650 Norton Vibration and Balance Factor

I competed restoring my 1966 650SS last summer and was disappointed with the vibration levels. I have owned the machine for close to 30 years and had ridden it quite a bit up to about 1995 when I decided to build a full house 750 Dunstall replica. I always liked the 650 and decided to restore it properly several years ago.

The crankshaft was reground , cylinders bored .060 over, and everything balanced to 82% balace factor by a noted Norton tuner here in the USA. I had used the same guy for my 750 and it was remarkably smooth up to 70 mph or so (I think the short rigid clip ons helped). Anyway I had problems with thhe aftermarket 650 pistons, or more specifically the wrist pins, as they were too tight in the rods. What to do! After getting some advice I decided to use some NOS wrist pins which fit the rods properly but obviously were a bit looser in the new pistons. The new pistons (JP or GPM? I can't recall) seemed a fair bit heavier than the original (Hepolite?) as well. I then assembled everything, got it running, and put probably around 100 miles on it, tightening bolts, tweaking carbs, hoping for ring seating, etc.

The motor was a bit rough on the left side and fired intermitantly at idle although is picks up once under load. In general the motor is just rougher and vibrates more than I recall. One problem may be that my daily drivers for the past 5 years has been a BMW R75/5 and a Ducati 900ss. In spite of that I think there is room for improvement.

This is what I have done:

1. Found one loose engine bolt (tightening this helped)
2. Using good condition (not sleeved) 376 Monoblocs with 1 1/16" sleeves
3. Timed magneto to 28 deg BTDC using cig paper method and degree wheel.
4. Checked primary chain tension
5. Secured loose alternator rotor with large washer
6. Using flat 27" wide bars (currently with soft grips and filled with buckshot)

Things I plan to do:

1. Check timing on both cylinders using timing light (once I find small diameter degree wheel)
2. Try different set of known to be good, sleeved 389 Monoblocs
3. Fit 22T engine sprocket (21T current)
4. Replace alternator rotor
5. Perhaps shorten bars to around 26"

Questions:

1. Could the heavier pistons spoil the smooth(er) potential of the 650? Do I basically have a 650 Atlas now (less power but with same vibs)?
2. Could the slightly looser fit of the wrist pins in the pistons be a problem (no noises..yet)
3. Should the 650 use a different balance factor? I recall the Dunstall book called out something different (lower) but I wonder if that would help at low to medium cruising speeds.
4. Are lighter pistons available? I plan to save a search for "650 Norton Hepolite" on Ebay

As a aside I had the frame aligned by a guy on the west coast who had a featherbed jig and it really handles as if it on rails, even with rather tired suspension!

Thanks,
Paul R.

The 650 Norton had a 70% balance factor from the factory. When they were new they would be pretty smooth up into the 3000 rpm range then towards 4000 they would start to blur the instruments. Gearing them up helps if you can get it so the rpm at cruising speed is down to 3500 rpm or lower.

With an 82% balance factor, that is something someone with a race-bike would use. My guess is it would make the engine vibrate more at low rpm, and it would still vibrate at higher rpm but maybe not as bad as with the 70% factor. So it sounds like someone put a racing balance factor in a street bike.

The Atlas supposedly came with a balance factor over 80%, and it was a famous shaker too. The heavier the pistons you run of course the more it will shake at all rpms.

Jim Schmidt sells super-lightweight pistons and connecting rods for the Dominator Nortons now, but I am pretty sure his pistons will not work with the standard connecting rods so you end up having to spend a lot of money for the two Carrillo rods plus the matching pistons. I think it would be well worth it though for someone who is bucks-up or very passionate about their Norton Domi.

I don't see how a 650ss would vibrate more than a 750 Dunstall Atlas unless whoever did the work on the engine screwed up.

My favorite setup for my old original 650 Norton Manxman was when I had a 23 tooth gearbox sprocket on it. I could cruise on a level highway at 70mph with my rpms down where the bike was very smooth. This summer I should be riding around on a 62' 650ss that right now I have a 21 tooth sprocket on, but I have the 23 toother in reserve for after I am sure I have all the bugs and teething problems taken care of after completing a lot of serious maintenance on the bike.

