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.

[Sesman]

Hallelujah. Glad it worked out. I genuinely didn’t know there was a right and wrong way round for the carb rubbers. Is this just a 400/4 thing?

[taysidedragon]

Hallelujah. Glad it worked out. I genuinely didn’t know there was a right and wrong way round for the carb rubbers. Is this just a 400/4 thing?

On the original 400f rubbers they are numbered, 1&4 and 2&3 so they get fitted in the correct position.

[Sesman]

So it’s just a 400/4 thing?

[Scottish Badger]

Gareth, the adaptors were in the right positions, but one was mounted 180 degrees out! Someone must have had one heck of a time and really persevered to force the complete carb set back in initially.......!  I've found a few "bodges" as I've been working my way through the bike, at least I now know it's safe to go out on (when it warms up a bit, lol).

Just ventured back out to the workshop - fully cold, started first prod of the button with 3/4 choke. Happy days!

[taysidedragon]

Gareth, the adaptors were in the right positions, but one was mounted 180 degrees out! Someone must have had one heck of a time and really persevered to force the complete carb set back in initially.......!  I've found a few "bodges" as I've been working my way through the bike, at least I now know it's safe to go out on (when it warms up a bit, lol).

Just ventured back out to the workshop - fully cold, started first prod of the button with 3/4 choke. Happy days!

Great news. All ready for the good weather. 😎

[Sesman]

Sound news. Bit of a faff, but glad it worked out ok.

[Johnny4428]

The 550 ones are the same, got to be the correct ones on the outside, with small tabs that seem to point inwards if correct, so 1 and 2 tabs pointing inwards to face each other and 3 and 4 facing each other. This way you know they are in the correct position and also you know they are not 180 degrees out.

I must add that these were replacement rubbers so can’t vouch for originals.

[Sesman]

Apologies, was confused between carb inlets and inlet stubs.

[PatM]

Im just going to add my tuppence-worth as mine does exactly this fault and ive never been able to find the reason behind it.

It will start like it did when it was brand new after its been running- but leave it a couple of hrs till it gets cold, and its a real bugger. its also running week on one pot, yet when I did a compression test, it was the same for all 4. Ive even tried another bank of carbs- and same problem.

Now, due to ill health and such personal reasons- Its not turned a wheel for nearly 3 years and I think the cam chain tensioner may be siexed- as the nut/bolt has been repalced at some time and loosening it has little effect. My fear is- snapping off bolt/studs- but my next move would be to tape the top cam cover off and check the cam-chain and horizontal mark to make sure its NOT a work cam-chain.

[Sesman]

I’ve found in the past that old carb rubbers can be the cause of all sorts of running/starting problems.