I've just fitted a Mk1 premier concentric onto my 1967 Thunderbolt. It has a No 4 slide 230 main jet and the needle is in the 2nd groove. The bike has straight through high level pipes. Can anyone give me base settings to start from. The bike will start with the throttle wide open but needs reving to keep it running.
A good starting point would be the same as used in the original 389 carb:
Main: 310
Pilot: 25
Slide: 3 1/2
Needle Pos: 3
Needle Jet: .106
is needle position 3 top or bottom slot? And what about mixture screw position. I set it at 1 1/2 out.
I believe the correct needle on a 389 carb has 5 slots and Pos 3 is therefore the middle one whichever way you look at it. Normally you start with the mixture screw as indicated and then adjust it along with the throttle stop screw until you get the best even tickover at the right tickover speed.
Hi,
The '68 Thunderbolts were fitted with 28mm Concentric carbs and the manual has the jets etc. listed.
Is this a new carb. or was it on the bike? If it had been running properly before then it may need a good clean so that the pilot jet etc. works properly. The slide should be a 3 1/2. The 4 will make the bottom end weaker and may cause the problem or it may have been considered necessary because of the pipes. Is there an air filter?
Dave.
Hi Chris,
The needle position is counted from the top down.
You stated you have a concentric prem carb, from the Amal website here are the recommended fittings and settings. As Dave mentioned a clean, paying attention to the pilot jet may sort your issues out.
Link to Amal https://amalcarb.co.uk/
Best wishes...
Thanks for all the ideas.
The carb is brand new, it came with a No 3 slide, I fitted the No 4 as I found info somewhere to say it was better for straight through pipes. It had monobloc fitted before which was quite worn. Also when running before the points gap was 40 thou, I have re-gapped to 15 thou. Also it was running a 12 volt coil on one side and a 6 volt on the other. I have replaced the 6 volt with a new 12 volt.
I guess I need to adjust the slide screw and possibly fit the No 3 slide that it came with.
Once running properly, I would see how well it picks up from idling which is what the slide cutaway is designed to adjust. In general, if the engine hesitates picking up from idling, the cutaway could be too large, whereas, if it feels a bit lumpy picking up from idling, the cutaway may be too small. I suspect that if a 3 1/2 cutaway is ideal, you may not notice the difference using either a 3 or 4 cutaway.
Again, thanks for the advice, will check this when we next get a dry spell. :D