One old timer I knew would set the points on the mag, then see where each cylinder fired and split the difference. You are supposed to stone the points cam though to try and get each cylinder firing correctly, but it takes a patient and skilled hand to do it without screwing up the points cam profile and getting points bounce at high rpm.

The .060 pistons are usually the max overbore for the 650, and the pistons for that size simply have very thick walls, they are not machined down as far. So they will be the heaviest of all sizes of course.

Pulling the inlet port sleeves out and putting 389 carbs on the bike might make some of your low and mid-range torque disappear and it might not pull a large gearbox sprocket at low rpm.

Blah blah blah......sorry.

Heavy pistons are the problem with vibration.

The new lightweight JS pistons for the 67mm bore weigh only 170 grams (unfortunately they are special order and not cheap). Original Dominator pistons are probably near 220 grams or more. You have a choice of compression ratio. They are available for 650 cc and you want to special order the JS "ultralight" bushless Carrillo rods with them.

See my earlier post " New product -Domni 500 lightweight pistons and Stuart's Velo"

80% balance factor is too high. Its easy to think that higher balance factors are for higher RPMs - but I no longer believe this after extensive tests. 68% (70% max) is more like it.

For the scoop on how to determine correct balance factor - go here (pay attention to the top post and the scratch tool):

balance-factor-scratch-test-tool-t15134.html?hilit=BALANCE%20FACTOR%20SCRATCH%20TEST%20TOOL

lightweight Dominator pistons below.

Paul, since you cannot change the balance factor or pistons and rods without a lot of expense and effort, I would look to everything else that rotates and can be balanced.
I also have a 650 SS overbored 60 thou with GPM pistons. It came to me in a fully restored/rebuilt state. The vibration level at 70 was awful, at 80 it was even worse. The problem was that the professionally built wheels had not been balanced or trued.
The rear was .130 out of round and .250 out side to side. The front was out almost as much. I trued the wheels to within ten thou then balanced. The bike was instantly so much smoother. There is still some engine vibration creeping in around 70 mph, but it is quite tolerable.
I would have expected vibration from the wheel problem to be a slow frequency type, but it seemed very fast and would numb the hands and feet quickly.
To test for this problem, just run the bike up to or a little above the speed at which vibration is bad, then pull in the clutch and let the engine idle along. If the wheels are true and balanced, then it should sail along (on smooth pavement) with no vibes.
My apologies if you have already done all of this, it is a simple detail but often gets overlooked as we focus on the engine itself as the source of all vibration.
You might also look to the clutch, it is a heavy old lump and likely is out of balance. Adding the Newby belt drive has reduced the vibration levels on my bike somewhat.
Finally, I have noticed with my 650, unlike the other bikes I ride, it smoothens out a lot once warm. I ride it at 50 mph for ten minutes, then up to 60 for ten minutes, then it will slide along at seventy quite smoothly. IOn the other hand, if I go to seventy in the first few minutes, the engine is quite buzzy. So it is telling me to take it easy until fully warmed up.

Glen

My 850 commando engine is balanced at 72% and is extremely smooth up to 7,000 RPM. However it literally shakes at low rpm. You need to choose your operating revs, and balance to suit, or your bike will fall apart. Lighter pistons would be a good move . If you add them, and change nothing else , you get a performance boost, the motor spins up quicker, and will be smoother at higher revs. Balancing your wheels and aligning them with string are good moves too. Also make sure you have a well secured head steady, preferably using rose joints and shoulder bolts, so the pulses from the motor are directed fore and aft. It is easy to make a pair of straight edges from angle iron and get them ground parallel, Hold them in a vise and check your balance factor yourself. Phil Irvings Tuning for Speed tells you how to do it, all you need is your wife 's good set of electronic kitchen scales and a level. I think you will enjoy doing the exercise, but be careful if you need to drill the flywheel to get the desired factor - don't make a grenade out of it.

