cb125tdc bigger carbs

old1eye · Borekit Bruiser Joined: 01 Sep 2011 Karma :

hi guys, i am gas flowing the head on my cb125tdc and port polish, skim.

i am wanting to put bigger carbs on as well since i have heard that they are a dog to set up with out the air box? (my cd250 twin carb was a doddle)

i want to know which would be best as i know that a smaller carb gives better air pressure than larger but less feul so theres a fine line between too big a carb.

not sure what the standard size is (think 22m), i am looking at trying ither 2 mikuni vm24 (round or flat slide?) or the mikuni vm26.

any thoughts?
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old1eye · Borekit Bruiser Joined: 01 Sep 2011 Karma :

also does any one know what the flex ally exhaust tubing is like?
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Teflon-Mike · tl;dr Joined: 01 Jun 2010 Karma :

CB125TD-C engine wont respond much, if at all to head flowing. It already has 50% more flow capability than it can utilise, curtecy of UK bikes having 'reduced effect' camshaft profile. & Honda down-sized carbs from PD26's to PD24's to improve mid range carburation to suit de-tuning.
Full power, with rare as rocking horse cam, and PD26's these motors can make 17bhp.

Therfore the weal-link in the power train isn't port flowing, threfore unlikely to do anything.

Also suggest you mic up some of the port wall sections before you start trying to take metal off them.

There is NOT a lot of metal in some places, particularly the region of the valve stem.

From memory, there is also a 'lump' of metal in the corner of the port between the inlet tract from the carb as it 'knecks' into the valve gallery.

Very tempting to look at that and think 'We'll have THAT' off, make port bigger, straight away!" but in fact its pretty critical and is a pretty important flow 'spoiler' smoothing turbulance accross teh discontinuity, helping increase flow, and more, baffling back flow when air hits back of valve gallery and tries to come back.

I looked at the head in detail for what might be done, and its not got a lot of scope, for anything really. Ports are pretty large, to begin with, and as said, not a bad shape... and not a lot that could be done to improve it without building up with ali-weld, and re-cutting to a more modern profile.

Valves too, are incredibly close together and about as big as you can get into the combustion chamber.

Fitting bigger valves would be a right pain, you would have to drill out the guides, fill guide holes and re-drill, not JUST off set, to get bigger valves in, but also an a new angle as its a Hemi-Head.

Move valve around 1mm, so you can get bigger valves, becouse of the angle end of valve stem would be shifted more like 5mm, and you would have to extend the rockers to work it.....

You then have the valve lift and cam timing, and as standard they run very very close, inlet actually opening 'over' the face of the closing exhaust..... severely limiting possible cam lift and duration...

In short, its not an easy engine to tune, as a generation of teenagers discovered, buying RD125LC's when they gave up trying!

& Even Honda sort of gave up. The gains to be found over the full power variant were are so small, engine bing in so high a standard state of tune, to get more, they simply went back to reduced effect states of tune and bored & stroked the bugger... theres no replacement, for displacement!
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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?'

old1eye · Borekit Bruiser Joined: 01 Sep 2011 Karma :

Probably a silly question but what are your thoughts on a cb250 3valve head going on then? Will also be looking at one of the 142cc barrel kits
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stinkwheel · Bovine Proctologist Joined: 12 Jul 2004 Karma :

I'll tell you what made mine run vey smooth and better than it ever had done.

That was taking the twin carbs off and fitting a single carb using a CM125 2-1 manifold and a pod filter.

Those bikes are almost impossible to balance properly on the twin carb setup they have and it's something that gets worse as they get older.

I've never seen one that fires up on both pots straight away so one cylinder is constantly slaving off the other.

They carburate perfectly adequately on one of the standard carbs on standard jetting with a pod filter and run clean and smooth as you'll hear one. So while the carburettion isn't optimal on paper, it's actually better than it was on the twin carbs in reality.

So my suggestion would be to fit a single carb of whatever size.
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old1eye · Borekit Bruiser Joined: 01 Sep 2011 Karma :

Teflon-Mike · tl;dr Joined: 01 Jun 2010 Karma :

CB250N 'Super-Dream Delux' 3valve is completely different family of engine. Without a heck of a lot of machining I doubt it would even fit, and with centres distanced to go over a pair of 200+cc barels on the 400, & 450 models, the 250 lump as sleeved down from, doubtful whether you would even get the combustion chambers in the same county as the barels on the 125 engine!

