sudden the revs scream up to 4K and i have to choke back considerably. What seems like ages but is probably no more than 1 or 2 minutes, downpipe 4 starts to warm and 3 comes along shortly thereafter. Choke eventually goes off and she idles okish at circa 1100 but occasionally stalls. Now warm she kicks first time and when properly warmed up the engine runs as sweet and free as any engine I have known. A little spin up the road and she is running beautifully and pulling powerfully (for a little 350). I could now ride her all day with no issues.

The next day, and the next, and the next repeat the above - very frustrating.

Incidentally, I have watched dozens of videos on how to check coils but none of them look anything like mine and I haven't got a clue where to stick the probes !

Any thoughts or guidance would be much appreciated.
Cheers
Graham

[Mikep328]

MY 400 does essentially the same except that it's a single cylinder (#4) that does what you describe.  The issue appeared after last year's winter layup.  Same as you noted, the bike runs perfectly on the road from any RPM/gear up to redline and idles fine at around 1100 RPM though, just as you said, occasionally at idle it will quit but restarts instantly when the start button is pressed.  After a run, all pipes are equally hot but if it sits there idling for a bit, #4 cools while the others remain hot.  Seems clear to my troubleshooting ability that it's definitely the carb and it's only an issue at closed throttle (idle).

TBH, I didn't feel like removing the carbs so I operated the bike this past year and ignored the issue since it didn't really affect normal riding.  Maybe next year I'll pull them.

[K2-K6]

The ignition isn't common to 3/4 and so has half-a coil, point etc involved in each .... suggesting they are ok up the line.

I've had plug caps give similar to this for individual items before. Dead cylinder at cold, chiming in when running, to ultimately run ok when all up to temp. One of those was a disconnect from HT lead in top of cap. Jumping across effectively when it got going. Took some time to find that 

Possible to swap #1 & 4 leads over  ? (Enough length available  ? ) to give functionality check either much work.

Start it in the dark, then look for "tracking" sparks around caps snd HT leads on 3 & 4 that may give you indication of leak that is improved by heat etc.

If the problem stayed where it was, that would point more at carbs.

Assuming ignition checks out:-

Cylinder cold, is either too little fuel or too much if ignition is OK.  Before starting, can you check drains on 3 & 4 float bowls to see if they are dry  ?

Or, turn off fuel when hot to run a few minutes with it no supply, then start next day cold WITHOUT  turning fuel on, this to check for flooding risk. See if they all chime in together.

[exvalvesetdabbler]

My 400 has always started a bit rough and spluttery on choke. Set it for a fast idle the after about  40 seconds to a minute the revs will shoot up and need the choke lever backing off a bit.  Exhaust temps warm up the same though.  I have always put that down to a bit of fresh fuel making its way through.

A gentle tap with a screwdriver handle on the float bowl  might settle the sticky float but ultimately there's possibility of crud stuck in  the valve. Don't rule out a wonky float frame that may have got bent and not straightened properly. I had it with mine where the float would stick if it  sank halfway down and got stuck, my fault I probably twisted it up trying to free it off when it got varnished up after being parked for a couple of years. Easy to verify, with the bowls off, turn the carbs right way and giive them a jiggle, then turn them upside down and see if one sticks.


Regards
Dave

[Otis]

Thank you for some really good pointers there.

I will check out those options and possibilities and report back.

The carbs were stripped down and bench balanced (not vacuum gauged) a year ago and afterwards, whilst still not perfect she would fire up lovely after a couple of kicks. I'm leaning toward your spark suggestions.

[Eldrick]

When I had a sometimes hot sometimes cold exhaust pipe it was a sticking (bent) inlet valve. Only found that out when I cleaned the pistons and that specific valve was tougher to remove.