Help CBX250 Twister Race Bike

Hi all newbie here. A bit about myself which will explain the question to follow. Based In Port Elizabeth South Africa and have been active in circuit racing at club level for 5 years. I compete in the 650 twins class I am compaigning on a SV650S Suzuki and an ER650 Kawasaki. Recently I had a guest ride on a CBR150 great fun and close racing but just physically too small for my size. This year they have opening up the class to include a breakfast class which is non championship until the field grows. Regs permit a max 250cc air/oil cooled carburetor bike. I have chosen to build a bike based on a Honda CBX250 Twister. They race these in Cape Town for cheap racing. I am quite far along having 1st tested in stock deciding what needs sorting (mods done and proposed will be in a separate post.

Background
The stock 34mm VEA2A keihin CV carb is a major week link to performance , speaking to people in the know several 34mm carbs were suggested as replacement. I settled on a VM34 Mikuni copy which is pretty exact to original even taking the genuine jets. Info suggested on others experience to use an airbox of some sort to create a still air barrier and the same size jets as the stock cv carb. I examined the jets in the supposedly stock carb which are keihin no 142 (hole dia 1.35mm) main and 45 pilot (0.45mm). Now the crutch of the mater there is no common way of measuring jets between different manufacturers. Mikuni jets are mostly measure in fuel flow ie cc per minute, some others use hole diameter in mm, whist keihin another standard?. I have found several charts that permit conversion between the different standards which will be close for a starting point ( but not exact as different lead in angles on jets and length can affect flow delivery quantities. my VM copy carb came with metric sized jets ie 1,28mm main and 0.46mm pilot. The problem I am experiencing is the bike will only start and run even when warm on choke. this tends to suggest too small pilot jet, it runs clean from above midrange ( a little stutter at cross over point so maybe raise the needle one position. If based on metric equivalents the the main is also to small for top end.

I just need someone that has the stock carb that hasn't been rejetted to confirm my starting point jet sizes are correct?

 

Thanks very much Grey Import at least now I have a starting point and know both their id's wihich puts my idle jet almost perfect and the main a bit small ie original carb jets 1.35mm and 0.45mm new carb jets 1.28mm and 0.46mm so now to play with A/F screw to see if I can get it to idle and get bigger main.

Mods to date remebering that biike must be able to return to stock. Ali refers to Aluminium.

Suspension : Chop off progressive part of fork springs, make fork oil restrictor my semi emulators to control rebound (10mm thick ali disc od to match fork tube id with 12mm hole in center like a big washer, fork springs tube spacers made to match with some preload dialed in and 20W oil. Front end 100% better, as for the rear I'm, busy rebuilding an old thunderace shock 340 vs stock 345mm. That 5mm length difference can adjusted internally by shims. Just need to grind about 2 mm off top mounting bracket for clearance on top shock eye and also had to have about 0.4 mm ground off each side of top eye bush. bottom clevis has 12mm holes so will make a bush for each side to bring back to stock.

Body Work : zx250 race fairing, seat unit might be R6 and make all bracketry and fit rivnuts to frame.

Exhaust remove stock 6.7kg can cut back pipe closer to bolting flange and fit aftermarket ali one.Will get custom high rise pipe made up in the future. Made home made rearsets and reversed brake rod lever to the top of the drum. Clubman bars, fitted race lever protector bar ends and adjustable race quick throttle or you can do the poor mans version by fitting R6 throttle tube to std switch gear. Waiting for narrow aftermarket kill / starter switch. Make new airbox to fit new carb. Made side stand switch locking plate and relocated it and removed side stand (can't just cut and join wires as it has 3 not 2 and talks via EC. Remove LHS switch gear (choke lever on carb). Using stock clocks as I I need a wiring diagram as with certain plugs disconnected it wont start. Made shark fin for swing arm, and paddock stand bobbins and their mounts that were ali brazed to the swing arm. All race loock wing done Sintered frt pads and dot 5.1 brake fluid. Must do braided hose. Might skim the flywheel. I heard to leave the head stock but learn't some very important info. Looking from the front of the bike to where the exhaust header bolts on you we see a small hole at the back of the barrel indent on the top rhs of the header. This can get partially covered up by flashing in the sand castingprocess. Open it up with a small screwdriver. It's an air passage hole for cooling the head. People who have had these closed have been known to have the head crack in half.

 

The aim is to push to a true 160kph or more top end from the stock 145kph gps measured top end. its never going to be the 200+ I see on my ER650 and SV650S at the end of the straight but at least I can be on par with the youngsters on CBR150 with 50kg pilots.

 

As promised some pics. The 1st pic is from testing last year before I fitted the fairing upper (zx250). Bike now shod with Dunlop Alpha 14's 110/70 and 140/70. Quick throttle fitted and smaller kill switch and starter button and clutch switch removed. I will remove lhs switch gear once new VM34 carb is rejetted and fitted as I need the choke lever. Also on the cards is a custom full high rise pipe and a R6 race seat once I have made up mounting brackets and fitted rivnuts . Got the thunderace rear shock rebuilt but spring is to soft so busy getting a stiffer rate one. One quick question I would like to replace the stock dash with an Acewell 4X54W-XX single pod type. Does the stock dash have any circuitry (chip) inside that is needed to run?

 

Yes already got that looking now to compare pilot jets

 

Bike handles surprisingly good with the new tyres, even with std rear shock. Found some springs from the breakers yard for the thunderace shock just need to get their kg/mm rate measured against the stock shock spring. I can confirm you don't need the dash to run. Busy wiring up the acewell 4X54W-XX single pod type. Looking through the english / 1/2 portuguese version of the cbx / f 250 twister manual, I see they have a 16 pin plug going to the dash. It toke a while to find as its hidden under the rear of the dash housing cover. It's a short harness with 2 different 9 pin plugs fitted on the other end that connects to the main harness. I seams that something is lost in translation as normally when you read wiring colours, black/ brown it means the cable is black with a thin brown marker. The manual seams to be the opposite. The wiring diagram in the pdf manual is illegiable. Anyone here have a better wiring diagram?. A great help would also configuration of either the 16 pin dash plug. eg pin 9 , red +ive from battery or for the 2 x 9 pin plugs.