V50 mk 2 Starting issue

Have a V50 2 in North London and it has recently had a top end rebuild by professionals. Bike has good compression needless to say but before and after the work we are having problems with one side not firing when cold. Take plug from offending side out and warm up in oven and it fires fine and runs fine. Plugs are apparently new. Tried disengaging choke on offending side to rule out over fuelling at the suggestion of helpers in Epping forest a few weeks ago but to no avail. Had bike for nearly 30 years and it used to start easily on the button. Son now riding it and its never been quite right. Fuel or electrical? Some damage to HT lead sheathing which has been taped. Bike appears to have spark on both sides. Any ideas welcome.

Just a quick idea, having just been through a similar, but somewhat different problem.

Do you have access to a colourtune kit ?

I am afraid not but my son has mentioned this. What is it?

Try giving the carbs a really good clean. Take everything apart and soak it in carb cleaner, WD40 etc. Blow through all the jets, airways holes etc. Get some fine copper wire and poke it down all the holes to make sure they are clear and free flowing. then re-assembleMake sure the mixture screws are the same number of turns out, usually somewhere between 1 & 1/2 to 2 turns. Check you have the O rings on the end of the springsMake sure the throttles are balanced. When you shut the throttle, they should both click shut at the same time. You could initially try swapping the carbs over from left to right to see if the problem swaps over. Ditto the ignition coils.Check the fuel is flowing freely through both taps and that the banjo’s are clear.Then try an old set of plugs. Don’t trust the plugs in it, just because they new.The Mk 2 has electronic ignition, try swapping the black boxes over from one side to the other see if the problem moves with that.

It’s an old fashioned way of checking fuel mixture, absolutely ideal for these old carb bikes with no electronics involved in any mixture settings.

You can get one from Halfords (that’s where mine came from)), but it isn’t listed on their web site, you need the car one due to the size of your plugs (the motorcycle ones will be to small).

On mine the issue was swelling floats, the nearside one would occasionally stick open, and these were new floats, something to do with some eco friendly poohe in petrol these days.

This gave me all sorts of different issues, from poor starting, not needing choke … ever, plugs fouling, poor running, and even occasionally fuel leaking from the nearside carb.

We just shaved some of the plastic from the floats (where the shaft goes through), so they could move sideways a little, and we actually set them to close 0.5 mm earlier, and bingo, all the issues were solved in one go.


I will pass on to my son Josh but he is fast approaching his finals and I’m not sure whether he has the time or the gear to carry out said works in his street in Crouch End. Certainly very helpful suggestions and will probably end up here back in Cornwall before I we can set to work on it. The Colour tune sounds useful regardless as I have a Lemans 2 as well. I have had a look at them online. Thanks fore all the help.


Further to Don, suggests to me the cold-start enrichment (choke) is not working fully on that side, this system is surprisingly sophisticated with an atomiser for the starting jet, I suspect ‘fall-out’ from modern fuel (brown or green caked hard crud) might have blocked the atomiser air holes or the channel in the bottom of the float chamber where the jet gets its petrol from. In my experience carb cleaner often doesn’t touch this stuff so has to be scraped off and/or buy a set of new jets. HTH

Thanks for this. Motocorsa did send the carbs through the ultrasonic cleaner back along and more recently they did query the state of the brass plunger on the choke on both carbs and suggested I replace them ASAP. Another friend suggested that as the heads were reconditioned over 500 miles ago it will be ready for a torque down and reset gaps on the rockers. He thought this could explain the problem if the heads had settled and gaps had closed more so on one side.

I’d replace, or at least test by swapping over, the suppressor caps and also the leads, if you haven’t already done so.

Yes unfortunately if deposits are very bad no amount of fancy aerosols seem to work, never tried US cleaning but don’t believe that will totally shift it either, so forget about such “short cuts”, get in there and do it by hand, has been my experience. Yes choke pistons can get crudded up, corroded, scratched or scored, and the seals in the bottom can get indented and then leak. Last year I ended up having to replace ALL my jets (on VHB 30 carb’s) to be absolutely sure they were completely clear of any residues and thus the correct ‘size’. Problem being you can’t tell if a jet hole is constricted in size by deposit UNTL you get a new one to compare it against, only then it becomes (sometimes glaringly) obvious. HTH