cb125twin struggles to go over 40mph

[Alonda]

I have a 1978 cb125twin I have replaced barrel and pistons, ignition coils, condensers and contact breakers. It is very sluggish to get to 40mph and will not go over that. I have tried new carbs which it did not like just kept spluttering so recleaned old ones and put back on. It will start and idle great just lacks power have checked and rechecked timing nothing seems to work any help would be grately appreciated.

[Triton Thrasher]

Are you revving it out in every gear?
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Rrrrrrrrrrrrrrrrrr

[Rogerborg]

The Workshop
Technical discussions and mechanical advice. Need help with your bike?
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Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike

[swampy]

[Teflon-Mike]

We cant do diagnosis by voodoo and 'bikes a bit slow' doesn't really give us much to go on!
Check inlet rubbers & manifold gaskets; that's a known bug-bear. rubbers perish & get torn by improper carb removal, gaskets & o-rings often don't get replaced, while bolts that clamp it all up are a pita to get at and do up evenly to ensure a good seal.
Then there is the notorious carb balancing, and on these, the infamous choke link to sync.
Next; post top end rebuild; Barrel & head are on VERY low torque studs. 16NM off top of my head; lower than the first number on most 'work-shop' torque wrenches, and very easy to strip a stud if you do up a work-shop tQ-Wrench to 'sixteen' somethings! There's also the valves, which on a top-end rebuild would usually benefit from a re-grind or lap to get them sealing properly. If not done, valves dont seal, biuke runs crap. Cam-timing is also critical, and easy to get wrong, and can shift when you clamp down the cam-caps to torque up the cylinder head & barrel. Tappet clerances are also bludy small and easily set to the wrong gap, with an imperial feeler, or a .25 instead of .025 etc; Piston rings can also get snagged when fitting pistons...... did you lube everything up with two-stroke or light oil on rebuild before starting it up? Have you done enough 'gentle' running to burn that all off? Havew you cleaned the plugs since burn-in? Are the plug threads stripped?.... Raft of potential 'faults' that can be created in a top-end rebuild is long, and this could be caused by any number of them... a compression test would be a good start.
Also exhausts; a lot of presumed carb-faults aren't; they are in a rotten or badly modded exhaust pipe; check for holes in the header; incorrectly seated exhaust gasket; leaking exhaust gasket (re-used old one perhaps), big holes in the silencer by the hanger bracket and loose baffles in the muffler.
Ought to give you some targets to start looking at... but, its any-ones guess really; if you have taken half an engine apart it could be that hasn't 'fixed' original problem, could be you have made a problem... and we don't have the bike at to prod & poke to find problems, only you do.
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My Webby'Tef's-tQ, loads of stuff about my bikes, my Land-Rovers, and the stuff I do with them!
Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[Nobby the Bastard]

Which wanker didn't post a CB125T post in workshop?

THAT'S SO SIMPLE!" you say. "WHY do you need a Photo-How to for unscrewing a spark plug?!?!" That's what I thought; & even Tef, who will never give the 'short' version if a 10,000 word essay will suffice, didn't think it was THAT worth mentioning.... BUT, after answering the FAQ "Stripped Spark Plug Hole, What's a Helicoil' umpety hundred times...... he thought it MIGHT actually warrant a mention! Prevention is better than a cure, after all! So!

EXCESSIVE FORCE IS NOT REQUIRED

& Socket Sets are your ENEMY! Far to easy to apply far too much force. But, often only tool in the box that will fit a motorbike spark-plug, and buried deep in a casting with double overhead camshafts either side it can sometimes be awkward and the extensions & knuckle joints, can be useful, but be warned, more between your finger tips and spark plug, less 'feel' you have to avoid over tightening or cross threading.




Keep the socket as square to the spark plug as possible. Let the socket 'fall' over the plug, grip the ratchet 'loosely', and apply a gentle even pressure to the end to start it turning, and hold the centre of the socket over the centre of the spark-plug.


As soon as the initial pressure is taken off the plug, STOP using the leverage of the ratchet. Remove it and if you have to use the socket or extension, twist it like a screwdriver, with 'finger force' only, until you feel the plug starting to 'wobble' in its threads.

THEN wind the last way out, wherever possible PURELY with your fingers.

Refitting plugs is pretty much the reverse.

Putting the plug in; START the plug VERY gently. Avoid just 'dropping' the plug down the spark plug rebate. You do NOT want the electrode on the bottom holding the critical 'electrode gap' to get bashed on the cylinder head and closed up. Engine wont run well if it does. So try and lower it in, preferably with your fingers, and get it central in the hole, before you get metal touching metal.

