SOHC.co.uk Forums > CB350/400
Really bad cold starting. Ign or fuel?
Scottish Badger:
Cap, it's running from a separate fuel supply bottle at present, with fresh super-unleaded.
K2-K6, the coils are 4.8 Ohm so ought to be ok......
Not had a chance to do any further investigation this last week, the clutch went on one of my cars so that's been taking up my time. Turned into a nightmare of struggling to bleed the system, finding out I'd been supplied incorrect parts and now delays getting the right parts! Hopefully it'll be resolved early this coming week and I can get back to the bike!
Scottish Badger:
It's fixed!!!!
Hi folks. I finally got to the bottom of the bad starting. It was nothing to do with the ignition system, nor the carbs themselves.... but everything to do with the carb adaptor elbows on the head and the filter box connecting rubbers!
In frustration, I took the carbs back off and went through them meticulously - and found nothing untoward! I'd ordered up new mounts and rubber stubs in advance for the new engine (when I get round to building it!) but decided to fit them anyway. That was when I discovered that someone in the past has fitted the stubs the wrong way round and basically distorted them forcing the carbs into them - the combination of time and heat has kind of "set" them into that position, and most likely not sealing correctly. I also found silicon sealant on the o-rings between two of the adapters and the head, not good! The rubber connectors between airbox and carbs were hardened with age as well, and probably not sealing onto the carbs all that well - which I appreciate will cause midrange and possibly top-end fuelling issues, but shouldn't really have much effect on cold start.
My theory - the elbows onto the head weren't sealing properly and the cylinders were drawing air past them when cold, but they were sealing up after a little thermal expansion kicked-in, which is why I wasn't finding any leaks, I was checking with the engine hot!
After putting it all back together this afternoon, it started straight up within 2 seconds of pressing the starter button. Re-balanced the carbs again, engine purring like a kitten! The needle on no.1 gauge appears slightly different, that's how the camera "froze" them, the needles were constantly fluctuating a couple of degrees. [ Guests cannot view attachments ] [ Guests cannot view attachments ]
McCabe-Thiele (Ted):
So essentially an air leak on the induction side that reduced as the engine warmed up - you live & learn - good that you have posted the cause/fix.
Johnny4428:
Yes always interesting to hear of running issues but better still to hear of the fix.
K2-K6:
Great you've got a solution, always surprised at how even a small leak of uncontrolled air can play havoc with running an engine.
These (well most small engines) have such a small concise flow through the idle circuit that any spurious air supply route can make quite a difference.
All the better when you get something running on point.
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