Cpi sm 50 engine has been rebuilt. i have fitted new auto choke,new battery. carb has been stripped and cleaned, slightly bent the float to allow more petrol in thinking the bike might be starved of petrol,still the original low level exhaust on it. it starts well and ticks over well ,the throttle needle is at its highest position, the bike has a power band when riden but splutters and has no power. the previous owner said it would do 60mph

SOURCE: XR 100 didnt move for a year.My bad so i cleaned

First Make sure you have all new fresh gas,and new fuel filter,Then check the intake tube between the head and carb for cracks or leaks,And You'll need to go back through the carb again as this is a carb issue,But without doing the above first,a clean carb won't be clean for long.

SOURCE: Suzuki GSF400 Bandit problems with tick over.

as you have replaced carbs you will need to retune for fuel intake also check its getting enough air ie.. air filter has been changed or is clogged

SOURCE: I need tuning specs for my 1982 suzuki gs 750.

You need to fix the fueling first - without fuel it won't run!
Your other problem sounds like an air leak - either the manifold or the carb. Check by spraying WD-40 or CRC at the manifold and carb join while the bike is running - if the revs change (either up or down) you have a leak.

SOURCE: 81 xj650lh midnight maxim black exhaust, sooty plugs keep fouling

hi,fairly common problem with modified exhaust,start with the pilot screws,turn right in untill the screws are firm then back out 1 and 3/4 turns,this is a fairly standard rule of thumb that will get most things going,as far as the jets go,the original size would be a good starting point,but you should only need to go up 10-15 sizes to compensate for the exhaust mods(ie stock might be 127 may need to go to138 or 140 etc),the other thing that i would recommend is getting hold of a set of damped carb balancers these will enable you to get the mixtures and carb balance dead right,they are relitively cheap and easy to use and will have paid for themselves the first time you use them.(i regularly get 4-8 sometimes 10-11 extra hp over stock by tuning the carbs up properly).hope this helps

SOURCE: rebuilt all four carbs cleaned and replace needle

With Mikuni carbs, the idle circuits affect engine performance up to 3000 rpm under light loads. If the idle circuits are restricted (by fuel gum, rust dust from the tank or dirty fuel, the results will be 1) the engine will run with "choke" on. 2) poor or no throttle response below 3000rpm,3) back firing and missing on the effected cylinder(s). Cure is to blow out the idle circuits and idle screw adjustment jets with several applications of carb cleaner and compressed air. Failing that, strip the carbs.