... Or so I thought...
The same thing happened today. After riding him with the tank pretty much full it was fine.
Today as I rode of I felt it stumble in 4th again, then 3rd, then cut out, it had gone about a mile at this point.

It would t run reliability after that so I took it home.

I removed the online filter, just in case as it was horizontal and wanted to eliminate that. I noticed after a few seconds idling there is pressure in the fuel line, bubbles forming around the tap.

What would cause pressure here? And is that causing any issues. I am going to check the float bowl now to see if anything is wrong there

Ah, good point. I was trying to tie the gear change to the cutting out. It may be that the reg/rec I have is not great, (wassell), or the charging system is not giving the electronic ignition enough ? The battery is fully charged though as I had it on a trickle charger overnight.

Hello, I have a bit of an issue as I keep using my B40 to get to work, natively believing that the issue will clear itself!

It hasn't and after a very sweaty and sweaty trip to the office I feel like the thing has given up.

It was running very well a couple of weeks ago, last week I noticed once I got it into 4th and was travelling at along at about 40 it stated to misfire. I didn't think too much about it as it seemed to clear itself up.

I checked the plug and it's a good colour.

Then last Friday I was on 3rd and it started to splitter and bog, switching down to 2nd and revving seemed to sort it temporarily ,then on going back up to 3rd or 4th was misfiring badly and not revving.

It then stalled and was a pig to kick over, bit once it did it idled.

Pulling away it spluttered bit keeping it in a low gear and revving it seemed to be the only way to keep going. As soon as I went up a gear it just splitters and dies.

Same story today but more of it, easy to start first thing, will run in 1st and second with the odd misfire, 3rd patchy and 4th unrideable. 

Carb is original amal, was ultrasonically cleaned and has new jets,
It has vape ignition and is 12v.

It idles nice and cleanly, it revs up on the stand in neutral ok.

It now pops and bangs on the overrun where I believe it wasn't so much a couple of weeks ago, sounds like an air leak.. bit why not all the time, and why does changing gear affect it so much?

I did notice an rear indicator bulb has blown .

Thanks for any help!!!

Thanks for the input, yes I made my own double thick gasket and it is now sealing nicely. I can get it to start quite well on the second "live" kick  after a few priming kicks with the fuel on.

It will idle fine once warm, bit then after a while it will stall, especially at junctions!
I guess clutch drag may not help in first with the clutch pulled in at a junction.

It's strange that idle is more likely to die when fully warmed up!
Yes the answer is probably a new carb, but I'm a sucker for punishment!

Thanks Dave, I agree... I'll start saving my pennies! Unless anyone has a nearly new Monobloc lying around?!

I guess the ultrasonic cleaning would make things worse in this case, as it's removing material and potentially widening any passages. 

Thanks Dave, yes I have one on order!
It already has new jets, a needle and a slide.

Right, engine is all apart, the head gasket was pretty oily on both sides but it's inconclusive as it could have happened when the tension was released on the studs. I cannot for the life of me get the studs out, so my hopes are if the they're that tight then it's unlikely the oil is being pushed up from the crankcase.
Also, logically the lower fin would be the only wet one if the oil was coming up, as it would spread out as soon as it got to open air.

Rhyatt, you mentioned a "cylinder spigot upstand" in the assembly process, now am I being stupid or am I missing a part?

I noticed that the base gasket had migrated slightly and was half obscuring a drain hole. The drain holes seemed to be clear otherwise.

I am going to do as Rhyatt says, and make sure everything is sealed up top, bit it seems like of I could get the studs out then I could put a bit of thread sealer on them just to be sure.

Thanks is for the info, looks like I at least have the correct pilot now, the main is a 200 but then it's a +40 9:1 piston so I think that will be ok. I have the 190 as well to put back if not.

I bought the pilot from Burlen, based off the correct carb for the bike!

If after all this cleaning and rebuilding it still doesn't work, I may just buy a new one, the Keihin copy that is currently on there works well, but I can't tune out the massive backfires on the overrun!

Out of interest, the 376/8 that was on the bike looks a slightly better design, it has more strengthened areas at angles and where things screw into the body...  If the slide bore hadn't have been worn, are all 376 bodies (dependent on bore size) the same?