Hey all, I'm on my second cbf250. I picked this one up for a steal at $650 with 95000kms on it. I suspect the carby is about to die, and parts are hard to come by. So has anyone tried a different carby on it? I have a couple of 30mm OKO flat slide carbys lying around. Would it be worth it? I'm not sure if 30mm is big enough. Obviously I'd have to bypass the vacuum, adapt the choke and jet it properly. Has anyone tried it?
What is excatly wrong with the carb that you think it is going to die? Have you checked compression and valve clearances yet?
Yep all good. The diaphragm is looking worse for wear and I cleaned out alot of thank corrosion. Regardless of the condition of everything else I'm curious if anyone has used another carby. They aren't performance machines, but using a racing flat slide carb would be interesting
Don't the carburettored versions have a TPS ? That may be your biggest problem with a replacement carb. Otherwise something like a 32mm VM Mikuni would suit. And yes, i see the stock CV carb is bigger.
As @maelstrom says, many late model carbed bikes have TPS sensors. My old R6 had one, ZX6R/ZX7R/ZX9R all have them and I'm sure many others do as well.
A bit more complicated but yes that's part of it. Combing information like engine speed, temperature and throttle position the bikes computer makes adjustments on the fly. Pretty advanced for carby bikes, but nothing much compared to today's fuel injection. Even early injection stuff has issues, for example outside air temp / atmospheric sensors on early R1s only took one reading when ignition on, people ran into issues when going up mountains due to air density. A cycle of ignition would re-read the sensor and the bike runs again, until you go down to sea level again.
I can understand that. I have a 2005 fz6 that has early injection but still needs the throttle bodies balanced for some reason. Quite temperamental. Luckily, I've checked the CBF250 and it doesn't have a TPS. It's very simple. I might just hook it up and see how it goes
Update: looks like my flat slide carburetors that I had lying around are too small. I used the original carburetor and adapted it to fit pwk jets by swapping out the emulsion tube. This allowed me to use jets I had on hand. I cut out the stock airbox and put on a foam filter with a long snorkel in between the filter and the carb. That combined with the semi straight exhaust has de restricted the bike quite a lot. I settled on a 155 jet up from the stock 142 and a 50 pilot jet. Just with those changes it has completely changed the bike. It has way more power and is more responsive. There is a flat spot at mid throttle so I will play with different needles when I have some spare time but very happy so far. Sounds like a dirt bike too which is nice
If the CBF 250 is the same as the CBX250 Twister then you can go for a VM34 copy. Busy doing that myself at the moment. The chaps I have been speaking to say use the same keihin carb jet sizes. The problem come is that Keihin jet sizes are not measured the same as Mikuni (based on flow rate). After some research I found some charts that convert Keihin main jet size to straight hole diameter (I measured the hole size with the aid of tapered round jet round smoothing broaches to check) and another chart to convert hole diameter to Mikuni flow rate sizing. Its not exactly perfect as they are have a 90 deg bevel on the top of the jet which has an effect, but it at least gives you a start point. The marked jet sizes on the copy carb was based on hole size and not the same as Mikuni jet sizes. I will either have to go to the dyno or borrow a wide band oxygen sensor. One thing to take note of that is a big difference between a 2 stroke and 4 stroke designed carb. Many buy the wrong type. I have seen it often with the OKO's (TMX copy). Another route is CRF150 CR slide carb with stock jets or a PZ or PJ 34mm.
