I'd like to know if other pre 1971 (Pre OIF) A65 owners, find the vibration uncomfortable at cruising speeds 55-65MPH and around 3500 revs.
Many moons ago, I had such a model. The style, riding position, weight distribution, power and acceleration were just right for me. The big issue however was vibration at speed. It made long journeys on A roads and dual carriageways a pain. In the end I sold it on. During my period of ownership I rebuilt the motor, having the crank balanced by Bassett Down etc but this made little difference.
On the other hand, my OIF Thunderbolt that I had for 15 years was much better particularly when I had the crank balanced on that too. I am wondering if that was down to the Rubber Mounts on the handlebars or was it just pot luck?
Your thoughts and experiences would be most welcome.
Chris
Hi, was your pre oif also a Thunderbolt? I remember my Lightning had noticably more vibration if the carbs were not in absolute perfect sync. If they were in sync the vibes were acceptabel at 55-65 mph.
cant remember my Thunderbolt vibrating, the A10 does but thats probably the plunger suspension and all other work done. the Thunderbolt did have the thicker grips.
Hello,
I think there is a degree of luck. I once had an A10 combo and on sidecar gearing it would get up to an indicated 80 mph quite easily and was very smooth. It had low compression pistons and a spitfire cam. When I rebuilt it with a standard cam it was never as smooth. My current 650 A65 was great at 55 to 65 and still smooth at an indicated 95 mph but on changing the pistons from 7.5:1 to 9:1 it all changed and despite reducing the weight of the pistons not as smooth.
I have yet to see if I have been lucky with my long stroke 750 A65. The compression is about 8.5:1 and it has been dynamically balanced. Once I re-torque the head at 200 miles I will find out as I open it up more.
There is also a possibility that head steadies have role to play in that they could be too good or not good enough. How to work this out I do not know.
Dave.
hi editor, i had a spitfire with the vibration you describe. but if you go beyond 70 mph the vibration becomes noticebly less? however i felt the vibs moved from the pilot to the pillion seat and my future wife gave me a punch if i went above 65mph. have you tried cruising at 75mph?
Good morning,
Just remembered another thing that might help. Has anyone noticed a bit of flat spot or rough patch at about 55mph on standard gearing so possibly at 60 with a 21T gearbox sprocket? It may be just a carb jetting issue but I believe that it was present as standard. The quite subtle issue seems to stem from the length of the inlet tract and by simply fitting a spacer between manifold and carb cures the problem. The spacer needs to be long enough short enough so that the filter can still be fitted so about 1.5" to 2". I discovered this by accident when fitting a spacer to accommodate a Scotoiler fitting. This was spotted by a couple of knowledgable gents at a BSAOC Open Day and Rally who explained the advantages to me.
The fitting of a Mikuni carb will achieve the same as the mounting spigot adds to the inlet tract length.
With such a spacer the engine does seem to run smoother at cruising speeds.
On twin carb heads spacers were often fitted to improve gas flow etc.
Dave.
Thanks for all your help/advice/ideas on this.
I think two possibilities have transpired, apart from balancing the crank which is very expensive and sadly wasted if the pistons are changed due to a rebore.
First is the carburation. On my OIF, that used to vibrate a lot for the first 10 years of ownership. I then had the crank balanced via Steve McFarlane and it was noticeably better and 'livewithable'. It improved dramatically when I put a brand new Amal Premier on it (best £150 I've ever spent on a bike). However, the longer inlet tract is definitely worth a try as it's a relatively cheap option.
Second is the pistons. I thought about lower compression but a loss of power would likely result. However, it's worth a try as, depending on the state of rebore, I would imagine second hand pistons from the likes of John Harding would not be too expensive.
My underlying reason for this post is I have been tracking the prices of '66-'70 A65 Thunderbolts lately as quite a few have been advertised in The Star at around £4000 to £4500 and they haven't sold. I struggle to find a reason and wondered if their reputation for the vibes could be it?
BTW a 1970 Lightning in excellent condition ready to ride sold at the week-end for £4500!!
Chris
Good afternoon,
A bit too hot for me but I did get a 60 mile run in this morning. A high mileage bike so some vibes to be expected. Not too bad at 65mph but could be better. I agree that the A65s vibrate more than the A10s but I think that the weight of any twin seems to increase as time goes by and this could be putting people off the bigger bikes. So equal on that point. I have found kick starting is easier on the A65 so a bias there but swaying the buyer could be that an electric start can be fitted to swinging arm A10s, ditto swinging arm B31, B33 and Goldstars. I do know that someone did specifically buy an A10 because and electric start could be fitted. Roll on the electric start for A65s and and plunger A10s.
Dave.
Hi,
Just a comment on the lower compression. Apart from the last few K my high mileage A65 had low compression pistons and it was smoother. There is a difference in the power but not that much under normal riding. For most of the time I was riding loaded with camping gear and the only really noticeable lack of power was climbing Alpine passes and the big gap between 2nd and 3rd gears sometimes meant a lack of pull through slow up hill corners and up long steep hills.
Dave.
ps Lowering the compression by using two or a thicker head gasket may be worth a try and a lot less cost and work compared with a full top end strip down.
i am under the impression vibration is caused by rocking of the piston and weight above the gudgeon pin makes vibration worse. a solution is to use longer conrods and modern pistons with the gudgeon pin as high as possible. at least one norton race shop does this conversion using corrillo rods and special gudgeon pins.
Good morning Eddie,
Yes. It is the bits going up and down that tend to cause the vibration so I suppose that the advantage of the flat top lower compression piston is that there is a bit less piston above the pin.
Compression could be regained from head modifications.
Dave.