40mm cv carbs - might help someone

[Oilyspanner]

Just thought I'd share something I did to finally get my 40mm BST carbs' on/off throttle response really smooth. The bike has been freed up on intake and exhaust, plus a bit of porting. I have a Factorypro config 3.0 kit (normally for pods) and had got the carburation pretty good -  good tick over, nice pick up on the needle and pulled good and strong on the mains, but there was a hesitation as the butterflies were just cracked open - fine when having fun, but it was always there when pootling round (has to be done sometimes !). Over time I fiddled around with all the normal things to sort it, different pilots, every option of the pilot screws, check/recheck of fuel levels/float heights, carbs balanced several times - even checked electrics.....

Anyway - my bike has never had any carb vent hoses fitted and just for the sake of it I fitted some and found they affect carburation - the hoses are meant to be pretty long and fitted over the top of the airbox. I didn't have enough tubing to do both, so experimented with different lengths, 4 inch lengths of 6mm bore with plastic pipe narrowing the ends to 4mm have completely cured the slight hesitation on/off throttle (would've been easier to fit 2 x1foot lengths of hose). The bottom end is stronger, really good smooth urge and the mid/high rev performance is as good as ever - 'kin result..

I think the big cv carbs are sensitive to the vent hoses, especially when the bike starts to be altered from standard, as when it was standard, it was pretty smooth - if heavy and not that powerful.

Sorry I didn't take any photos - the vents are the upper ones between 1/2 and 3/4 carbs, they keep the float bowls at atmospheric pressure, I guess the standard hoses cause a slight restriction/damping effect compared with open vents.

[boilerdude]

the 38s are finnicky in exactly the same way. ive got 20cm long hose on them as per factory pro readings. improvement but still not perfect.. perhaps I'll try restricting them like this next.

[Oilyspanner]

I was surprised how much effect they made BD - I guess by moving away from standard engine set up means it's worth altering the amount of resistance caused by the vent pipe length/or slight restriction - at least it's quick and cheap to experiment, makes a change !

[Gixer1460]

I believe it's a lot to do with positioning of the hose end as the under tank area is generally a 'low pressure' zone but does vary so may be adjusted ad infinitum. My CV's used a double tee arrangement to pair them up and vent down by the cases - never any stumbles, just pulled from idle so there is something in it.

[Swirl]

Excellent definitely something I wouldn't of thought of

[nlovien]

9 hours ago, Gixer1460 said:

I believe it's a lot to do with positioning of the hose end as the under tank area is generally a 'low pressure' zone but does vary so may be adjusted ad infinitum. My CV's used a double tee arrangement to pair them up and vent down by the cases - never any stumbles, just pulled from idle so there is something in it.

 pressure differential is what makes these things work - add in damping effect ( hose size etc.). to achieve the desired rate of pressure change signal

going to post the nice intro onto a certain Gurls blouse CB forum - these guys need to learn this stuff, probably time wasted  - just get another lecture on why not to remove the airbox