I commend you Stewart for "fessing up"! ,too many owners are unwilling to admit to being dissapointed by their exspensive "improvements" that were a backward step. Done a few myself. Too embarased to mention them.

My 650 vibrates horribly in the lower rev range. My mistake was getting a rebore and then failing to get the crankshaft rebalanced for use with the new pistons. Which are possibly heavier than the original set. Check out the words of wisdom from the late John Hudson....see below. Also bear in mind that crankshafts can be balanced statically, dynamically, dry or oil filled. Each affecting the factor that the shaft should be balanced to.

Balance Factor - Crankshafts

The balance factor to which all twin cylinder cranks were balanced at Birmingham was 70% of the reciprocating weight, but at Woolwich at an Atlas engine number which I do not know they increased this percentage to 85. I certainly have no proof that this figure was or is an improvement and in a conversation with Doug Hele, quite recently, I was discussing balance factors for the twins because another friend had obtained a Nourish crank for a 500cc twin for racing. I therefore asked Doug If we should raise the 70% factor to 80 which is the figure to which flywheel assemblies on Manx, G50 and 7R engines are balanced. (Incidentally the Norton International engines had their crankshafts balanced to 70% and not 80.)However Doug would not recommend a figure higher than 70% because of the greater distance the main bearings are apart compared with the much shorter distance which separates these bearings in a single cylinder engine, and a higher factor on a wide crank assembly for a twin would be likely to contribute to a flexing of the assembly at high rpm.As I understand it there should not be a mechanical reason why the 750cc Atlas engine should vibrate more than its 650cc counterpart. I would doubt if the hollow crown 7.3:1 Atlas pistons weigh appreciably more than the almost 9:1 pistons in the smaller engine.

Have you lightened the crank as well asbalancing to 70%, these are very different things

First check all mounting fasteners, head steady etc, before blaming crank balance

I have found some engines rough until they run in and smooth out amazinly

Laurence

I always thought that crankshaft balance was a compromise between the oscillating and rotating forces,somewhere "halfway up the conrod" was considered the happy medium. In other words 50 to 55%?

That is not the balance factor. What you describe is the rough way we decide what will be reciprocating mass. A 50% balance factor means you will counterbalance exactly half of that (whatever you decided was reciprocating). So, deciding on 20% up the con rod does not give you a 20% balance factor, it just means you are adding more con rod to reciprocating mass and less to rotating mass.

The amount (or percentage) of reciprocating mass you decide to counterbalance is the balance factor. There is a better explanation on this forum somewhere (probably written by Phil Hannam, I don't remember).

Previously stewart_denton-giles wrote:

an offset crankshaft to it and claimed that it would rev to 8500 before it started to shake! I thought "That`s what I want!"

Well, to make a long story short, it turned out to be the biggest disappointment I have experienced in a while. IMO the lightened crank is totally unsuitable for the motor. It revs really well, BUT it shakes just as bad and it gets worse as you go faster.

I have decided to cut my losses and head back to the firm that did the original balance job, with my pride well and truly swallowed, and ask them to rebalance it to something close to "standard".Thanks, Stewart Denton-Giles

Unfortunately, there is no "standard" for an offset crankshaft (90 degree or 270 degree). Norton never used them in the Dominators. You should follow the advice of the person who made it. You didn't say who you got the crank from.

Can anyone recommend the "best" balance factor percentage for my `66 Dommie 650? The bike is currently on the road after a rebuild which included a crankshaft rebalance, which now is something I wish I had left well alone. The resultant vibes made the bike almost unrideable between 2 to 3500 rpm. As the engine was still running in I was not game to rev it any harder than that. It later dawned on me that the balance factor of 70 % recommended in Mr. Dunstall`s tuning manual was for RACING. Doh! It probably would have been ok at 5000, not really practicle for riding around the street. I then read an article in a certain magazine that featured a late model BSA 650 that someone had fitted an offset crankshaft to it and claimed that it would rev to 8500 before it started to shake! I thought "That`s what I want!"

Well, to make a long story short, it turned out to be the biggest disappointment I have experienced in a while. IMO the lightened crank is totally unsuitable for the motor. It revs really well, BUT it shakes just as bad and it gets worse as you go faster.

I have decided to cut my losses and head back to the firm that did the original balance job, with my pride well and truly swallowed, and ask them to rebalance it to something close to "standard". Yes, I know, I can hear someone saying I should not have touched it in the first place, sorry I couldn't help myself.They say the road to hell is paved with good intentions.

Any help here would be greatly appreciated,

Thanks, Stewart Denton-Giles

well the balance factor is 72% and the crank has a off set balance so it helps it go over top dead centre so it a bit heaver on the front side than the back side by a few grams its all to do were you do the drilling in the centre weight, But for me I leave it with Steve Maney . race engineering middleton near Wakefield , he dose have a web site

I didn't know what you meant by "offset". I thought you meant 270 degree. Please disregard.

I have decided to cut my losses and head back to the firm that did the original balance job, with my pride well and truly swallowed, and ask them to rebalance it to something close to "standard".Thanks,

They will probably say they can't add the weight back so you may have to find another flywheel. I recently learned the 750 flywheel uses the same casting as the 650 and these are plentiful so it should not be a problem.

Reading through this, did Stewart have hisflywheel lightened? That's what it seems to me. In my book, that is always something to be done with trepidation. A seriously lightened flywheel can make an engine unusable at lower rpm. As for the vibration, a balance factor of around 70% should be about right. A missing or loose head steady can result in vibration which can shake the plates off a battery and destroy chronometric internals.

Remember to clean out the sludge trap before re-balancing!