1981 Honda CB125tb engine "running away"

[undernutter]

Hi Folks,

I am recommissioning the above bike after 4 years or so of inactivity. The previous owner gave the engine a partial rebuild- with a new cylinder barrel and pair of pistons.

We had it up and running just before xmas. It ran like an absolute sack of spanners and was incredibly difficult to start- although we did have it running, driving and stopping once after dismantling and cleaning the carburettors, and fitting a new battery.

It turned out that the valve timing was out by a tooth- and this has been corrected.

New points were fitted and correctly gapped today (old ones were miles out), along with new plugs. It fired up instantly and absolutely purred- a million times better than before. It started eagerly with no choke, and idled fine when given light throttle.

However, shortly afterwords, the revs "ran away", shot up very quickly out of nowhere to well over 10k and rising, and could only be stopped by turning the key off.
We tried re-starting the engine, no throttle no choke, and the revs ran away again instantly.

The throttle cable is in good condition, and runs and springs back freely. The carb pistons move freely in the barrels.

This "running away" is significantly lessened by putting the choke fully on- then the revs will top out at around 4k- rising as the choke is released (no throttle). It sounds lumpy when you do this.

Is it perhaps running really really lean? something in the inlet tract stuck open?

When running As described above with the choke fully "on", the right hand pipe expels white smoke but the left hand pipe doesn't.- Bike is a twin-cylinder twin-carb setup whereby each cylinder is fed by a seperate carb and the intake tract is not linked.

Perhaps the right hand side is being mis-fuelled but the left hand side isn;t?

I'd like to renew the air filters and the rubber boots that link the carbs to the block- can anyone suggest any good sources of spares for these bikes?

Thoughts are very much appreciated

Thanks

Dave
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1981 Honda CB125tb :: 1986 BMW 520i

[Robby]

Air leak between one or both of the carbs, and the engine. A split inlet rubber is likely.

[stinkwheel]

If you've checked the throttle valves are in position, the right way round and with the needles correctly seated in them, then next suspect would be an air leak as Robby suggests.

It should be noted that if you "missed" putting the throttle valve back in totally straight first time and the needle touched the inside of the carb instead of going directly down the emulsion tube, it is very easy for the throttle needle to be pushed back up inside the throttle valve. The clip that holds it down in position isn't very strong. Worth checking.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[bradh511]

Is it a slide type carb a make sure the piston in carb is the right way round and b needle is seating down properly.. Didn't read what stinkwheel put feel like an idiot now lol
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Theory 11-10-12 lets hope i get mod 1 and 2 done by the 18th of jan 2013

[Teflon-Mike]

Ah! A Full-Effect model.

The carbs should be Keihin PD's; the TD Super-Dream's PD24's have a rather notorious 'flex-link' between the choke flaps which is CRUCIAL to get linked, and synchronised so both flaps open and close fully together, or you end up with one choke never letting the choke 'off' or worse, randomly flapping about screwing the mixture and strangling the pot; the T's PD26's have a rigid rod-link, I'm not so familiar with, but again, needs critical care to set up.. possibly free-up, first; or you are on a hiding t no-where trying to set carbs up on these motors.

The cab-mounting rubbers are the ext thing to worry about; the PD series of carbs is used on LOADS of small Honda's, but the 'twin-PD's' on the CB's are an anomaly, to the series; they have that alternative choke mechanism to be mounted as a pair; but they also have a curious asymmetric flange pattern; the two stud holes are on a centre line off-set from the centre line of the carbs main barrel by about 4mm.. looks the same as the PD's on CG's ad other bikes, but it ent! Which makes finding replacement rubber's rather hard... cos to my knowledge of five years searching, there ent any new-ones, and the pool of serviceable used ones is small... they are so frequently ripped, by owners pulling the carbs out of them, without first removing the air-box(s) to let them be tugged out 'straight' so they twist them out and tear the rubber. Bad enough on common as muck TD's , for which there are at least enough about you stand a chance of finding a couple that are usable.. good luck finding a pair of 'god' rubbers for a T, that have a bigger mouth for the larger carb.

Other-wise; IF everything else is as it ought; the carbs can be set up dam close on the 'book' settings; but very easy for them to get twiddled out of all sensibility.

BUT, my experience is that far too often, carb problems don't lie in the carb, but in the exhaust. Not sure whether you have 'linked' twin-pipes on that bike, but if so, I would be looking closely down the headers for pin-holing, and in the balance pipe for rot around the T junction and in the joint between the two.

Double check tappet clearances; they are effof tiny; 0,025mm from memory; and very easy for folk to pick the wrong feeler gauge and set 0.25mm or 25thou approx 0.6mm...

But my best bet would be that you have a mis-set choke link, and more than very likely, old, perished and leaking carb-rubbers, possibly, torn and badly repaired, quite possibly later TD rubbers 'flared' shoving a bigger manfold stub into them, also perished & torn, or eve mixing and matching CB 'bits' later TD manifolds that pot 'in' rather than 'out' as well as being a dfferent diameter to the T versions.
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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?'