dodgy

[Scotch10]

Hi
I have a 1979 honda cb 125 t2 with only 4550 miles on the clock. Trouble is it starts and idles fine but between 5 and 7000 revs it sputters and lags but then kicks back in and accelerates fine. Timing and fuel mixture are correct it also has new points and plugs. Somebody please HELP!!!!!!!

[Ste]

[The Shaggy D.A.]

[Ste]

[piazza]

Sounds about typical.
A cosmetc make-over, some notions of cafe-sxrambler type stuff going on, and "Oooh! It dont run right... lets blame carbs...."

1/ These motors are not particularly carb sensitive; they dont like pod filters much, but meh! They run pretty hapily without air-box or filter, and pods do a lot to strangle them, nipping flow at the mouth... but even with a pair of the smaller PD24's, with a pair of moped sized pots to feed, they aren't restricted much if at all carb-side.

2/ You will never set up a good carb on a crap motor. Believe me I have wasted far to much time trying, and have tried just about all perms of small honda carbs, singles and twins, and muggered about with the jets and needles, carb-cleans and overhaul kits to now not bother.

These old engines have tiny pots, they don't show obvious bore or ring wear on tear down, they show it by refusing to run right.. on such a small over-square motor, they do not make much manifold vacuum, they like sharp rings and good bore to get any decent 'suck' at the carb...

Constantly fouling plug, to ME suggests the thing is burning oil, probably not a lot past the rings, probably past well perished stem-seals...

As matter of course, 'now', I cut to the chase; I pull the motor and tear-down. A Chinese barrel kit, with new rings and gaskets is chaper than a rebore, at last look aprox £120. Head comes off and gets stipped; valves get lapped in, and new stem seals fitted. Then it goes back together on the Chnky barel, attension payed to the cam and cam bearings, rockers and state of the likely chewed up tappets. Cam gets critically timed in on rebuld, and hint the Chinky replacements are crap... cap bearings are worth the swap though. A-N-D the top-tip, IF the cam-chain loops onto the sprocket easily, and you dont have trouble timing it in... then the cam-chains likely fcuked!

That work done, motor back 'in', the exhaust is the first thing to look at before carbs; few OE pipes will have survved by now, I certanly haven't found any! After market Micron is pretty tough, but the Motad I think better, but I have yet to find any system that hasn't got dings in the down-pipes at kerb-height and few that have avoided pin-holing at the lower end of the header, usually on the outside of the bend.

Again; you are on a fools errand trying to set up a shit-motor on a questionable exhaust, even a decent moto with a crappy pipe... so make sure the pipe is good, and you dont either have two or three crush washers squashed into the port-flange, or no washer at all, omitted by some numpty believing it the mystical 'restrctor'....

Replacing the exhaust studs whilst the head is loose is also a good precaution, and wonderful ad to actually gettng the exhaust to seat, and seal!! But nspect closely whilst 'off'.. not unknown for heads to be butchered by folk with errant hack-and-wrecker drill trying to drill out old ones.. also not unknown to find any odd assortment of M6 olt used in substitution, or for the exhaut ports to be botchred with imperial roofbolts or the like used as self tappers in the alloy, or for M8's to have been forced into stripped holes, and flanges to have been drilled to take them!

With good head, barel, and 'zorst, set the carb up to the book, and away you go.....

Beware carbs overhauled with chinky overhaul kits; they often have the wrong sized jets in them, some marked with mikuni sizing rather tha Keihin, which ca cause some consternation, but most are CG derived and contain the wrong float valce needle, slide needle and wrong sized jets!!! Pilot air-screw however IS useful, especially as the very fine needle on the end is want to sieze in the carb-body and shear when you try adjust.... Remove check and when cleaning check carefully that you do HAVE a fine needle on the end of the air-screw.. A-N-D there's no 'old' sheared needle end left in the carb body!

It's all pretty easy to do, just takes time and a bit of wallet gulping to man up the cash for the barrel kit. and gonad appraisal to get the motor out and venture into the oil-covered bowels B-U-T one over that mental hurdle, its a hell of a lot more likely to actually 'work; and work well, and long-term be less work, hassle and cash, than faffing around with the bits outside the motor hoping that some-how problems will 'adjust' out.

[jaffa90]

Does the spark mechanical advance work freely?
Are both carb slides working together?

[Fizzer Thou]

I am in the process of slowly (read VERY slowly ) rebuilding my CB125T2.This was how I first bought it many years ago.Bit of a state

https://i666.photobucket.com/albums/vv23/WiNot_Rhencullen/CB125T2/LatterDays039.jpg~original

I have had the engine running,only to find that the piston skirts are severely scored and that the valve guides are beyond help.

https://i666.photobucket.com/albums/vv23/WiNot_Rhencullen/CB125T2/2013-02-26201707_zpseb016ef7.jpg~original

But before I tried to start the engine,which I had never had apart,I set the carbs up by eye and made sure that everything was clean.

https://i666.photobucket.com/albums/vv23/WiNot_Rhencullen/CB125T2/Picture387.jpg~original

It does sound as though the pilot jets are blocked.If you can squirt some carb cleaner through the air jets and the fluid comes out of those jets,then you know that they are flowing okay.
Have you measured the float height?
____________________
Just talk bikes.What else is there?

Always have a 'Plan B'

[Scotch10]

Thanks for that. I have stripped carbs but had no compressed air to blow out the airways. I am going to strip them again and borrow a garages compressor for 5 mins if I can. By the way does yours have a side stand they were an OPTIONAL EXTRA when new but I cant find one anywhere.

[Teflon-Mike]

Side-Stand bolts on via bracket/sub-frame on the bottom of the engine.

CD benleys and big-bore CB/CM's all use the same arrangement, and I believe they were standard on the Benley... may need bending to length.
____________________
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?'

[Scotch10]

Yes Teflon Mike I have seen a picture of it in a parts index have you any idea where to look for one

[Fizzer Thou]

[Teflon-Mike]

[Scotch10]

Yes Teflon Mike I have seen a picture of it in a parts index have you any idea where to look for one

[Scotch10]

Thanks fizzer thou will try carb cleaner

[Scotch10]

Thanks Teflon mike I will get hunting

[Nobby the Bastard]

[dydey90]

[jaffa90]

Scotch10 states,
""Trouble is it starts and idles fine but between 5 and 7000 revs it sputters and lags but then kicks back in and accelerates fine."""

How could it be the pilot jets blocked as mentioned????
I would say the needle jets need checking!

[Scotch10]

Thanks Jaffa90 but it has new needle jet and holder plus main jet, I cant get slow jet out as it does not have a slot for screwdriver but it seems clear. the ATU has been cleaned and oiled. Any ideas

[jaffa90]

Between 5, to 7,000rpm is mainly the needle jets and needles position so what are the needles positions?
Also what air filters are fitted?

[Scotch10]

Needle positions are as per Haynes manual and air filters are original and clean

[Scotch10]

Needle positions are as per Haynes manual and air filters are original and clean

[jaffa90]

Well if your happy about the advance and retard spark timing working correctly and no air going in in between the carbs and engine i would be tempted to lift the needles one notch BUT what colour are the spark plugs showing???

[Scotch10]

Not good one is black and a bit oily the other dry but still dark

[jaffa90]

Oh dear, signs of a worn engine, nearly 40 year young with no mileage.
Sounds like somebody has screwed the b*llocks off it or just starting and stopping it umpteen times.
I take it that the crankcase breather is breathing?