GT380B left pot only wont run on choke

[yeadon_m]

Folks,
One I should be able to fix but its been maddening me for ages. Symptoms is per title, the bike starts easily on choke (today, one kick) but it never fires at all on the left pot until after the choke lever is returned to the run position, when the bike idles on 3 cylinders beautifully, as slowly as 1000rpm. Its sweet on the road through the entire throttle opening range.
The carbs have been balanced while on the bike per the Suzuki / Mikuni manual, float heights (when last checked) were stock and I cannot think of something other than the choke jet is blocked - but it wasn't last two times I took it to bits. So while I am happy to pull the carbs again, I just don't know what I'm looking for.
What might I be missing?
Its not a problem as such, but its annoying.
Cheers,
Mike

[wardy]

Mike; I reckon it's got to be a fuel issue, as if you've got a spark and it runs well otherwise as it clearly does, the only thing left is that the fuel is not getting through for some reason when the choke is on.
very perplexing one this. Has it always done it or has it just recently developed.?
perhaps the answer is to have the l/h carb ultrasonically cleaned; that should clear out anything that should'nt be there.
Does it strike up straight away when the choke is turned off?
( by the way.. just ordered some ht lead from tim; thanks for that; 2 meters should be enough for both bikes)_

[BAZ]

I may be wrong but does the choke work on all cylinders or just two seem to remember something about this from way back.

[wardy]

Baz; the choke just operates on the l/h and centre pots on mine (1976 gt380a)
think it was different on the j and K with the separate carbs and handlebar mounted choke lever.

[yeadon_m]

Andy,

I think you must be right - came to the same conclusion myself but cannot blessed well find whats different about the setup or cleanliness of L and C carbs. To show how anoraky I have been I noticed that the size of the choke conducting holes in the C carb are bigger than the L - makes sense, as the C choke feeds C and R carbs as you say.
It has done this before (a couple of years back when I kept far less good notes!) - and then sometimes its OK - and I cannot say what the difference is.
It must be one or more of:
-float height (unlikely as it otherwise runs so well)
-choke jet in bowl (sure I tried swapping bowls to no effect - the problem didn't migrate)
-choke 'straw' that projects down into the well from the carb body (all clear, including the two pairs of tiny side holes)
-choke fuel/air flow when choke lever lifted (hmm...wonder if the choke fingers have fallen off the lifter - that'd be funny but I don't think so)
-throttle slide too open or closed (unlikely, as idles so well off choke)
None of the rest of the carb is involved in the choke phase - I think.
Is this the whole list or can any Inspector Morse's think of more to consider as I strip the carb again?
Cheers!
Mike

[wardy]

i suppose that all you can do is strip them again and triple check everything is clean and clear!
I never knew that the centre choke also fed the right one.... just presumed that there was no choke on the right!

[yeadon_m]

Andy,
If you look closely at this pic (GT380B carbs) you can see the tube that joins the outlet of the R and C carbs. That is only open to air when the choke lifter is up (choke on) and then it conducts rich mix from C carb outlet scross to the R carb. What an odd method!
Cheers,
Mike

[yeadon_m]

Funnily enough, having left the fuel pipe disconnected from the carbs for a few days and instead connected to a drain pipe to catch a few drips (fuel tap O-ring on the vacuum side has probably failed or got grit under it) it started on all three pots on choke. Not so good on left as others, but distinctly hot header while choke was still up. Very different than before, where not a peep from L until choke lever was down.

I'm now leaning towards a flooded carb on left, due to a slowly leaking tap as a probable cause, so will address the tap before pulling the carbs!

Float valves are not completely fuel-tight - they don't need to be in normal use - so a slow leak will allow fuel height to slowly rise. That its the left carb only might be related to the fact that the fuel pipe goes to left carb first then flows to C then R. A slow leak may bias carb overfilling on left as its just a drip now and then?

Cheers,
Mike

[Kettletimes3]

I have never owned a 380, but from my experience of carbs not running without choke, is blocked pilot jet.

[yeadon_m]

Thanks KT3,
I will re-check that one - though its idles off choke wonderfully well.
Cheers,
Mike