General => Douglas Motorcycles - General Discussion => Topic started by: Brian Cape Town on 24 Mar 2009 at 19:34

Title: Mark V vibration
Post by: Brian Cape Town on 24 Mar 2009 at 19:34
The Mk.V has finally been registered for the road, if anyone feels they have too much bureaucracy then come to South Africa, it took 3 months and 25 sheets of documentation to road register the bike.

The fine tuning after the complete rebuild is proving difficult for although I have new carb bodies the slides are old and I am having trouble with slow running and tickover. I will persevere and will no doubt resolve the issue, the main trouble is vibration which appears to be excessive, now I know it is a very long time since I rode a Duggie but I cannot remember vibration being so noticeable. Could it be unequal firing? the flywheel is not loose! any thoughts?

Cheers Brian
Title: Re: Mark V vibration
Post by: eddie on 24 Mar 2009 at 19:57
           Unbalanced carburettors may be part of the cause. Loose engine mountings will also exaggerate the problem. The front engine bolt is not usually a problem but the gearbox/ footrest bolt may not be clamping the gearbox. There is a frame cross tube just behind the footrest bolt and this may be preventing the frame from clamping the gearbox. Try releasing the footrest bolt and see if you can fit any shims between the frame lug and the spacer - then re tighten the footrest bolt. There is often a depression in the side of the gearbox where the spacer rests, making it difficult to fit a longer spacer - but once in position, there may be some end float, which must be shimmed out.
        Regarding carburetion, here in the UK, with our unleaded fuel, I have found that Mark engines run better with a slightly smaller than standard cutaway on the slide with the needle lowered by one groove to compensate and the main jet increased to a 85 or 90.
Title: Re: Mark V vibration
Post by: trevorp on 25 Mar 2009 at 12:46
put an icypole sticks under each slide and it is an easy setup to visually check the balance of the 2 carbs and then just open throttle and adjust cables to both move at the same time engine not running of course. its a bit bodgy but works fine
Title: Re: Mark V vibration
Post by: richson on 25 Mar 2009 at 21:41
Hi Brian,

If its a low revs vibration which happens as you open he throttle I would say that its down to unbalanced carbs. As Trevorp says, it's simply corrected - I use drill bits - just put them underneath the slides then open the trottle to see if they both move at the same time. If the slides are worn you will notice that after setting an even tickover the revs die if you just tickle the throttle open - the slides straightening up in the carb bore and shutting off the extra air getting past them in the rest position. There is no cure, you must replace the slides with new ones. Same with the amal concentrics - I spent ages messing about with them on my Nortons but at the end of the day worn slides or bores will not ever be any good. I had this problem with my comp, and when i fitted new slides and balanced them it disappeared straight away. Jetting is a different matter - it depends on what you have fitted to the inlets and exhausts, but it is fairly simply sorted by looking at your plugs after a ride.


Title: Re: Mark V vibration
Post by: Doug on 26 Mar 2009 at 01:21

I hope it is just a matter of carburation, as that is easy to fix.

Other possibilities, and not quite so easy to fix are:

Uneven timing; I have seen badly machined end covers for Lucas magnetos where the cam ring was not concentric to the armature. The result was the points broke sooner for one cylinder than the other, and resulted in low speed vibration that would smooth out slightly (or was masked) at speed. Usually it is easier to just modify the cam ring to get even timing.

Bad crankpins/rollers. When my professional (expensive) rebuilt Mk3 crank started to fail, it started to run rough and vibrate; worse under load. Sort of like the sensation of taking a hill with out retarding the spark but the engine is not quite to the point of pinking. On disassembly the front crankpin bungs had fallen out, starved the unit of pressure lubrication (it was surviving on splash), and the roughness was from the flats worn on the rollers.

Crankshaft twisted/shift. This I would think would vibrate worse the faster the engine turned. I have not had it happen directly to me, yet.

I repeat, I hope it is just unbalanced carbs!

Title: Re: Mark V vibration
Post by: Brian Cape Town on 04 Apr 2009 at 22:16
Thanks all for the advice although I didn't like the sound of what Doug was talking about!!.

Investigations resulted in a leaking near side cylinder gasket caused by dodgy studs, near side carb way out of tune, the cable was hooked somewhere and was not moving until the throttle was half open. All now fixed with the vibration much reduced, but still apparent. Once again thanks to all