hey sundance
good question. i am absolutely not any kind of authority on monoblocs, so my suggestions will be general. here's what was originally associated with the 1967 chassis you have:
the two carbs had #260 main jets, #106 needle jets, a Type D needle (standard), a #3 throttle valve cutaway, and a #25 pilot jet. i suspect that these may work just fine with your 800 cc motor, and in any event you should test them and see. main jets are numbered by orifice size- the bigger the number, the more fuel they can flow. needle jets are also sized by number, throttle valves have a higher number the leaner they are, and pilots get richer with number.
these may all work fine, but if anything were to change i would think that the increased engine capacity might require slightly richer jetting in several places. when you order the new bits. i would ask for the stock 260 main jets, but also get two more pairs going up--280 and 300, or maybe 270 and 280. you might never need them. the 106 needle jet and the standard D needle will very likely be fine-- you can adjust the needle up and down to richen or lean the mixture. there is a possibility that you might find the #3 throttle valve to be too lean, in which case you could try out a #2.5 with a smaller cutaway, but i wouldn't buy them in advance. finally, the #25 pilot jets might be lean, so i would add a pair of #30s to the lists.
but do ask burlen for their recommendations on the jet numbers. theyve been around these carbs for years, and theyve seen it all. tell em what you have, and they will know more than me for sure.
in total, i think you can probably tune it correctly if you have in hand the stock jetting and two numbers up in mains and one number up in the pilots. needle jets in amal concentric carbs are worthless after 10,000 miles or so-- the tapping from the needle wears them away. so i would also make sure you get two new 106 needle jets. the needles almost never wear out.
tuning them will be straightforward. all the different carburetor circuits come into play at different throttle settings/engine loads, so its pretty simple to tell which jets or adjustments need to be fiddled with by testing at constant throttle opening and load. amals are pretty easy to tune, in general, and i think the original 649 cc settings are going to be close..
one caution-- heavy loads at lean settings can cause the motor to overheat and detonate or melt a hole in your piston. an easy way to avoid this is to start testing with the too-rich jets and settings and work your way down to what runs best, rather than starting lean and running towards richer. when you get to where you are testing we should take stock of where you are and work through the procedure you will use to finalize the jetting.
