BSA Owners' Club Forum

The BSA Workshop => Twins => Topic started by: Rob Atkinson on 12 July, 2017, 18:16:03

Title: air/choke worth fitting on concentric carbs?
Post by: Rob Atkinson on 12 July, 2017, 18:16:03
I am having trouble starting an A65 Spitfire fitted with new concentric carbs. My local garage can start it no trouble and it runs very well when started, but I struggle to fire it up. It does not have choke slides fitted and I wonder if anyone has experience of starting with choke slides in place. Does it cold start much easier with chokes on? I had no trouble starting my A10 without a choke fitted.
 It could be age catching up with me or I haven't mastered the technique yet. To me, the kickstart has a completely different feel to the A10 with a lot more travel. Maybe I need a box to stand on!
Any thoughts or tips much appreciated.
Title: Re: air/choke worth fitting on concentric carbs?
Post by: STAR TWIN on 12 July, 2017, 20:24:57
Fit chokes. They are there for a reason. If your A10 never needs a choke to start, you are probably running a bit rich on pilot jet and/or cutaway.
Title: Re: air/choke worth fitting on concentric carbs?
Post by: Robspitfire on 20 July, 2017, 20:15:26
They may all be a bit different depending on ignition timing, points or electronic ign.etc, but my Spitfire won't start if the throttle is open at all. Closed throttle, no choke and it starts hot or cold 1st / 2nd kick...... (usually)
Title: Re: air/choke worth fitting on concentric carbs?
Post by: Rob Atkinson on 21 July, 2017, 15:50:55
Thanks for the replies to my post, some very useful advice.
 In the original post I had omitted to say the engine is newly rebuilt and the carbs. will need some tweaking after a good run. I will fit a pair of choke slides whether in time they get a great deal of use or not. The tip from Robspitfire is particularly interesting as I had been using a part open throttle and find it much easier to start the bike with the throttle fully closed.
Title: Re: air/choke worth fitting on concentric carbs?
Post by: Pippinz on 25 July, 2017, 23:33:45
I have only had my twin carb thunderbolt a few weeks but already I have learnt to inspect everything after the first job was to put equal amounts of plates in the clutch and in the correct order so I don't assume and take nothing for granted.

My bike has had the chokes removed and the cable gland have not been plugged. I was informed it always starts first time.... well as the plugs and filters were completely black I'm was not surprised and obviously it was running rich and was awful and unresponsive to ride. I replaced the pancakes which were filthy, cleaned the plugs and checked the gap and went for what I though were factory base setting on the carb. I found 1 turn out on the mixture screws to be a good starting point. Still a disaster, bike over reving, all over the place stalling at lights etc etc. As the pancakes were off I opened the throttle until one of the slides completely cleared the intake and adjusted that slide cable or the other until they both disappeared together... carbs now synced and one less thing to worry about.

Today managed to get it running right after reading a few posts and engaging a bit of common. I removed the pilot jet ( the horizontal one ) sprayed the hole with carb cleaner and snipped the end off a B chord guitar string. This is about .014 of an inch slightly smaller than the hole and had a good poke about until I was sure there was nothing obstructing the jet. The passage is quite deep but then hits a 90 degree bend but I was confident I had cleared any obstruction if there had been one.

All put back together then held the ticklers in for a couple of seconds to get some fuel into the engine. Kicked it over once then turned the ignition on and with the throttle fully closed kicked it again. Once fired mine needs operator input on the throttle to warm it up.

Once warm I adjusted the tick over/slide ( angled screws ) to get it ticking over above it stalling. Now the fun started. Pick either carb gently turn the horizontal mixture screw in or out until the revs increase. Do this very slowly blipping the throttle between adjustments, now take the associated angled screw out a little to lower the revs back to where they were if nothing happens go to the other side and turn the angled screw out anti clockwise to drop the revs to the previous level. Do the same with the opposite carb and adjust the mixture screw until you get a rise in revs then anticlockwise on the tickovers to bring it back down. Go back and forward increasing the revs with the mixture screw until there is no increase in revs. If you go past the sweet spot the engine will feel like it is going to stall so turn the screw back a little. Once there is no increase using the mixture screws you can bring the revs down to where you want it to idle by adjusting the angled screws a little at a time. If there is no immediate drop in the revs go to the other cylinder adjust the stop until you get a drop then back to the other side . A little at a time produces the best results. My bike now runs as it should and I was about to order new carbs..!

Although I ordered the choke and cables etc I won't be putting the chokes back in my particular bike as it can be feathering on the throttle once it is started until it warms and settles. It is nice that these machines almost have there own personality and I'm sure everyone is different. I hope this may help someone else but as a newbie I'll wait to be corrected by a more knowledgeable member. My bike is running better than expected and having to stop at lights no longer creates the panic it used to.

I may use a set of compressable ear plugs to block the glands and go through the set up again to see if it has had any effect. I can't believe what is being charged for a bolt on the net.

Hope this helps someone