Cbr 125rw revs jumping at high throttle

[DragonBoi]

Hi, first time posting, I have a CBR 125RW that I've bought and am fixing up from none runner. I cleaned out the car and it started up, it idles fine and revs drop quickly when I let go of the throttle but when I try to rev her up past about 6-7k rpm the revs just start jumping up and down like its trying to rev up but something is cutting it off. I put a K&n Air filter in it and there's an aftermarket exhaust on it. The air/fuel mixture screw doesn't seem to be doing anything and I'm a bit lost as to what else to do with it. Any ideas?

Thanks in advance for any suggestions!

[martin734]

If you still have them I would put the stock exhaust and filter back on and see if you still have the problem. That will at least confirm whether or not the new filter and/or exhaust are causing the problem.
If everything works well as standard then either keep it as standard or change one part at a time. By changing one part at a time you are reducing the number of variables that affect how the bike runs. Change one part, see the affect it has on the engine and adjust if necessary to get the engine running correctly. Then change the next part and repeat.

If the engine is not running correctly as standard then there is clearly something wrong with the bike. I would start by checking for spark. For this I use a spark tester similar to https://www.amazon.co.uk/LINE-IGNITION-SPARK-PLUG-TESTER/dp/B00XPMN7AU testing the spark by holding the plug against the engine and looking for a spark is not always reliable as a plug can often spark in atmospheric pressure, but fail to spark under load. I would also pull the plug out and have a look at it. Are there any sooty, oily or chalky deposits? is the insulator cracked or burnt looking? the condition of your spark plug can tell you if your engine is running rich or lean, or burning too much oil. While you hve the plug out, test the engine for compression with a compression tester (a finger in the spark plug hole is not a reliable method of testing compression), most bikes run a 10:1 compression ratio or higher so you should be looking for at least 145-150 psi. If it is lower than that then there is likely to be a problem with the engine.

If the spark is ok, start checking the fuel system. Check for pipe blockages or cracks and splits that let air in. check the fuel tap is functioning correctly and isn't blocked, check the fuel filters there may be an inline one and/or one in the tank on the fuel tap.

If you still find no problems check your air filter isn't blocked. Check there are no cracks, splits or holes in the airbox. Check the hoses from the airbox to the carb for splits or air leaks and check the gasket between the cab and cylinder head for leaks.

If after all of this you still can't get the engine to run properly then I would start dismantling the engine and looking at its internals. if you don't know what you are doing with this, then I would take it to a garage, but things like valve clearances are fairly easy to do.

Engine fault finding can be very frustrating but just do one thing at a time, adjust one thing at a time and replace or change one thing at a time.
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Current bike: ST1100 "Big Red"

[Robby]

First off, stop messing with the mixture screw. This only really affects the mixture at idle. Set it back to the factory setting, which is located in the Haynes/workshop manual.

I'm assuming you're trying to rev it at a standstill, rather than on the road, under load. This would suggest an electrical problem rather than a fuel supply/carb problem, because it is using bugger all power just to spin the engine up so the carb is barely opening.

Exhaust and air filter are not going to cause this problem. They could cause the mixture to be slightly lean, but in reality this will not be an issue. It isn't going to run lean enough to cause a problem. Also assuming you're running an air filter in the standard air box and not a pod filter - pod filters cause all kind of problems.

So, electrical, its making a good enough spark at low revs but not at high revs. You're fixing up a non-runner, and it's a CBR125, so it's been fiddled with by teenagers before. Bits to check:

1. Spark plug, HT lead, plug cap. All are cheap, replace.
2. Earthing for the wiring loom and the coil. You need to have a good clean contact between the metal of the frame and the wiring loom, no rust or corrosion. Hunt down the points where the green wire (earth in Honda wiring looms) connects to the frame, and clean up the connector and frame with some emery paper. A light smear of grease helps to slow down corrosion. Same goes for where the ignition coil bolts on to the frame.
3. Ignition switch. On this particular model of bike, I have seen problems with the ignition switch wiring getting pinched or worn around the headstock. This wears through the insulation until the wires can short out on the frame. Check the wiring from the ignition switch (only two wires I think, could be 3) to see if it shorting on the frame, or broken internally. Also check the main wiring bundle that loops around the headstock for any chafing.

Before doing any of this though, make sure it has oil in the engine. CBR 125s either drink a bit of oil, or the owners never check it, but we're always getting threads on here about knackered engines that were run out of oil.

[Easy-X]

If it's rideable you might want to just go up and down the road a bit just to see if it's the same under load. I would imagine it'd be much worse But more information is always good.
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Royal Enfield Continental GT 535, Husqvarna Vitpilen 401, Yamaha XSR700, Honda Rebel, Yamaha DT175, Suzuki SV650 (loan) Fazer 600, Keeway Superlight 125, 50cc turd scooter