Firing on one cylinder only

[Oggers]

Gents

Another  fault developed with 66 Triumph Bonneville Post the clutch issue - and thanks for all replies - I moved on to service the carbs (Amal 930s).  Just a visual of floats,main jets, slides etc to see if all was well. I removed the tank, removed the carbs, replaced everything as per. It's all pretty new, so didn't notice anything, and didn't really expect to as the bike was running fine. Another thing I did was simply to remove the rounded oblong rocker box covers - it has a later head - with the intention of checking the valve clearances. I didn't bother in the end as all seemed well. All reassembled as per manual and woe! Right hand cylinder is not firing - well mostly, it seems to now and then when testing on the road, but basically there is no power there. Left hand seems fine.

What I have done so far

Checked carb for float faults, needle, needle holder - all seems well. Bowl gasket fine. Carb seems to be delivering petrol to the engine and not flooding - I can see the spray through the inlet. All seems well here.

Checked for spark at plug. Fine. Good spark. Changed the plug just in case. No difference.

Checked the coil - all connections appear fine. No real reason to suspect this. Coils are pretty new.

Gave the exhaust valve several tap on the head- stuck valve perhaps. No change. Admittedly did not do inlet.

It's all very odd - it ran fine a day ago! I may have disturbed something, but no idea what.

All suggestions are most welcome
 
 

[Rex]

This may seem an off-the-wall question, but are you running a Boyer ignition unit and is the battery fully-charged? It's a well-known characteristic of Boyer-equipped bikes that with a low-ish battery voltage they'll only fire on one cylinder in use.

[Oggers]

Yes Boyer, and battery reads 12.5V when not running.

I can only think it is something I have done when either servicing the carbs or removing rocker covers. Would fitting the rocker covers upside down foul the tappets I wonder? I 'll take a look....

Swapped the HT leads - no difference.

Yet to try

Swap the coils - dont see why, but you never know
Run the bike with IN/EX oblong rocker box covers off - too see if valves are sticking. How do you unstick them!
Check O-ring on carb inlet manifold. I may have disturbed it when refitting.

Anything else?

Many thanks

 

[Rex]

Rocker covers upside down?? Is it only firing on one all the time, or now and again?
Put a voltmeter on the battery and read the voltage with the engine running. Blip the throttle and check that the voltage actually rises.

[R]

Its a well known feature of the earlier Boyers that they can break the wire(s) (inside its insulation) where it goes to the pickup plate.

If you waggle that wire/those wires while its running, it may
1. chime in again on both cylinders.
2. cut out on both cylinders !
3. reset itself, reconnect the wire (inside the insulation) - and cut out on one or both cylinders sometime in the future !

The answer is so resolder the wires with known good uns ...

It may be something else of course, so don't be too rough on them . !

[iansoady]

This may seem an off-the-wall question, but are you running a Boyer ignition unit and is the battery fully-charged? It's a well-known characteristic of Boyer-equipped bikes that with a low-ish battery voltage they'll only fire on one cylinder in use.

I've never heard of this and can't see how it can be true as the Boyer is a wasted spark system which fires both cylinders at the same time.....

What does happen with low voltage is that they flop onto full advance which can be trying when starting.

[Oggers]

Right - It seems I have an answer. Carbs are Amal 930s, and the copper clip which secures the needle is split at the very point where the needle clips into it. Thus the needle is not engaged at all with the slide. To compound that, the needle/clip assembly was entagled with the carb spring - rather than resting on the slide body under the spring. The mix would have been hugely over-rich!

No idea where I went wrong. I was very careful on disassembly. The only thing I can think of is that I hit the slide assembly/needle inadvertedly when it was out of the carb and dislodged the needle.

Many thanks for the replies in any case. 

[Rex]

This may seem an off-the-wall question, but are you running a Boyer ignition unit and is the battery fully-charged? It's a well-known characteristic of Boyer-equipped bikes that with a low-ish battery voltage they'll only fire on one cylinder in use.

I've never heard of this and can't see how it can be true as the Boyer is a wasted spark system which fires both cylinders at the same time.....

I've personally experienced it when a rectifier died and stopped the battery charging on an old T120V I had. That said, it's a recognised (at least, to me) symptom when a bike is fitted with a Boyer.
After all, when the battery voltage is reducing the wasted spark system will keep on working....until the voltage is too low, then it won't.

[DM]

Spark plugs work better with a -Ve polarity voltage applied to the electrode, according to this write up applying a +ve polarity voltage to the plug requires up to a 45% voltage increase to fire.
 
http://www.dragonfly75.com/motorbike/plugpolarity.html

So it's not surprising that a wasted spark coil will misfire one plug when the battery voltage is too low.

[R]

That means that a twin cylinder magneto - say a K2F - with one +ve spark and one -ve spark
will spark stronger on one side ??

[DM]

That means that a twin cylinder magneto - say a K2F - with one +ve spark and one -ve spark
will spark stronger on one side ??

No, I think the K2F would have been designed to provide sufficient HT voltage to fire both sides reliably.
But if the magneto deteriorates giving a lower HT voltage I'd expect the +ve spark to give problems first.