Lemans 2 B&G Carb issue

Managed to get the lemans back together after full engine rebuild. Fired it up in the workshop and she is sounding great. However, I will need to do some tuning of mixture etc. Timing has been set and tappets adjusted. My question relates to a slight wafting of smoke/vapour from the air intake of the right hand carburettor when the engine is warm and then switched off. Seems to occur only on the right carb and I am also getting a bit of spitting on this side. Any suggestions welcome.

sounds to me like choke open a bit or the inlet valve not fully sealing when hot have you checked valve clearance when hot to see if you have any play


you mean pop the rocker cover off and check gaps when hot.

You could also check compression when warm. Valve not seating would drop compression.


I will do some digging at the weekend. Is it possible for the carburettor to be at fault? Its just that with reconditioned heads, valves guides and professionally lapped in plus setting the gaps when cold a few days ago I find it hard to think the gaps could be out. I will keep you posted. I did find when I raised the revs of the engine and then shut off the throttle the revs took a while to return to tickover. I have crudely checked the carbs are opening equally on the twist grip and idle screws are equally 1.5 turns out. I will read my Carb manual again this evening.

Have you fiddled with the accelerator pumps during the rebuild. these can make it rich as they squirt huge volumes of fuel down the inlet tract. It is a v long time since I had a Lemans but seem to remember the haynes manual suggests squirting 10 full pumps of the throttle into a jam jar to check the quantity being delivered.Mine was also a pig for spitting back if the timing was gnats whisker off perfect.


I did wonder about timing because there is a little play in the drive between distributer and camshaft gears. I have only set this in the static state. I think that’s what you call it. A friend helped.

Don’t bother changing the fibre gear on the bottom of the distributor in attempt to get rid of the play in the drive. I did it on mine and the new one was no better.Just set the timing with a strobe, but make sure you use the full advance marks at about 4000 revs (Do it when the neighbours are out !! )

I've had that delay for revs to drop and it was because the throttle stops (and hence slides) were too high. But it wouldn't idle if lower ~ why? 'Cause I'd set the mixture screws at 1.5 turns like Haynes says. But this is wrong, if you read it properly it's a starting point. 1.5 isn't enough, mine are now about 2.25 - 2.5 turns out. Then you can make slides lower then it will shut off properly and still idle on the stops.   HTH    PS: 'spitting' also suggests it's too lean, i.e. idle mixture.     

Mike H2014-05-23 16:25:29

I will see if I can get my mate to help me with this this coming weekend and report back. I had the carb apart this evening, cleaned and checked all jets etc. Put back on and when starting I notice a lot of fuel collecting at the bottom rear of the intake(I have the conical grills off at the moment) and I also notice more pronounced sucking noise from that right carb. The main needle hasn’t been replaced and I am wondering whether this is not seating well. This carb was originally far more clogged and required more cleaning than the left side when I first acquired the bike last year. Lots of green gunge. They haven’t been through the ultrasonic cleaning but they all seem clear with compressed air. The main jets have been replaced and perhaps I should have replaced the needles at the same time.


Spot on on the 2.5 turns and yes it seems to help with tickover. Revs drop back as normal. As mentioned the left side just seems to have a louder sucking noise on each stroke than the right side and I’m getting fuel depositing at the mouth of the intake area.

Check the cold start jets (Choke) Take the assemblies out and have a good look at the rubber seal on the bottom. That has to drop down and close off the fuel supply when the chokes are of. They can get perished with age.

You might be right needle valve is leaking. Another possibility is leaky float.