Expansion chamber setting up

Could I trouble you all for more advice?
After running in the cafe racer bantam, I'm starting to fettle up an expansion chamber exhaust. As I've never made one before, I'm trying to keep it simple-I found a chamber at Stafford show (Codnor light fabrications limited, an early micron!). I've worked out the lengths using formulae from the haynes two stroke tuning book, and have made it so I can adjust the header pipe length and fit different bleed pipes.
My question is how do I go about setting the carb up to run with a different exhaust? I've got it running about right on the standard one, should I make everything richer then work back from there or try it as it is first? I realise it is easy to cock up, so I will proceed carefully.
Thanks
Phill

Hi
always, but always start by richening the mixture, even if it wets plugs, as that is much less expensive than a holed piston (or worse), and aluminium everywhere!
Bring the jetting down gradual and check the plug readings as you go along... traditional stuff is best.
Good luck, and hope all goes well.
Alan

alan wrote:

Hi
always, but always start by richening the mixture, even if it wets plugs, as that is much less expensive than a holed piston (or worse), and aluminium everywhere!
Bring the jetting down gradual and check the plug readings as you go along... traditional stuff is best.
Good luck, and hope all goes well.
Alan

Thanks for that. Do I do it as per the book (it still has the original amal 626)-full throttle to check the main jet, then idle etc etc? Also, anyone got a collection of amal jets I could borrow, they're a tad expensive when you need lots and lots to try.

Past experience and the quiet advice of a long time two stroke maestro has taught me that the old standard practice of trying main jet in an Amal first does not always work well. If the needle jet is big enough it will, but if the needle jet is too small ( as is often the case with modified Bantams ) it will be the flow limiter and the engine will pull upto 7/8 throttle and changing the main jet will not make any difference.
It took me several years of crook advice, 4T stuff, mucking around; in 5" Ginger had it sorted.
Take out the main jet, fit the biggest needle jet you can find, or drill out and put the needle clip in the bottom / richest slot. Making sure you have a 3 code ring or D20 needle (don't use crappy Spanish or pattern Xor U types, they run weak) My longstroke 185 uses a .108 for long fast circuit at 150' asl and 107 for short circuit at 1600'asl; Short stroke 125 has a 107 in Mk2 Amal. Modified D14 Road engine has a 106.Start the engine and if it gases up easily after 1/4 throttle the needle jet is too big, after 1/3 then then its big enough. If the engine revs out , the needle jet is too small.
Once you have got the needle jet combo in the right sort of area, put the clip back in middle slot, then go out on track or quiet uphill straight road with massive main jet, it should rev well then gas up on full throttle. Come down 1 size, #10 at a time, usually end up with 200-210 for road (<6000rpm) and 270 - 310 for racing
In 30 years with a number of different Bantam engines, they have all ended up with very similar jetting. Needle jet size seems to affect low to midrange more than clip position, cos its working on the tapered part of the needle. You can make fine tweaks with washers under the needle clip or jet.
Another cunning plan that transforms a standard motor is to fit a Mikuni carb.

Thanks for that, I've just got a load of jets to try out. Hopefully I've bought enough! I have taken the bike for a very careful run with the new pipe (half choke to prevent it going lean), she seems a lot more eager. I've got a load of stuff to sort out for a steam rally this weekend, hopefully I'll get time to have a play beforehand. It is bloody loud too!

Well, that worked rather nicely! Currently on 190 main (which gives a nice brown colour), 107 needle with needle on middle notch and #3 slide.
The bike accelerates quite cleanly at all throttle openings. However, when trying to stay at a constant speed (around 45-50 at 4000rpm, 1/3-2/3 throttle) it starts spluttering and slows down Once you get it over 4500 it is fine, but that usually means 55-60 in a 50 limit and derbyshire police are rather keen round here! Am I correct it is a bit rich, and thus need to lower the needle? I will try it in due course but I'm off to a steam rally soon, and I think the neighbours are deaf enough for one day