Non-Starter really.

I sort of disagree with Stink, that CB125 twin-carbs are a pain to balence...... never had a problem myself....

Carbs are a pita to set up, though, and choke linkage is main pain. But properly set up, they DO run a hell of a lot better than you'll ever get them to on a single branch CM/CD manifold, regardless of carb on the end.

I have had two dream engines with single carb 'conversion'.... its a cop out!

Manifold is very restrictive, but just about flows enough air for the reduced effect 125 engine. Its same manifold as 200 benley, and 'good' just for up to 15bhp.

In thoery the PD24 on the Super-Dream has enough flow to supply one pot of reduced effect engine. And becouse its a four stroke, its only being sucked on for 1/4 of the time, when the motors on the induction stroke. Hence, slamming one on a two branch manifoild theory holds it has capacity to flow enough air to supply two pots of the engine, utilising suck in the down time of redundant strokes....

Reason why four pot engines can get away with silly small carbs on four branch manifoild.... where the manifold evens out the suck pulses and directs them to the single carb, where becomes almost constant 'flow', evening out metering insconsistancies of big air speed changes.

BUT, the Benley and CM motor its intended for has 360 crank, so its sucking on one branch then the other with equal gaps between. CB motor is 180 crank, giving asymetric pulses through the plumbing.... giving more 'confused' signal to the carb.

Use CM's CV carb on one, and even thoiugh its not the most sophisticted fuel metering device ever, it can get rather vexed by it.

Oft used is the PD26 carb of the CG125, or earlier CD-Benley 125, n single branch manifold, as its flow rating is a closer match to 12bhp flow requirement.... but the PD24, can actually work slightly better, giving higher air speed, and less low speed 'fluff', on twin branch manifold.

However, its still a cop out, and it does rob engine response, and 'hold back' the engine. Its not something you would see on a dyno, running engine at steady state, but the pick up is less eager, with the single branch manifold, compared to 'well set up' twins..... though possibly better than badly set up twins (very common) and likely it will work better on shagged engine, that doesn't carburate cleanly no matter how well set up the carbs are! (also very common!)

THAT asside.... single carb on early twin shock, there is niggle that the carb mounth is very close to where the frame spine curves round behind the engine, limiting filter options. On Super-Dream, there is 'some' space, the cradle having twin hoops either side of teh carbs in that area, BUT, support rail for the mono-shock is close where the carb mouth would sit on a single manifold. Manifold puts carb about 1/2" further back, and slightly lower than single 'stubs' and its touch and go whether a stubby K&N will go on a carb withouit fouling the shock mount. (Bike I scrapped had dire T-Shock conversion, and the mono-mount actually hacksawed out to make space for single carb conversion!) AND you have to ditch the air-boxes and fabricate support for battery, or cut up the air-boxes to make space for carb and retain battery tray.

Its a lot of hassle for questionable benefits, and certaionly not a performance mod!

Meanwhile, why do you keep asking about Mikuni's?

Honda fitted Keihins....
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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?'

old1eye · Borekit Bruiser Joined: 01 Sep 2011 Karma :

i used to build and mod monkey bikes with the 'honda nice' engines, always used mikuni and im very good at jetting them right. they also have quite a high air flow rate for the bigger sizes.

the vm 24 and 24 flatslide and the vm26 are what i am looking at twinning onto the cb engine. i need new piston and barrels but im not sure what the ones on flebay are like. any comments on those? 142cc or stock?

the cmx 250 and cd250 barrels are the same as the ca125 and there heads were interchangble. would that work onto the cb as the barrels are the same as the ca125s?
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Teflon-Mike · tl;dr Joined: 01 Jun 2010 Karma :

old1eye · Borekit Bruiser Joined: 01 Sep 2011 Karma :

ok cheers for that, the 142's are on ebay for about £92, posted 3-7 days. The stockers are on for around £80 same postage.

I am having new throttle cables made up to be slightly longer as i would like to mount the mikuni's on 45 degree manifolds, coming slightly out of the sides of the bike so the is more room inside the frame.

I shall stick with the stock head then and see if i can find the cam for the T model, there on ebay from china but i dont think there the full power version as some are listed as tdc and T models.

Would like to get over 17hp, the cd250 i popped in my ca125, i tunned on the dyno for 20bhp but due to the bikes design it wouldnt top 78mph, but i was keeping the gearing slightly short for round town.
I dont realy like the CV carbs though.
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