Hitchcocks Forum

Afternoon all, though not an Enfield problem, I feel someone will have an answer. I have just acquired a Honda cg125 which starts easily and runs well except when coming off the throttle and reving to change down, when I encounter a huge flat spot as I open the throttle. It's only momentary, but definitely not right. Pottering about at low speed it doesn't seem to occur, just when cruising at medium throttle opening and changing down. I'm wondereing if the needle wants to be raised a notch or two. It feels as though I'm about to run out of petrol. Thanks in advance for your learned comments.

Reving to change down?I am old school but why are you doing that?

There’s nothing wrong with revving to change down. I’ve always done it; it helps match engine revs to the gearbox layshaft gear speed. Some modern bikes have auto- blippers specifically for this purpose, which should tell one something...
Re the actual question, flat spots are usually the result of a weak mixture. I would try richening the tickover a little. If it isn’t that then you could try raising the needle. Failing that the slide cutaway is too big. It could also be an air leak. I am unfamiliar with the carb fitted to the bike, but I doubt that different carb slides are available or even needed. You might want to dismantle the carb and give it a good clean, comparing the actual settings with those specified in the manual. Good luck.

Yep, a good clean is probably called for paying particular attention to the needle jet atomisation holes.

Never done it never had to do it and my downshifting is smoothly fine.In my plastic fantastic 100 bhp bikes I used to do clutchless upshifting.Dont do it anymore.

Not really the point is it? The OPasked for help with his carb not for an opinion on the merits of blipping the throttle on downshifts.

If the bike is high mileage then a worn slide/carb body will cause this.

As bikes don't have a syncromesh gearbox, it is recommended to blip the throttle on downshifting.

If it's got a lot of mileage, it could also be drawing oil through a worn valve stem on the overrun.



How does the plug electrode look?



I agree with revband, rev-matching on the downshift gives the gearbox a much easier time and keeps things smooth, especially with a pillion on board.

what the OP needs is a slipper clutch conversion

If it used to run OK but doesn't now then something has happened to cause this. Altering the needle position won't fix the problem but may mask it.
If you got the bike with this problem already existing then it's a bit more difficult.
Flat spots can be too much fuel as well as too little. I'd clean the carb first and check the float height.

Clutchless changing is a good may to wear out a gearbox incidentally. It may feel smooth to you as the rider but the shock to the gear dogs and selector forks is pretty severe. I know racers do it but they often use an ignition cut out to take the torque off the 'box as they shift.
Blip downshifts, on the other hand, reduce shocks in the 'box.