Hello folks. I'm looking for advice on what changes to AMAL carbs provide the best tune after replacing stock exhaust. The bike is a 69 a75 (20 thousand original miles) with original 27mm 626 amals and stock internals and jetting. Im replacing the rayguns with a Big D 3 into 1 header(purchased new 1973). The engine is stock with good compression and oil pressure. The ignition is Boyer digital power. I suspect most of the changes should be in mid range to top end likely requiring larger main jets,perhaps a smaller # slide cutaway and possibly raising the needle position. I don't anticipate changes to the needle or pilot jets or for that matter the float levels. Im sure there are quite a few members familiar with this header who in the past have installed it (or similar units) and arrived by trial and error at the right combination. Im asking because all these AMAL bits and pieces are quite pricey (read exorbitant) and here in Canada, at least in British Columbia, no longer readily available at your local brit bike shop(there aren't any) so Id really appreciate any help getting to that sweet setup faster with less expense and without a handful of parts that prove useless.
I am pretty sure you wouldn't want a smaller cutaway, go 3.5 or 4, middle needle position, and if you do a lot of high speed riding [+80mph] then maybe go to 160 mains, but if you're road riding [traffic, speed limits etc] then the 150's are fine. Sport exhausts tend to unleash the sweet note and any hidden ponies without warranting any radical carbie changes.
I have a 1000 kit with taper turbo's on a Viking big bore exhaust and all I did was 3.7 cutaways, drop my needles one position [note NOT raise], and go up to 180 mains. And I think that's generally a popular starting point for those with bore and/or stroker kits.
Until now I used 3 in 1 Jardine exhaust from the same period on my T150V with completely stock carbs configuration and the engine worked very well. Only problem I had it took a long time to get to operating temperature ( 10 - 15 min of riding in the morning ) and when below this temperature wasn't happy going into revs higher than 3 thousand, bogging down when pushed. But was faultless when hot. However bike was very responsive and fast with this exhaust, noise it produced was very loud and metallic ( big angry wasp ), so I decided to switch to "beauty kit" exhaust, perhaps sacrificing some top end speed and acceleration I don't really need and saving my neighbors and riding together buddy hearing:). Finding and changing this "beauty kit" exhaust was difficult and quite expensive experience for me, But I'm really happy with deeper and quieter sound.
My '72 T150 came to me with a 3-1 header. Don't know who made it, but it looks pretty much like pictures I've seen of the Big D exhaust. 3.5 slides and new 106 needle jets. Someone here suggested 170 main jets. I've only ridden it once, but it runs out to red line smoothly with these mains.
Thanks for the advice and feedback guys.I was thinking it would require a fair amount of changes from stock jetting but seems to be quite straight forward. As a good header system should pull more ponies from a given displacement albeit at generally higher revs I would have thought at least larger mains would be required and advisable.High revs and lean condition makes me wary. I will proceed cautiously. Cheers!