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.

[Otis]

Today was the first opportunity I have had to consider the solutions put forward by the very kind contributors to my post.

What I found today was quite different to my previous posts.

The bike had been sat for a few weeks and knowing that the electric start doesn't work I knew I would need to kick start it. On this occasion, she fired up on the third easy kick but this time cylinders 1 & 4 warmed up quickly and 2 &3 Lagged behind, very different to the previous scenario!

I had noticed that cylinder 4's carburettor overflow was pouring. I tapped the side of the bowl and it stopped. After running for a few moments it settled into a smooth and seamless idle and couldn't have sounded better.

I tend to think this is a carburettor issue and not a spark issue?

[McCabe-Thiele (Ted)]

When I have been out on my 400 I turn off the fuel tap a short distance from home, by the time its on my driveway its starting to splutter a bit so I keep the revs up until the engine runs out of fuel, this drains the bulk of the fuel out of the bowl.

In my relatively short experience with my 400  I find that on turning the petrol tap back on it sometimes floods as the float valve has stuck open. I have found that just rocking the handle bars from left to right when off the stand immeditely stops any flooding.

If I am planning to ride out several days in the same week I just switch off as normal leaving the bowl full of petrol. It does not flood in this situation.

[exvalvesetdabbler]

I had that in one carb where the float would stick down, a gentle tap with a screwdriver handle would in most cases sort it out.  Like you I turn the fuel off about 3/4 mile from home if the bike isn't going out until the next weekend.  Saves on it stinking out the garage  a bit as well. It also starts a lot easier when the carbs have had a drink of fresh fuel.

In the end it was found to be the frame of the float has got twisted when trying to dismantle a gummed up pin after being parked for a couple of years. 

Dave

[kevski]

Sounding like a carb problem, ie pilot circuit, pilot jets maybe gumming up, the overflowing down to float needles either particls jamming them open or jamming and not closing, I took the needles out of mine and polish the 3 ribs with a 1200 grade paper and polished the tips lightly with solvol along with the seats using solvol and a cocktail stick, overflowing cured.

[exvalvesetdabbler]

A thin cocktail stick with cotton wool wrapped around the end to make a skinny cotton swab does work well to clean up the float valve seat with a minimum amount of metal polish.  It's usually the ridge worn into the pointy end of the float needle that spells trouble.

I wish you could get the float needles with a viton tip like Ford used in their carbs.


Dave

[DomP]

I managed to find viton tipped float needles for my xs650 that were intended for another bike entirely, often wondered if you could get them for cb's

[Seabeowner]

My CB550 came with a mix of float needles, a couple are rubber tipped (probably not viton but they have caused no issues) so I am sure they are available.