bsa lightning carb set up

[marty 31]

my 1969 bsa lightning approaches completion, engine, professionally rebuilt, electronic ignition etc, brand new amal concentric carburettors, correct for the model, the bike is bog standard, with the original type pan air filters etc the bike starts and runs but takes a bit of work sometimes, when stopped it doesn't always start again without a bit of messing about, it has not been tried on a run yet, the carbs are jetted correctly to the makers manual and the makers set up directions have been used, the needle position 1  the confusion is, is that position from the bottom of the needle, or the top? can anybody help? thank you in advance, it would be great to get the bike to start easy, hot and cold, and not be such a pig

[R]

Have you tweaked the idling settings, so that it idles nicely and evenly on BOTH cylinders ?
Without this, starting could well be 'difficult'.

Also that the ignition is perfectly timed at full advance, so that the ignition is retarded at idle speeds, which gives easy starting.

Also2, twin carb setups ALWAYS require the carbs to be 'synched' (synchronised),
so that both cylinders are pulling their weight - and one is not doing all the work.
This mainly entails ensuring and adjusting the throttle cables so that both slides lift evenly.
hth

[L.A.B.]

the needle position 1  the confusion is, is that position from the bottom of the needle, or the top?

'1' is the top groove, so with the clip in that groove, the needle is set at the lowest/weakest position.

[mini-me]

Also2, twin carb setups ALWAYS require the carbs to be 'synched' (synchronised),
so that both cylinders are pulling their weight - and one is not doing all the work.
This mainly entails ensuring and adjusting the throttle cables so that both slides lift evenly

and the best way I have ever found to do that is the old lollipop stick trick.

[iansoady]

When I had twin carbs on the Commando (replaced after several years by a single Concentric which was far better for my uses) I used to set each cylinder separately first - take (say) the left hand plug out and rest on the engine with the HT lead connected then start on the RH cylinder and tune normally. Then repeat for the LH cylinder.

After this put both plugs back and start normally. Tickover will be too high but unscrew both throttle stops an equal amount till speed is correct. Make sure the cable adjusters are not hanging the slides before they touch the stops.

Then you need to get the cables synchronised so they lift at the same time. On the Commando, if you put your left hand under the carbs you could touch the throttle stop screws on both sides with thumb and middle finger. By opening and closing the throttle you could then adjust the cables until both slides hit the stops at the same time.

Of course the layout of the BSA may not allow this so mini-me's lollipop sticks (or small drill bits) may be necessary but of course this means disconnecting the air filter.

[marty 31]

Have you tweaked the idling settings, so that it idles nicely and evenly on BOTH cylinders ?
Without this, starting could well be 'difficult'.

Also that the ignition is perfectly timed at full advance, so that the ignition is retarded at idle speeds, which gives easy starting.

Also2, twin carb setups ALWAYS require the carbs to be 'synched' (synchronised),
so that both cylinders are pulling their weight - and one is not doing all the work.
This mainly entails ensuring and adjusting the throttle cables so that both slides lift evenly.
hth

the ignition was set by the engine rebuilder (srm) and is electronic, there is slack on both cables and the slides lifting at the same time, I have tried as the original manual says taking one plug cap off, and tuning the tickover and pilot jet, then swapping over and doing the other cylinder, then when running on both cylinders, adjusting down, you can mess with it for ages and think its right, then bingo, its a bitch to start, sometimes kicking back, which when 59 and two not so clever knees is a bit scary when I was about 20 I had a bit of a wreck of a 500 a50 star and as I remember it started easy and ran smoothly, if it hadn't been twin carb I would have been tempted to have gone for the miluki conversion, but twin milukis would mean a extension to my mortgage, but they were supplied with concentics new so they should run on them now one would think thanx again for the help

[R]

Kicking back vigorously is often a sign the ignition isn't fully retarding at starting speeds, or is simply not set correctly.

Professional rebuild or not, it sounds like getting hold of a strobe timing light and checking that the timing is all correct and shipshape could be worthwhile - assuming this BSA has a facility to do this - Commandos certainly do.  And, if the timing isn't correct, this has the potential to damage the engine with extended running, so likely very worthwhile having it checked.
hopethishelps.

[Rex]

Absolutely, and also check the battery voltage/charging too. These bikes were never hard to start and this one shouldn't be either

[iansoady]

Yes, especially if the electronic ignition is the Boyer variety.

[mini-me]

Every bike I have ever bought that had electronic ignition fitted, I binned it and went back to original.

[marty 31]

Every bike I have ever bought that had electronic ignition fitted, I binned it and went back to original.

its my first, but was thinking the same, how do you know if its the right unit? come to think back when I first bought the bike the engine was knocking, and had the old carbs on, but it was still a pig to start, also I have seen bigger sparks at the plugs, than on this bike, what next ?

[marty 31]

the needle position 1  the confusion is, is that position from the bottom of the needle, or the top?

'1' is the top groove, so with the clip in that groove, the needle is set at the lowest/weakest position.

that's a big help thanks,

[Rex]

also I have seen bigger sparks at the plugs, than on this bike, what next ?




Get yourself a workshop manual and go back to basics, checking everything.
Asking for clues and pointers ( as so often seen on the 'Net) will just take longer than a methodical approach.