High Idle

Jul 5, 2008
Have a 69 Lightning. Noticed that it kicks over and idles great but after a run around town, it settles to about a 2000rpm idle.

I have backed the slide screws back but am worried that if I take them too far back, the starting idle will become a problem.

Have rebuilt the carbs and cleaned them all up.

Any ideas as to why it would do this?
An AA unit that sticks at full advance will do this so check that. A loose Amal carb slide that can cock on the idle setting screw tip will do this so check this. Bad old cables with bad old abutments can also give this trouble. A leaking flange that gets worse as the engine heat transfers to the carb body could do this. There are some other less likely but possible causes.
Well got the idle sorted by just tweaking back the slides.

Question for you NorBSA, I notice that you are running both BSA and Norton. Does your lightning vibrate pretty good through the hand-grips? The BSA is new to me and I took her out on the highway for a little burn. Could not believe how much engine vibe was felt at the grips compared to the Commando.

Is this normal from your experience?

I dunno much about BSA's, but The Commando is rubber mounted, so my Triton shakes alot more the my Commando, but is Ok until you get to about 65mph, then the vibes come in.
The thing I remember about BSA's is a bush instead of of a bearing on the timing side mains...is this the same on your Lightning? I remember seeing a piston come out of a BSA head (or should I say through!) coz of this. The bush wears and then you get big end oil starvation. SRM have a mod(£££$$$) that sorts it. Whilst you had it apart you could get the Crank dynamically balanced...that makes a big difference.
Mr. Stone, If you release the pressure on the long bolt underneath the motor ( lower motor mount) you may find a air gap has shown up between the engine and frame. BSA's should have distance pieces installed to take up this air gap.(metric washers are good for this) The next thing to check would be the head study same thing really, loosen it off to be sure that it is formed in a way that the tightening of the bolts does not reform the head steady tube at each end. Tweak it for a good flat fit before any bolts are used. The trick with 69's is to find the sweet spot were the motor is balanced nice and gear it to the speed you like to go at on the highway. The bike needs careful tuning to find the sweet spot. 1969 and on BSA's were balanced better that the previous motors. Keep looking and you will find your reward.
Thanks for the informed replies. Will check the front mount and head steady.

Back off the front and the bottom at the same time if gaps show themselves fill them with just the right distance piece. The front is normally good but the bottom may show a gap.

Dumb question but what about filling any gap with rubber washers? Would this be option?

Not for this, too much Pressure is needed. The shimming is to get the spring out of the frame and the head study needs to be very tight.