THEN, again, with your fingers, GENTLY get the spark plug square and twisting into the threads, without cross threading. This high up the hole the plug can 'wobble' a bit, so don't force it, tickle it into the threads.

As removal, if you need to use socket and or extension like a screw driver to wind the plug to the bottom of the hole. ONLY when it has bottomed out, do you need to use the ratchet, and THEN only with finger force to just 'nip' the sealing ring between the plug and head.

Again, cradling the ratchet to keep it central over the plug, and using 'just' finger tip pressure on the handle to apply that 'nip'.

This handy guide was offered on the matter of what that 'nip' may be! Discussion was also had on the topic of using a Torque Wrench, but if you know how to use a torque wrench, you probably don't need this How2!

The Tightening torque on a Spark plug is VERY small; maybe 8ft-lb/10nm/1.1kgm, and most Torque Wrenches, sold to do up cylinder head bolts and things, don't go down to torque settings that low. So the Danger is if you were told to use one, you would very possibly pick up a 'High Range' torque wrench, and confused by the scale, pick a number that is 'similar' to the specified setting, but could be 10, 15, 100 or even 150 times what's needed, and actually be MORE likely to strip your cylinder head threads than NOT using a Torque Wrench!

So be careful of your spark-plugs and gentle in their handling, and your cylinder head will thank you by NOT stripping its thread.

If you are clumsy, brutal or lazy, with them? Well, then the likelihood is you will suffer a cross or stripped thread.

If you do, and you are lucky, the thread may merely be cross threaded, and a 'cleaner die' might be used to reform the damaged threads.

If not, then it will probably require helicoiling; which is a process of drilling out the damaged thread, and cutting a new larger one, the same pitch, and winding in a coil of metal wire to sleeve the thread back to the spark plug size.

If you are very lucky, then this might not need the engine removed from the bike, or necessarily the engine stripping so that the cylinder head can be drilled. But don't bank on it!

And once helicoiled; make the same mistake again, and you can strip the insert AND thread in the cylinder head, and do THAT, and chances are there is probably no cheap or easy fix!

So, prevention is better than the cure, and ALL it takes is a little care and attention.
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trevor saxe-coburg-gotha:"Remember this simple rule - scooters are for men who like to feel the breeze on their huge, flapping cunt lips."
Triumph Sprint ST 1050

[Alonda]

Thanks for all your input everything you suggested has been checked and rechecked. The exhausts are new as the old ones were full of soot and carbon due to the original problem when bought bike was worn piston rings and barrel hence new barrel and pistons. The bike revs and idles great it just really seems to struggle under load.

[Teflon-Mike]

[Alonda]

Have tried all that but still no joy. I am beginning to wonder if although I have good spark at low rpms could it be breaking down under higher rpms could this cause the same symptoms as what I am having.

[Nemo]

Is it running on both cylinders?
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Previously owned: Kawasaki KH125, Suzuki GP125, Suzuki GX 125, Honda CB125 TDC '83, Honda ntv 600, Kawasaki ER5, Kawasaki ZZR600, Aprilia RSV 1000 Mille
Now own: Ducati 848 evo

[Alonda]

Yes it is running on both cylinders.

[Fizzer Thou]

Once the engine is hot,perform a compression test.Then drop some engine oil down each spark plug hole and do the same test again.If there is a big difference between the readings then the piston rings may well have not been run in sufficiently.

Also,check the resistance of the plug caps.On my CB125T2 the plug caps that were on the bike when I bought it were seriously leaking some of the coil output.So I installed new NGK caps and this cured the problem.

https://i666.photobucket.com/albums/vv23/WiNot_Rhencullen/CB125T2/Picture387.jpg
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Just talk bikes.What else is there?

Always have a 'Plan B'

[Alonda]

How can I check compression without a compression tester is there a way to make one and do you know what the correct compression should be on this bike.

[unitynotsocri...]

Check the timing chain and the advance retard mechanism.
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nearly a normal tax paying tosser.......with ferrileness suzi100,cg125,cb125scb100n,cb175,cd100,cj250t,kh250,c15,125 bantam,super 6,rickman gs750,xt500,250rs,dt175,lifan125,dolomite1850,metro,Morris220ld,morrisfg,leyland princess,range rover,corsa,vw camper .now struggling with legs. MORE ORDER = MORE CHAOS