As far as choosing the balance factor is concerned, there is a lot of info about on what works at various rev ranges. The standard factors for various road motors is useful info, but if you decide you are going to ride your bike quickly and often, you should think about the higher factors. It is worse (more destructive) to have the crankshaft out of balance at high revs. It is usually tolerable if it happens low down the usable rev range.

You need to look into that intermittent firing at low rpm, FIRST. ?
If both cylinders are not firing evenly - at all rpm - who knows what is happening ?

A magazine survey some years back apparently found that 70% of twin cylinder bikes were not running 100% on both cylinders.
Unsynched throttles, maggies, worn engines.
Vibration causes, all of em...

Thanks for all the advice. That is just the sort of experienced input I was looking for.

On a theoretical note, why SHOULD the 650SS be smoother than the Atlas? Is it solely due to piston/recipracating weight if both engines are carefully balanced or did balance factor play a roll? Why should the 650 be balanced at 70% and the Atlas at 84%?

As I said in an earlier post I had built up a Dunstall Atlas with 10.5 : 1 Dunstall pistons, balanced at 82% that was remarkably smooth. However, It I did have to stone the mag ring on that one as the timing was 10 deg different between the two cylinders! It has pretty rough before that. I used a fine drum sander and polishing bob if I recall. The Dunstall also had clip ons, which I am guessing have a natural frequency well removed from the engine forcing frequency, which could have helped too.

I plan to work on the easy stuff and get the 650 as smooth as I can without replacing expensive bits and see if I get things to an acceptable level.

If I did find lighter pistons, could I expect any improvement by installing them since I currently have a balance factor of 82%, and I assume the lighter pistons would just make this number higher?

Thanks!
Paul R.

Rohan said:

You need to look into that intermittent firing at low rpm, FIRST. ?
If both cylinders are not firing evenly - at all rpm - who knows what is happening ?

A magazine survey some years back apparently found that 70% of twin cylinder bikes were not running 100% on both cylinders.
Unsynched throttles, maggies, worn engines.
Vibration causes, all of em...

Click to expand...

Magneto should be checked with timing disc and corrected to fire at 180 degrees true on parallel twins -it's easy when you know how, and doesn't involve a stone of any description.
Lucas K2Fs are nearly always out regarding the above.
Correct size diameter engine bolts, correctly tighten up can dampen out vibes.
As can Tyre fitment on rims and wheels out of true

Good points on tightening things up and tuning for smoothness before chasing after BF.

I worked on a fellows P-11 once where there was poor handling and vibration. The steering head bearing was terribly loose. Nipping it up proper transformed the bike. Might be worth a look in your circumstance.

Hmmm...I would like to hear how you can correct the magneto to 180 deg firing without reprofiling the cam ring. It my simplistic view there are two "lobes" on the cam ring, one for each cylinder. What am I missing?

Thanks for your thoughts!

Paul R.

I would not have thought that the difference in piston weight from an over bore would change the balance factor so much, that vibration appeared in the usable rev range of a nearly standard bike. I'd weigh the pistons and have a think about it. There is a lot of mass in a crankshaft.

650SS said:

Hmmm...I would like to hear how you can correct the magneto to 180 deg firing without reprofiling the cam ring. It my simplistic view there are two "lobes" on the cam ring, one for each cylinder. What am I missing?

Thanks for your thoughts!
.

Click to expand...

We've had this discussion here before about this.
No details were forthcoming.
Conclusion was that someone knew squat about it...

'Conclusion was that someone knew squat about it...'

Did you get out of the wrong side of the bed this morning ?

650SS said:

Hmmm...I would like to hear how you can correct the magneto to 180 deg firing without reprofiling the cam ring. It my simplistic view there are two "lobes" on the cam ring, one for each cylinder. What am I missing?Thanks for your thoughts!Paul R.

Click to expand...

What Rohan was saying, was that since the magneto rotates at half engine speed, while it fires once for every 360 degrees of crank rotation, it is only turning 180 degrees itself. So to get the magneto to fire exactly every 180 degrees of it's rotation is the same as it firing exactly every 360 degrees of crank rotation.

