Apologies in advance if this is in the wrong section- but here goes.
Bike in question is a 1981 CB125 TB.
I am recommissioning it 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 out of nowhere, 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 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?
Thoughts are very much appreciated
Thanks
Dave
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Bike in question is a 1981 CB125 TB.
I am recommissioning it 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 out of nowhere, 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 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?
Thoughts are very much appreciated
Thanks
Dave
