Starting trouble

[Dizerens5]

I recently fitted new carb floats as the old ones were clearly very old and discoloured. Also new pivot pins as a PO had clearly been battling with pliers to remove them (why? they tap out easily). And new needles and gaskets. Since then I've had problems starting from cold, the engine fires a few times, then dies, try again, maybe it will stumble into life, coughing and spitting and dying at the slightest throttle opening. But after about a one-minute warm-up all is suddenly fine, it takes rpm easily and on the road, it's perfect. Using the choke enrichener improves things a bit, but not much. Varying pilot jet screw opening seems to make no difference at all. Sounds like a temporary fuel shortage, but why only temporary?
When I took a close look at the old floats I saw that a PO had bent down the tabs which control the needles, this would have the effect of raising the fuel level. I'm beginning to wonder if that PO had some good reason for faffing around with the fuel level. Has anyone heard of the old flat-top Bings behaving like this?

[donbmw]

I don't have the flat top carbs on my bikes. To me it sounds like the carbs need cleaning.

[tvrla]

Since the bike runs fine once warm, I'd guess the carbs are fine.

The system directly related to cold start and running is the enricheners (often referred to as the choke), and that's where I'd look. It's very easy to get the enrichener discs installed incorrectly so make sure they're right. The gaskets tend to suck in, and the mounting screws loosen, so check that. Perhaps the enrichener jets are plugged or the passage to them. The jets are in a corner of the float bowl, with the copper tube leading down there. Or maybe the cables need adjusting so they open evently and completely.

If you haven't gone through the enricheners, now's the time!

[Bob_Roller]

Did you have this problem before working on the carbs ?

If the enrichening system isn't the issue, have you checked the ignition system ?

A weak coil(s) can cause hard 'cold'  starting first start of the day .

I'm not sure what the resistance value of the dual coil set up that you have .

Are the ignition leads 'old' ?

[Dizerens5]

Both coils are near-new from Motorworks. Ignition leads also. Trouble started when new floats were fitted, so I'm guessing that there is a connection to the new lower fuel level in the float chambers. Certainly with the old floats and their bent needle tabs, the bike never needed enrichener use even starting at near-freezing temperatures. I recall the old Amal carbs -- using the tickler pushed the float down and raised the fuel level for starting -- very effective! I'll have another poke round. Thanks, everyone!

[Barry]

Fuel height in the carbs is one of the most difficult things to set precisely because short of a glass float bowl any other way of doing it is an inferred method which depends on the float buoyancy rather than a measurement of the actual level.

I presume you have the floats set to shut off parallel with the carb body. If the old floats had less buoyancy than the new ones there may be a difference in actual fuel level. Don't know if you tried measuring the level any other way. The nearest I've got is to remove the float bowl quickly and measure the fuel height from the centre well which should be 22 - 24 mm.

There seems to be varying views on how important the actual level is but I can easily tell the difference that 2mm makes to the way the engine runs so I try to set them to an accuracy of 1mm.  

[Sosyal]

Hi, did you change only the carbs or the handlebar switches also?

Sounds me the problem I recently had. Installed high handlebars with the used switches arrived on them and the carbs simultaneously. Then the bike did not start and showed almost the same symptoms with your case. I've done everything with the carbs (even hit them to ground) but would not rev. up until the engine gets warm.

A year later (!) discovered that the kill switch was faulty!!! In a simple way let me try to explain: the switch was so dirty that it did not let enough current (electricity) flow BOTH to the starter AND the coils. Starter drained all the current and the voltage of the coils were dramatically reduced and could not fire the sparks strong enough.

Check your kill switch first.

Ibrahim

[tvrla]

If the bike starts in cold weather without the choke, it's running waaay too rich. That's why we have chokes - to enrichen the mixture when the engine is cold.

So needing to use the choke is a step in the right direction!

Barry - you can see what the fuel level is by modifying an old float bowl. Drive the over-flow tube out the bottom, cut off what's left inside, attach a clear line to the bit on the outside and hold it up alongside the carb. You now have a sight tube when the gas is turned on.

I made mine to work on both left and right carbs by knocking out the plug to the other side enrichener well.

[Barry]

If the bike starts in cold weather without the choke, it's running waaay too rich. That's why we have chokes - to enrichen the mixture when the engine is cold.
 

Agree absolutely that this is how it should be - if  BMW got the jetting right but I wonder if they did with the flat tops ? There is something about flat tops and the choke. Mine is hopeless on choke and will hardly run with any on but does need some for a few seconds in cold weather to start. For the warmer 6 months of the year I don't touch it at all and I recently did 74.5 MPG on one tank so even if I'm a touch rich  there can't be too much wrong with the mixture.  

Maybe the early European bikes were set up a bit rich. One difference which for me would explain this behaviour is a 45 pilot jet where some US bikes if not all have a 40 pilot jet. That would make no difference to the mixture at idle if the mixture screw was set correctly but it would richen the transition and small throttle open running through the transfer ports which are fueled by the pilot jet but not controlled by the mixture screw.

Thinking about how we start an airhead by catching the engine with a fraction of throttle this starts to make sense. As soon as we apply some throttle the transfer ports come into play and in this scenario carbs with a larger pilot jet will have a richer mixture and need less choke than those with a smaller jet irrespective of the mixture screw setting.

I think this may partly explain Constellations previous easy cold running although perhaps it shouldn't have been quite so easy as it was which tends to point to high float levels. Being aware that the pilot jets are a touch oversized I've aimed to set the fuel level at 22mm which is at the lower end of the range. I've also tried as low as 20mm which resulted in noticeable power loss and fuel starvation at high speed.