Thanks Ben.

This discussion actually goes deeper than this though - B was here before with twin maggies that don't need perfect cam rings to give perfectly symmetrical timing. Details were vague, to say the least. Although you can jiggle things about and split the difference - which is no substitute at all, really...

The manual tells to stone the cam ring until any wear is evened out, and the firing intervals are EXACTLY opposite, as they should be.

But you never know, maybe there is some simple magic fix out there.....

It is easy t o tell if you have a crankshaft balance factor problem in a bike with a rigidly mounted motor. The bike should vibrate at low revs and smooth out over the top half of the usable rev range, (if it doesn't have balance shafts to dampen the vibration). In commandos the isolastics were intoduced to hide up the destructive vibration at high revs, so this comment does not really apply. I would expect a fast 650SS to vibrate when ridden slowly, but once you get going it should be really smooth. If the vibration only comes in at high revs, you should rebalance the crank. If you pull the clutch in when you are going fast on a smooth road, you should be able to tell whether your wheels need balancing. I don't believe you will ever feel vibration from a difference in ignition timing between the two cylinders, there is too much mass in the crankshaft for that.

Unsynchronised carbs are a common cause of engine vibes.
Agreed, a slight difference in ignition timing between cylinders isn't going to make much difference to how it feels. But if one cylinder isn't running as strongly as it should - from low compression, air leaks, tight valves etc then ignition timing problems aren't going to improve this...

I've overhauled many magnetos for parallel twins over the last year. A fair number had unequal firing intervals. But not in a single case was it a problem with the cam itself.

The main culprit is the bearing insulator at the contact-breaker end. Remove the bearing outer race, re-seat it properly in a new insulator, and hey presto, equal firing intervals (unless of course somebody has already tried stoning the cam to solve the problem).

Ken
Brightspark Magnetos

That is a very interesting observation.
What material do you use for the insulator - and what did Lucas use ?

Is this what Bernhard was referring to earlier ?

Rohan said:

650SS said:

Hmmm...I would like to hear how you can correct the magneto to 180 deg firing without reprofiling the cam ring. It my simplistic view there are two "lobes" on the cam ring, one for each cylinder. What am I missing?

Thanks for your thoughts!
.

Click to expand...

We've had this discussion here before about this.
No details were forthcoming.
Conclusion was that someone knew squat about it...

Click to expand...

Oh really Rohan :?:

For the record, I have written this subject before on this website before Christmas last year, and got insulted for it, so this time….DON ‘T SHOOT THE MESSAGER :!: (You know who I am talking about!)
The Cure for this, and this was printed in the Motorcycle Mecs. Mag dated Aug 1969, as a Q&A section as follows;
Q; how can you set magneto points accurately on a twin cylinder bike?
A; the trouble here is that you can’t get right on both cam lobes within the 3thou tolerance allowed on modern points, as the cam is positioned slightly off centre, so while on side may be all right, the other may be way out. Cure this by shimming magneto end plate but only with a degree plate to ensure that the points break at exactly 180 degrees to each other.
Translated;
obtain .003 &.005 thou Lucas magneto shims and cut them into quarters, and insert required quarter/amount into the correct corner of Mag cover to push end plate so points will align to correct 0 & 180 degree timing Using a 360 degree timing disc.
Note; remove mag centre bolt if using battery/ bulb method on points to find point opening.

This was done for me in the 1970s by a magneto expert who successfully raced 650 Triumphs on track and dragstrip with same K2F / K2FC magnetos, and my engine had somehow got through 6 pistons on one side (the side that was out) until I clocked on to the fault. With this mag fitted, the bike didn’t chew another piston up after that. Wish I had known this sooner.
To those of you who disagree with this method, it is a tried and tested method, you can either spend 1 hour correcting the magneto as above or spend whatever time it takes you to stone the cam ring down, the cam ring which may or may not be correct… Don’t ask me how I know.